Ramaswamy, V, Yi Ming, and M Daniel Schwarzkopf, April 2021: Forcing of global hydrological changes in the twentieth and twenty-first centuries In Hydrological Aspects of Climate Change [Pandey, A., S. Kumar, and A. Kumar (eds.)], Springer, Singapore, Springer Transactions in Civil and Environmental Engineering, DOI:10.1007/978-981-16-0394-561-76. Abstract
The Earth’s climate system in the twentieth century has experienced significant effects due to human-influenced factors. In this paper, we focus on the manner in which anthropogenic aerosols have radiatively forced changes in temperature and precipitation and contrast the effects with that due to the influence of well-mixed greenhouse gases. We employ the NOAA/Geophysical Fluid Dynamics Laboratory 3rd generation global climate model to simulate and derive a mechanistic understanding of the response to the forcings. We find that, over the twentieth century, anthropogenic aerosols have counteracted greenhouse gas effects to a substantial extent with regards to climate forcing, temperature and precipitation. The manner in which this comes about is traced through the effects on the atmosphere and surface heat balance, with resultant effects on the hydrologic cycle. Understanding of the twentieth century precipitation change is a prerequisite for confidence in model-based projections of the effects in the twenty-first century in response to emission scenarios of greenhouse gases and aerosols.
We document the development and simulation characteristics of the next generation modeling system for seasonal to decadal prediction and projection at the Geophysical Fluid Dynamics Laboratory (GFDL). SPEAR (Seamless System for Prediction and EArth System Research) is built from component models recently developed at GFDL ‐ the AM4 atmosphere model, MOM6 ocean code, LM4 land model and SIS2 sea ice model. The SPEAR models are specifically designed with attributes needed for a prediction model for seasonal to decadal time scales, including the ability to run large ensembles of simulations with available computational resources. For computational speed SPEAR uses a coarse ocean resolution of approximately 1.0o (with tropical refinement). SPEAR can use differing atmospheric horizontal resolutions ranging from 1o to 0.25o. The higher atmospheric resolution facilitates improved simulation of regional climate and extremes. SPEAR is built from the same components as the GFDL CM4 and ESM 4 models, but with design choices geared toward seasonal to multidecadal physical climate prediction and projection. We document simulation characteristics for the time‐mean climate, aspects of internal variability, and the response to both idealized and realistic radiative forcing change. We describe in greater detail one focus of the model development process that was motivated by the importance of the Southern Ocean to the global climate system. We present sensitivity tests that document the influence of the Antarctic surface heat budget on Southern Ocean ventilation and deep global ocean circulation. These findings were also useful in the development processes for the GFDL CM4 and ESM 4 models.
We describe the baseline coupled model configuration and simulation characteristics of GFDL's Earth System Model Version 4.1 (ESM4.1), which builds on component and coupled model developments at GFDL over 2013–2018 for coupled carbon‐chemistry‐climate simulation contributing to the sixth phase of the Coupled Model Intercomparison Project. In contrast with GFDL's CM4.0 development effort that focuses on ocean resolution for physical climate, ESM4.1 focuses on comprehensiveness of Earth system interactions. ESM4.1 features doubled horizontal resolution of both atmosphere (2° to 1°) and ocean (1° to 0.5°) relative to GFDL's previous‐generation coupled ESM2‐carbon and CM3‐chemistry models. ESM4.1 brings together key representational advances in CM4.0 dynamics and physics along with those in aerosols and their precursor emissions, land ecosystem vegetation and canopy competition, and multiday fire; ocean ecological and biogeochemical interactions, comprehensive land‐atmosphere‐ocean cycling of CO2, dust and iron, and interactive ocean‐atmosphere nitrogen cycling are described in detail across this volume of JAMES and presented here in terms of the overall coupling and resulting fidelity. ESM4.1 provides much improved fidelity in CO2 and chemistry over ESM2 and CM3, captures most of CM4.0's baseline simulations characteristics, and notably improves on CM4.0 in (1) Southern Ocean mode and intermediate water ventilation, (2) Southern Ocean aerosols, and (3) reduced spurious ocean heat uptake. ESM4.1 has reduced transient and equilibrium climate sensitivity compared to CM4.0. Fidelity concerns include (1) moderate degradation in sea surface temperature biases, (2) degradation in aerosols in some regions, and (3) strong centennial scale climate modulation by Southern Ocean convection.
In this two-part paper, a description is provided of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). This version, with roughly 100km horizontal resolution and 33 levels in the vertical, contains an aerosol model that generates aerosol fields from emissions and a “light” chemistry mechanism designed to support the aerosol model but with prescribed ozone. In Part I, the quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode – with prescribed sea surface temperatures (SSTs) and sea ice distribution – is described and compared with previous GFDL models and with the CMIP5 archive of AMIP simulations. The model's Cess sensitivity (response in the top-of-atmosphere radiative flux to uniform warming of SSTs) and effective radiative forcing are also presented. In Part II, the model formulation is described more fully and key sensitivities to aspects of the model formulation are discussed, along with the approach to model tuning.
In Part II of this two-part paper, documentation is provided of key aspects of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode has been provided in Part I. Part II provides documentation of key components and some sensitivities to choices of model formulation and values of parameters, highlighting the convection parameterization and orographic gravity wave drag. The approach taken to tune the model's clouds to observations is a particular focal point. Care is taken to describe the extent to which aerosol effective forcing and Cess sensitivity have been tuned through the model development process, both of which are relevant to the ability of the model to simulate the evolution of temperatures over the last century when coupled to an ocean model.
We use transient GFDL-CM3 chemistry-climate model simulations over the 2006-2100 period to show how the influence of volcanic aerosols on the extent and timing of ozone recovery varies with a) future greenhouse gas scenarios (RCP4.5 and RCP8.5) and b) halogen loading. Current understanding is that elevated volcanic aerosols reduce ozone under high halogen loading, but increase ozone under low halogen loading when the chemistry is more NOx dominated. With extremely low aerosol loadings (designated here as ‘background’), global stratospheric ozone burden is simulated to return to 1980 levels around 2050 in the RCP8.5 scenario, but remains below 1980 levels throughout the 21st century in the RCP4.5 scenario. In contrast, with elevated volcanic aerosols, ozone column recovers more quickly to 1980 levels, with recovery dates ranging from the mid-2040s in RCP8.5 to the mid-2050s to early 2070s in RCP4.5. The ozone response in both future emission scenarios increases with enhanced volcanic aerosols. By 2100, the 1980-baseline adjusted global stratospheric ozone column is projected to be 20-40% greater in RCP8.5 and 110-200% greater in RCP4.5 with elevated volcanic aerosols compared to simulations with the extremely low background aerosols. The weaker ozone enhancement at 2100 in RCP8.5 than in RCP4.5 in response to elevated volcanic aerosols is due to a factor of 2.5 greater methane in RCP8.5 compared with RCP4.5. Our results demonstrate the substantial uncertainties in stratospheric ozone projections and expected recovery dates induced by volcanic aerosol perturbations that need to be considered in future model ozone projections.
Pincus, Robert, Eli J Mlawer, L Oreopoulos, A S Ackerman, S Baek, Manfred Brath, Stefan A Buehler, K E Cady‐Pereira, Jason N S Cole, J-L Dufresne, M Kelley, J Li, James Manners, David J Paynter, Romain Roehrig, M Sekiguchi, and M Daniel Schwarzkopf, July 2015: Radiative flux and forcing parameterization error in aerosol-free clear skies. Geophysical Research Letters, 42(13), DOI:10.1002/2015GL064291. Abstract
This article reports on the accuracy in aerosol- and cloud-free conditions of the radiation parameterizations used in climate models. Accuracy is assessed relative to observationally-validated reference models for fluxes under present-day conditions and forcing (flux changes) from quadrupled concentrations of carbon dioxide. Agreement among reference models is typically within 1 W/M2, while parameterized calculations are roughly half as accurate in the longwave and even less accurate, and more variable, in the shortwave. Absorption of shortwave radiation is underestimated by most parameterizations in the present day and has relatively large errors in forcing. Error in present-day conditions is essentially unrelated to error in forcing calculations. Recent revisions to parameterizations have reduced error in most cases. A dependence on atmospheric conditions, including integrated water vapor, means that global estimates of parameterization error relevant for the radiative forcing of climate change will require much more ambitious calculations.
The late 20th century response of the South Asian monsoon to changes in anthropogenic aerosols from local (i.e., South Asia) and remote (i.e., outside South Asia) sources was investigated using historical simulations with a state-of-the-art climate model. The observed summertime drying over India is replaced by widespread wettening once local aerosol emissions are kept at pre-industrial levels while all the other forcings evolve. Constant remote aerosol emissions partially suppress the precipitation decrease. While predominant precipitation changes over India are thus associated with local aerosols, remote aerosols contribute as well, especially in favoring an earlier monsoon onset in June and enhancing summertime rainfall over the northwestern regions. Conversely, temperature and near-surface circulation changes over South Asia are more effectively driven by remote aerosols. These changes are reflected into northward cross-equatorial anomalies in the atmospheric energy transport induced by both local and, to a greater extent, remote aerosols.
Fry, M, M Daniel Schwarzkopf, Z Adelman, and J Jason West, January 2014: Air quality and radiative forcing impacts of anthropogenic volatile organic compound emissions from ten world regions. Atmospheric Chemistry and Physics, 14(2), DOI:10.5194/acp-14-523-2014. Abstract
Non-methane volatile organic compounds (NMVOCs) influence air quality and global climate change through their effects on secondary air pollutants and climate forcers. Here we simulate the air quality and radiative forcing (RF) impacts of changes in ozone, methane, and sulfate from halving anthropogenic NMVOC emissions globally and from 10 regions individually, using a global chemical transport model and a standalone radiative transfer model. Halving global NMVOC emissions decreases global annual average tropospheric methane and ozone by 36.6 ppbv and 3.3 Tg, respectively, and surface ozone by 0.67 ppbv. All regional reductions slow the production of PAN, resulting in regional to intercontinental PAN decreases and regional NOx increases. These NOx increases drive tropospheric ozone increases nearby or downwind of source regions in the Southern Hemisphere (South America, Southeast Asia, Africa, and Australia). Some regions' NMVOC emissions contribute importantly to air pollution in other regions, such as East Asia, Middle East, and Europe, whose impact on US surface ozone is 43%, 34%, and 34% of North America's impact. Global and regional NMVOC reductions produce widespread negative net RFs (cooling) across both hemispheres from tropospheric ozone and methane decreases, and regional warming and cooling from changes in tropospheric ozone and sulfate (via several oxidation pathways). The total global net RF for NMVOCs is estimated as 0.0277 W m−2 (~1.8% of CO2 RF since the preindustrial). The 100 yr and 20 yr global warming potentials (GWP100, GWP20) are 2.36 and 5.83 for the global reduction, and 0.079 to 6.05 and −1.13 to 18.9 among the 10 regions. The NMVOC RF and GWP estimates are generally lower than previously modeled estimates, due to differences among models in ozone, methane, and sulfate sensitivities, and the climate forcings included in each estimate. Accounting for a~fuller set of RF contributions may change the relative magnitude of each region's impacts. The large variability in the RF and GWP of NMVOCs among regions suggest that regionally-specific metrics may be necessary to include NMVOCs in multi-gas climate trading schemes.
Results from the simulation of a coupled chemistry–climate model are presented for the period 1860 to 2005 using the observed greenhouse gas (GHG) and halocarbon concentrations. The model is coupled to a simulated ocean and uniquely includes both detailed tropospheric chemistry and detailed middle atmosphere chemistry, seamlessly from the surface to the model top layer centered at 0.02 hPa. It is found that there are only minor changes in simulated stratospheric temperature and ozone prior to the year 1960. As the halocarbon amounts increase after 1970, the model stratospheric ozone decreases approximately continuously until about 2000. The steadily increasing GHG concentrations cool the stratosphere from the beginning of the twentieth century at a rate that increases with height. During the early period the cooling leads to increased stratospheric ozone. The model results show a strong, albeit temporary, response to volcanic eruptions. While chlorofluorocarbon (CFC) concentrations remain low, the effect of eruptions is shown to increase the amount of HNO3, reducing ozone destruction by the NOx catalytic cycle. In the presence of anthropogenic chlorine, after the eruption of El Chichón and Mt. Pinatubo, chlorine radicals increased and the chlorine reservoirs decreased. The net volcanic effect on nitrogen and chlorine chemistry depends on altitude and, for these two volcanoes, leads to an ozone increase in the middle stratosphere and a decrease in the lower stratosphere. Model lower-stratospheric temperatures are also shown to increase during the last three major volcanic eruptions, by about 0.6 K in the global and annual average, consistent with observations.
Carbon monoxide (CO) emissions influence global and regional air quality and global climate change by affecting atmospheric oxidants and secondary species. We simulate the influence of halving anthropogenic CO emissions globally and individually from 10 regions on surface and tropospheric ozone, methane, and aerosol concentrations using a global chemical transport model (MOZART-4 for the year 2005). Net radiative forcing (RF) is then estimated using the GFDL standalone radiative transfer model. We estimate that halving global CO emissions decreases global annual average concentrations of surface ozone by 0.45 ppbv, tropospheric methane by 73 ppbv, and global annual net RF by 36.1 mW m−2, nearly equal to the sum of changes from the 10 regional reductions. Global annual net RF per unit change in emissions and the 100-yr global warming potential (GWP100) are estimated as −0.124 mW m−2 (Tg CO yr−1)−1 and 1.34, respectively, for the global CO reduction, and ranging from −0.115 to −0.131 mW m−2 (Tg CO yr−1)−1 and 1.26 to 1.44 across 10 regions, with the greatest sensitivities for regions in the tropics. The net RF distributions show widespread cooling corresponding to the O3 and CH4 decreases, and localized positive and negative net RFs due to changes in aerosols. The strongest annual net RF impacts occur within the tropics (28° S–28° N) followed by the northern mid-latitudes (28° N–60° N), independent of reduction region, while the greatest changes in surface CO and ozone concentrations occur within the reduction region. Some regional reductions strongly influence the air quality in other regions, such as East Asia, which has an impact on US surface ozone that is 93% of that from North America. Changes in the transport of CO and downwind ozone production clearly exceed the direct export of ozone from each reduction region. The small variation in CO GWPs among world regions suggests that future international climate agreements could adopt a globally uniform metric for CO with little error, or could use different GWPs for each continent. Doing so may increase the incentive to reduce CO through coordinated policies addressing climate and air quality.
Employing the Geophysical Fluid Dynamics Laboratory (GFDL)'s fully-coupled chemistry-climate (ocean/atmosphere/land/sea ice) model (CM3) with an explicit physical representation of aerosol indirect effects (cloud-water droplet activation), we find that the dramatic emission reductions (35–80%) in anthropogenic aerosols and their precursors projected by Representative Concentration Pathway (RCP) 4.5 result in ~1°C of additional warming and ~0.1 mm day−1 of additional precipitation, both globally averaged, by the end of the 21st century. The impact of these reductions in aerosol emissions on simulated global mean surface temperature and precipitation becomes apparent by mid-21st century. Furthermore, we find that the aerosol emission reductions cause precipitation to increase in East and South Asia by ~1.0 mm day−1 through the 2nd half of the 21st century. Both the simulated temperature and precipitation responses in CM3 are significantly stronger than the previously simulated responses in our earlier climate model (CM2.1) that only considered direct radiative forcing by aerosols. We conclude that sulfate aerosol indirect effects greatly enhance the impacts of aerosols on surface temperature in CM3, while both direct and indirect effects from sulfate aerosols dominate the strong precipitation response, possibly with a small contribution from carbonaceous aerosols. Just as we found with the previous GFDL model, CM3 produces surface warming patterns that are uncorrelated with the spatial distribution of 21stcentury changes in aerosol loading. However, the largest precipitation increases in CM3 are co-located with the region of greatest aerosol decrease, in and downwind of Asia.
Fry, M, Vaishali Naik, J Jason West, M Daniel Schwarzkopf, and Arlene M Fiore, et al., April 2012: The influence of ozone precursor emissions from four world regions on tropospheric composition and radiative climate forcing. Journal of Geophysical Research: Atmospheres, 117, D07306, DOI:10.1029/2011JD017134. Abstract
Ozone (O3) precursor emissions influence regional and global climate and air quality through changes in tropospheric O3 and oxidants, which also influence methane (CH4) and sulfate aerosols (SO42-). We examine changes in the tropospheric composition of O3, CH4, SO42- and global net radiative forcing (RF) for 20% reductions in global CH4 burden and in anthropogenic O3 precursor emissions (NOx, NMVOC, and CO) from four regions (East Asia, Europe and Northern Africa, North America, and South Asia) using the Task Force on Hemispheric Transport of Air Pollution Source-Receptor global chemical transport model (CTM) simulations, assessing uncertainty (mean {plus minus}1 standard deviation) across multiple CTMs. We evaluate steady-state O3 responses, including long-term feedbacks via CH4. With a radiative transfer model that includes greenhouse gases and the aerosol direct effect, we find that regional NOx reductions produce global, annually averaged positive net RFs (0.2 {plus minus}0.6 to 1.7 {plus minus}2 mWm-2/TgN yr-1), with some variation among models. Negative net RFs result from reductions in global CH4 (-162.6 {plus minus}2 mWm-2 for a change from 1760 to 1408 ppbv CH4) and regional NMVOC (-0.4 {plus minus}0.2 to -0.7 {plus minus}0.2 mWm-2/TgC yr-1) and CO emissions (-0.13 {plus minus}0.02 to -0.15 {plus minus}0.02 mWm-2/TgCO yr-1). Including the effect of O3 on CO2 uptake by vegetation, likely makes these net RFs more negative by -1.9 to -5.2 mWm-2/TgN yr-1, -0.2 to -0.7 mWm-2/TgC yr-1, and -0.02 to -0.05 mWm-2/TgCO yr-1. Net RF impacts reflect the distribution of concentration changes, where RF is affected locally by changes in SO42-, regionally to hemispherically by O3, and globally by CH4. Global annual average SO42- responses to oxidant changes range from 0.4 {plus minus}2.6 to -1.9 {plus minus}1.3 Gg for NOx reductions, 0.1 {plus minus}1.2 to -0.9 {plus minus}0.8 Gg for NMVOC reductions, and -0.09 {plus minus}0.5 to -0.9 {plus minus}0.8 Gg for CO reductions, suggesting additional research is needed. The 100-year global warming potentials (GWP100) are calculated for the global CH4 reduction (20.9 {plus minus}3.7 without stratospheric O3 or water vapor, 24.2 {plus minus}4.2 including those components), and for the regional NOx, NMVOC, and CO reductions (-18.7 {plus minus}25.9 to -1.9 {plus minus}8.7 for NOx, 4.8 {plus minus}1.7 to 8.3 {plus minus}1.9 for NMVOC, and 1.5 {plus minus}0.4 to 1.7 {plus minus}0.5 for CO). Variation in GWP100 for NOx, NMVOC, and CO suggests that regionally-specific GWPs may be necessary and could support the inclusion of O3 precursors in future policies that address air quality and climate change simultaneously. Both global net RF and GWP100 are more sensitive to NOx and NMVOC reductions from South Asia than the other three regions.
The Geophysical Fluid Dynamics Laboratory (GFDL) has developed a coupled general circulation model (CM3) for atmosphere, oceans, land, and sea ice. The goal of CM3 is to address emerging issues in climate change, including aerosol-cloud interactions, chemistry-climate interactions, and coupling between the troposphere and stratosphere. The model is also designed to serve as the physical-system component of earth-system models and models for decadal prediction in the near-term future, for example, through improved simulations in tropical land precipitation relative to earlier-generation GFDL models. This paper describes the dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component (AM3) of this model.
Relative to GFDL AM2, AM3 includes new treatments of deep and shallow cumulus convection, cloud-droplet activation by aerosols, sub-grid variability of stratiform vertical velocities for droplet activation, and atmospheric chemistry driven by emissions with advective, convective, and turbulent transport. AM3 employs a cubed-sphere implementation of a finite-volume dynamical core and is coupled to LM3, a new land model with eco-system dynamics and hydrology.
Most basic circulation features in AM3 are simulated as realistically, or more so, than in AM2. In particular, dry biases have been reduced over South America. In coupled mode, the simulation of Arctic sea ice concentration has improved. AM3 aerosol optical depths, scattering properties, and surface clear-sky downward shortwave radiation are more realistic than in AM2. The simulation of marine stratocumulus decks and the intensity distributions of precipitation remain problematic, as in AM2.
The last two decades of the 20th century warm in CM3 by .32°C relative to 1881-1920. The Climate Research Unit (CRU) and Goddard Institute for Space Studies analyses of observations show warming of .56°C and .52°C, respectively, over this period. CM3 includes anthropogenic cooling by aerosol cloud interactions, and its warming by late 20th century is somewhat less realistic than in CM2.1, which warmed .66°C but did not include aerosol cloud interactions. The improved simulation of the direct aerosol effect (apparent in surface clear-sky downward radiation) in CM3 evidently acts in concert with its simulation of cloud-aerosol interactions to limit greenhouse gas warming in a way that is consistent with observed global temperature changes.
Reducing methane (CH4) emissions is an attractive option for jointly addressing climate and ozone (O3) air quality goals. With multidecadal full-chemistry transient simulations in the MOZART-2 tropospheric chemistry model, we show that tropospheric O3 responds approximately linearly to changes in CH4 emissions over a range of anthropogenic emissions from 0–430 Tg CH4a−1 (0.11–0.16 Tg tropospheric O3 or ∼11–15 ppt global mean surface O3 decrease per Tg a−1 CH4 reduced). We find that neither the air quality nor climate benefits depend strongly on the location of the CH4 emission reductions, implying that the lowest cost emission controls can be targeted. With a series of future (2005–2030) transient simulations, we demonstrate that cost-effective CH4 controls would offset the positive climate forcing from CH4 and O3 that would otherwise occur (from increases in NOx and CH4 emissions in the baseline scenario) and improve O3 air quality. We estimate that anthropogenic CH4 contributes 0.7 Wm−2 to climate forcing and ∼4 ppb to surface O3 in 2030 under the baseline scenario. Although the response of surface O3 to CH4 is relatively uniform spatially compared to that from other O3 precursors, it is strongest in regions where surface air mixes frequently with the free troposphere and where the local O3 formation regime is NOx-saturated. In the model, CH4 oxidation within the boundary layer (below ∼2.5 km) contributes more to surface O3 than CH4 oxidation in the free troposphere. In NOx-saturated regions, the surface O3 sensitivity to CH4 can be twice that of the global mean, with >70% of this sensitivity resulting from boundary layer oxidation of CH4. Accurately representing the NOx distribution is thus crucial for quantifying the O3 sensitivity to CH4.
This study examines the impact of
projected changes (A1B “marker” scenario) in emissions of four short-lived
air pollutants (ozone, black carbon, organic carbon, and sulfate) on future
climate. Through year 2030, simulated climate is only weakly dependent on
the projected levels of short-lived air pollutants, primarily the result of
a near cancellation of their global net radiative forcing. However, by year
2100, the projected decrease in sulfate aerosol (driven by a 65% reduction
in global sulfur dioxide emissions) and the projected increase in black
carbon aerosol (driven by a 100% increase in its global emissions)
contribute a significant portion of the simulated A1B surface air warming
relative to the year 2000: 0.2°C (Southern Hemisphere), 0.4°C globally,
0.6°C (Northern Hemisphere), 1.5–3°C (wintertime Arctic), and 1.5–2°C (∼40%
of the total) in the summertime United States. These projected changes are
also responsible for a significant decrease in central United States late
summer root zone soil water and precipitation. By year 2100, changes in
short-lived air pollutants produce a global average increase in radiative
forcing of ∼1 W/m2; over east Asia it exceeds 5 W/m2.
However, the resulting regional patterns of surface temperature warming do
not follow the regional patterns of changes in short-lived species
emissions, tropospheric loadings, or radiative forcing (global pattern
correlation coefficient of −0.172). Rather, the regional patterns of warming
from short-lived species are similar to the patterns for well-mixed
greenhouse gases (global pattern correlation coefficient of 0.8) with the
strongest warming occurring over the summer continental United States,
Mediterranean Sea, and southern Europe and over the winter Arctic.
We employ a coupled atmosphere-ocean climate model to investigate the evolution of stratospheric temperatures over the twentieth century, forced by the known anthropogenic and natural forcing agents. In the global, annual-mean lower-to-middle stratosphere (∼20–30 km.), simulations produce a sustained, significant cooling by ∼1920, earlier than in any lower atmospheric region, largely resulting from carbon dioxide increases. After 1979, stratospheric ozone decreases reinforce the cooling. Arctic summer cooling attains significance almost as early as the global, annual-mean response. Antarctic responses become significant in summer after ∼1940 and in spring after ∼1990 (below ∼21 km.). The correspondence of simulated and observed stratospheric temperature trends after ∼1960 suggests that the model's stratospheric response is reasonably similar to that of the actual climate. We conclude that these model simulations are useful in explaining stratospheric temperature change over the entire 20th century, and potentially provide early indications of the effects of future atmospheric species changes.
We use the GISS (Goddard Institute for Space Studies), GFDL (Geophysical Fluid Dynamics Laboratory) and NCAR (National Center for Atmospheric Research) climate models to study the climate impact of the future evolution of short-lived radiatively active species (ozone and aerosols). The models used mid-range A1B emission scenarios, independently calculated the resulting composition change, and then performed transient simulations to 2050 examining the response to projected changes in short-lived species and to changes in both long-lived and short-lived species together. By 2050, two models show that the global mean annual average warming due to long-lived GHGs (greenhouse gases) is enhanced by 20–25% due to the radiatively active short-lived species. One model shows virtually no effect from short-lived species. Intermodel differences are largely related to differences in emissions projections for short-lived species, which are substantial even for a particular storyline. For aerosols, these uncertainties are usually dominant, though for sulfate uncertainties in aerosol physics are also substantial. For tropospheric ozone, uncertainties in physical processes are more important than uncertainties in precursor emissions. Differences in future atmospheric burdens and radiative forcing for aerosols are dominated by divergent assumptions about emissions from South and East Asia. In all three models, the spatial distribution of radiative forcing is less important than that of climate sensitivity in predicting climate impact. Both short-lived and long-lived species appear to cause enhanced climate responses in the same regions of high sensitivity rather than short-lived species having an enhanced effect primarily near polluted areas. Since short-lived species can significantly influence climate, regional air quality emission control strategies for short-lived pollutants may substantially impact climate over large (e.g., hemispheric) scales.
Biomass burning is a major source of air
pollutants, some of which are also climate forcing agents. We investigate
the sensitivity of direct radiative forcing due to tropospheric ozone and
aerosols (carbonaceous and sulfate) to a marginal reduction in their (or
their precursor) emissions from major biomass burning regions. We find that
the largest negative global forcing is for 10% emission reductions in
tropical regions, including Africa (−4.1 mWm−2 from gas and −4.1
mWm−2 from aerosols), and South America (−3.0 mWm−2
from gas and −2.8 mWm−2 from aerosols). We estimate that a unit
reduction in the amount of biomass burned in India produces the largest
negative ozone and aerosol forcing. Our analysis indicates that reducing
biomass burning emissions causes negative global radiative forcing due to
ozone and aerosols; however, regional differences need to be considered when
evaluating controls on biomass burning to mitigate global climate change.
Changes in emissions of ozone (O3) precursors affect both air
quality and climate. We first examine the sensitivity of surface O3
concentrations (O3srf) and net radiative forcing of
climate (RFnet) to reductions in emissions of four precursors -
nitrogen oxides (NOx), non-methane volatile organic
compounds, carbon monoxide, and methane (CH4). We show that
long-term CH4-induced changes in O3, known to be
important for climate, are also relevant for air quality; for example, NOxreductions increase CH4, causing a long-term O3
increase that partially counteracts the direct O3 decrease.
Second, we assess the radiative forcing resulting from actions to improve O3
air quality by calculating the ratio of ΔRFnet
to changes in metrics of O3srf. Decreases in CH4
emissions cause the greatest RFnet decrease per unit reduction
in O3srf, while NOxreductions
increase RFnet. Of the available means to improve O3
air quality, therefore, CH4 abatement best reduces climate
forcing.
Collins, William D., V Ramaswamy, M Daniel Schwarzkopf, Y Sun, R W Portmann, Qiang Fu, S E B Casanova, J-L Dufresne, D W Fillmore, Piers M Forster, V Y Galin, L K Gohar, W J Ingram, D P Kratz, M-P Lefebvre, P Marquet, V Oinas, Y Tsushima, T Uchiyama, and W Y Zhong, July 2006: Radiative forcing by well-mixed greenhouse gases: Estimates from climate models in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Journal of Geophysical Research, 111(D14), D14317, DOI:10.1029/2005JD006713. Abstract
The radiative effects from increased concentrations of well-mixed greenhouse gases (WMGHGs) represent the most significant and best understood anthropogenic forcing of the climate system. The most comprehensive tools for simulating past and future climates influenced by WMGHGs are fully coupled atmosphere-ocean general circulation models (AOGCMs). Because of the importance of WMGHGs as forcing agents it is essential that AOGCMs compute the radiative forcing by these gases as accurately as possible. We present the results of a radiative transfer model intercomparison between the forcings computed by the radiative parameterizations of AOGCMs and by benchmark line-by-line (LBL) codes. The comparison is focused on forcing by CO2, CH4, N2O, CFC-11, CFC-12, and the increased H2O expected in warmer climates. The models included in the intercomparison include several LBL codes and most of the global models submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In general, the LBL models are in excellent agreement with each other. However, in many cases, there are substantial discrepancies among the AOGCMs and between the AOGCMs and LBL codes. In some cases this is because the AOGCMs neglect particular absorbers, in particular the near-infrared effects of CH4 and N2O, while in others it is due to the methods for modeling the radiative processes. The biases in the AOGCM forcings are generally largest at the surface level. We quantify these differences and discuss the implications for interpreting variations in forcing and response across the multimodel ensemble of AOGCM simulations assembled for the IPCC AR4.
The formulation and simulation characteristics of two new global coupled climate models developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) are described. The models were designed to simulate atmospheric and oceanic climate and variability from the diurnal time scale through multicentury climate change, given our computational constraints. In particular, an important goal was to use the same model for both experimental seasonal to interannual forecasting and the study of multicentury global climate change, and this goal has been achieved.
Two versions of the coupled model are described, called CM2.0 and CM2.1. The versions differ primarily in the dynamical core used in the atmospheric component, along with the cloud tuning and some details of the land and ocean components. For both coupled models, the resolution of the land and atmospheric components is 2° latitude × 2.5° longitude; the atmospheric model has 24 vertical levels. The ocean resolution is 1° in latitude and longitude, with meridional resolution equatorward of 30° becoming progressively finer, such that the meridional resolution is 1/3° at the equator. There are 50 vertical levels in the ocean, with 22 evenly spaced levels within the top 220 m. The ocean component has poles over North America and Eurasia to avoid polar filtering. Neither coupled model employs flux adjustments.
The control simulations have stable, realistic climates when integrated over multiple centuries. Both models have simulations of ENSO that are substantially improved relative to previous GFDL coupled models. The CM2.0 model has been further evaluated as an ENSO forecast model and has good skill (CM2.1 has not been evaluated as an ENSO forecast model). Generally reduced temperature and salinity biases exist in CM2.1 relative to CM2.0. These reductions are associated with 1) improved simulations of surface wind stress in CM2.1 and associated changes in oceanic gyre circulations; 2) changes in cloud tuning and the land model, both of which act to increase the net surface shortwave radiation in CM2.1, thereby reducing an overall cold bias present in CM2.0; and 3) a reduction of ocean lateral viscosity in the extratropics in CM2.1, which reduces sea ice biases in the North Atlantic.
Both models have been used to conduct a suite of climate change simulations for the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment report and are able to simulate the main features of the observed warming of the twentieth century. The climate sensitivities of the CM2.0 and CM2.1 models are 2.9 and 3.4 K, respectively. These sensitivities are defined by coupling the atmospheric components of CM2.0 and CM2.1 to a slab ocean model and allowing the model to come into equilibrium with a doubling of atmospheric CO2. The output from a suite of integrations conducted with these models is freely available online (see http://nomads.gfdl.noaa.gov/).
Manuscript received 8 December 2004, in final form 18 March 2005
The global and tropical means of clear-sky outgoing longwave radiation (hereinafter OLRc) simulated by the new GFDL atmospheric general circulation model, AM2, tend to be systematically lower than ERBE observations by about 4 W m-2, even though the AM2 total-sky radiation budget is tuned to be consistent with these observations. Here we quantify the source of errors in AM2-simulated OLRc over the tropical oceans by comparing the synthetic outgoing IR spectra at the top of the atmosphere on the basis of AM2 simulations to observed IRIS spectra. After the sampling disparity between IRIS and AM2 is reduced, AM2 still shows considerable negative bias in the simulated monthly mean OLRc over the tropical oceans. Together with other evidence, this suggests that the influence of spatial sampling disparity, although present, does not account for the majority of the bias. Decomposition of OLRc shows that the negative bias comes mainly from the H2O bands and can be explained by a too humid layer around 6–9 km in the model. Meanwhile, a positive bias exists in channels sensitive to near-surface humidity and temperature, which implies that the boundary layer in the model might be too dry. These facts suggest that the negative bias in the simulated OLRc can be attributed to model deficiencies, especially the large-scale water vapor transport. We also find that AM2-simulated OLRc has ~1 W m-2 positive bias originating from the stratosphere; this positive bias should exist in simulated total-sky OLR as well.
Historical climate simulations of the period 1861–2000 using two new Geophysical Fluid Dynamics Laboratory (GFDL) global climate models (CM2.0 and CM2.1) are compared with observed surface temperatures. All-forcing runs include the effects of changes in well-mixed greenhouse gases, ozone, sulfates, black and organic carbon, volcanic aerosols, solar flux, and land cover. Indirect effects of tropospheric aerosols on clouds and precipitation processes are not included. Ensembles of size 3 (CM2.0) and 5 (CM2.1) with all forcings are analyzed, along with smaller ensembles of natural-only and anthropogenic-only forcing, and multicentury control runs with no external forcing.
Observed warming trends on the global scale and in many regions are simulated more realistically in the all-forcing and anthropogenic-only forcing runs than in experiments using natural-only forcing or no external forcing. In the all-forcing and anthropogenic-only forcing runs, the model shows some tendency for too much twentieth-century warming in lower latitudes and too little warming in higher latitudes. Differences in Arctic Oscillation behavior between models and observations contribute substantially to an underprediction of the observed warming over northern Asia. In the all-forcing and natural-only forcing runs, a temporary global cooling in the models during the 1880s not evident in the observed temperature records is volcanically forced. El Niño interactions complicate comparisons of observed and simulated temperature records for the El Chichón and Mt. Pinatubo eruptions during the early 1980s and early 1990s.
The simulations support previous findings that twentieth-century global warming has resulted from a combination of natural and anthropogenic forcing, with anthropogenic forcing being the dominant cause of the pronounced late-twentieth-century warming. The regional results provide evidence for an emergent anthropogenic warming signal over many, if not most, regions of the globe. The warming signal has emerged rather monotonically in the Indian Ocean/western Pacific warm pool during the past half-century. The tropical and subtropical North Atlantic and the tropical eastern Pacific are examples of regions where the anthropogenic warming signal now appears to be emerging from a background of more substantial multidecadal variability.
Observations reveal that the substantial cooling of the global lower stratosphere over 1979–2003 occurred in two pronounced steplike transitions. These arose in the aftermath of two major volcanic eruptions, with each cooling transition being followed by a period of relatively steady temperatures. Climate model simulations indicate that the space-time structure of the observed cooling is largely attributable to the combined effect of changes in both anthropogenic factors (ozone depletion and increases in well-mixed greenhouse gases) and natural factors (solar irradiance variation and volcanic aerosols). The anthropogenic factors drove the overall cooling during the period, and the natural ones modulated the evolution of the cooling.
Ramaswamy, V, J W Hurrell, Gerald A Meehl, A Phillips, B D Santer, M Daniel Schwarzkopf, D J Seidel, S C Sherwood, and P W Thorne, 2006: Why do temperatures vary vertically (from the surface to the stratosphere) and what do we understand about why they might vary and change over time? In Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences, Karl, T R, S J Hassol, C D Miller, W L Murray, eds., Washington, DC, A Report by the Climate Change Science Program/Subcommittee on Global Change Research, 15-28. PDF
The climate response to idealized changes in the atmospheric CO2 concentration by the new GFDL climate model (CM2) is documented. This new model is very different from earlier GFDL models in its parameterizations of subgrid-scale physical processes, numerical algorithms, and resolution. The model was constructed to be useful for both seasonal-to-interannual predictions and climate change research. Unlike previous versions of the global coupled GFDL climate models, CM2 does not use flux adjustments to maintain a stable control climate. Results from two model versions, Climate Model versions 2.0 (CM2.0) and 2.1 (CM2.1), are presented.
Two atmosphere–mixed layer ocean or slab models, Slab Model versions 2.0 (SM2.0) and 2.1 (SM2.1), are constructed corresponding to CM2.0 and CM2.1. Using the SM2 models to estimate the climate sensitivity, it is found that the equilibrium globally averaged surface air temperature increases 2.9 (SM2.0) and 3.4 K (SM2.1) for a doubling of the atmospheric CO2 concentration. When forced by a 1% per year CO2 increase, the surface air temperature difference around the time of CO2 doubling [transient climate response (TCR)] is about 1.6 K for both coupled model versions (CM2.0 and CM2.1). The simulated warming is near the median of the responses documented for the climate models used in the 2001 Intergovernmental Panel on Climate Change (IPCC) Working Group I Third Assessment Report (TAR).
The thermohaline circulation (THC) weakened in response to increasing atmospheric CO2. By the time of CO2 doubling, the weakening in CM2.1 is larger than that found in CM2.0: 7 and 4 Sv (1 Sv 106 m3 s−1), respectively. However, the THC in the control integration of CM2.1 is stronger than in CM2.0, so that the percentage change in the THC between the two versions is more similar. The average THC change for the models presented in the TAR is about 3 or 4 Sv; however, the range across the model results is very large, varying from a slight increase (+2 Sv) to a large decrease (−10 Sv).
The global distribution of tropospheric ozone (O3) depends on the emission of precursors, chemistry, and transport. For small perturbations to emissions, the global radiative forcing resulting from changes in O3 can be expressed as a sum of forcings from emission changes in different regions. Tropospheric O3 is considered in present climate policies only through the inclusion of indirect effect of CH4 on radiative forcing through its impact on O3 concentrations. The short-lived O3 precursors (NOx , CO, and NMHCs) are not directly included in the Kyoto Protocol or any similar climate mitigation agreement. In this study, we quantify the global radiative forcing resulting from a marginal reduction (10%) in anthropogenic emissions of NOx alone from nine geographic regions and a combined marginal reduction in NOx , CO, and NMHCs emissions from three regions. We simulate, using the global chemistry transport model MOZART-2, the change in the distribution of global O3 resulting from these emission reductions. In addition to the short-term reduction in O3, these emission reductions also increase CH4concentrations (by decreasing OH); this increase in CH4 in turn counteracts part of the initial reduction in O3 concentrations. We calculate the global radiative forcing resulting from the regional emission reductions, accounting for changes in both O3 and CH4. Our results show that changes in O3 production and resulting distribution depend strongly on the geographical location of the reduction in precursor emissions. We find that the global O3 distribution and radiative forcing are most sensitive to changes in precursor emissions from tropical regions and least sensitive to changes from midlatitude and high-latitude regions. Changes in CH4 and O3 concentrations resulting from NOx emission reductions alone produce offsetting changes in radiative forcing, leaving a small positive residual forcing (warming) for all regions. In contrast, for combined reductions of anthropogenic emissions of NOx , CO, and NMHCs, changes in O3 and CH4 concentrations result in a net negative radiative forcing (cooling). Thus we conclude that simultaneous reductions of CO, NMHCs, and NOx lead to a net reduction in radiative forcing due to resulting changes in tropospheric O3 and CH4 while reductions in NOx emissions alone do not.
Climate models predict that the concentration of water vapor in the upper troposphere could double by the end of the century as a result of increases in greenhouse gases. Such moistening plays a key role in amplifying the rate at which the climate warms in response to anthropogenic activities, but has been difficult to detect because of deficiencies in conventional observing systems. We use satellite measurements to highlight a distinct radiative signature of upper tropospheric moistening over the period 1982 to 2004. The observed moistening is accurately captured by climate model simulations and lends further credence to model projections of future global warming.
for climate research developed at the Geophysical Fluid Dynamics Laboratory (GFDL) are presented. The atmosphere model, known as AM2, includes a new gridpoint dynamical core, a prognostic cloud scheme, and a multispecies aerosol climatology, as well as components from previous models used at GFDL. The land model, known as LM2, includes soil sensible and latent heat storage, groundwater storage, and stomatal resistance. The performance of the coupled model AM2–LM2 is evaluated with a series of prescribed sea surface temperature (SST) simulations. Particular focus is given to the model's climatology and the characteristics of interannual variability related to E1 Niño– Southern Oscillation (ENSO).
One AM2–LM2 integration was performed according to the prescriptions of the second Atmospheric Model Intercomparison Project (AMIP II) and data were submitted to the Program for Climate Model Diagnosis and Intercomparison (PCMDI). Particular strengths of AM2–LM2, as judged by comparison to other models participating in AMIP II, include its circulation and distributions of precipitation. Prominent problems of AM2– LM2 include a cold bias to surface and tropospheric temperatures, weak tropical cyclone activity, and weak tropical intraseasonal activity associated with the Madden–Julian oscillation.
An ensemble of 10 AM2–LM2 integrations with observed SSTs for the second half of the twentieth century permits a statistically reliable assessment of the model's response to ENSO. In general, AM2–LM2 produces a realistic simulation of the anomalies in tropical precipitation and extratropical circulation that are associated with ENSO.
Stenchikov, Georgiy, Kevin P Hamilton, A Robock, V Ramaswamy, and M Daniel Schwarzkopf, February 2004: Arctic oscillation response to the 1991 Pinatubo eruption in the SKYHI general circulation model with a realistic quasi-biennial oscillation. Journal of Geophysical Research, 109(D3), D03112, DOI:10.1029/2003JD003699. Abstract
Stratospheric aerosol clouds from large tropical volcanic eruptions can be expected to alter the atmospheric radiative balance for a period of up to several years. Observations following several previous major eruptions suggest that one effect of the radiative perturbations is to cause anomalies in the Northern Hemisphere extratropical winter tropospheric circulation that can be broadly characterized as positive excursions of the Arctic Oscillation (AO). We report on a modeling investigation of the radiative and dynamical mechanisms that may account for the observed AO anomalies following eruptions. We focus on the best observed and strongest 20th century eruption, that of Mt. Pinatubo on 15 June 1991. The impact of the Pinatubo eruption on the climate has been the focus of a number of earlier modeling studies, but all of these previous studies used models with no quasi-biennial oscillation (QBO) in the tropical stratosphere. The QBO is a very prominent feature of interannual variability of tropical stratospheric circulation and could have a profound effect on the global atmospheric response to volcanic radiative forcing. Thus a complete study of the atmospheric variability following volcanic eruptions should include a realistic representation of the tropical QBO. Here we address, for the first time, this important issue. We employed a version of the SKYHI troposphere-stratosphere-mesosphere model that effectively assimilates observed zonal mean winds in the tropical stratosphere to simulate a very realistic QBO. We performed an ensemble of 24 simulations for the period 1 June 1991 to 31 May 1993. These simulations included a realistic prescription of the stratospheric aerosol layer based on satellite observations. These integrations are compared to control integrations with no volcanic aerosol. The model produced a reasonably realistic representation of the positive AO response in boreal winter that is usually observed after major eruptions. Detailed analysis shows that the aerosol perturbations to the tropospheric winter circulation are affected significantly by the phase of the QBO, with a westerly QBO phase in the lower stratosphere resulting in an enhancement of the aerosol effect on the AO. Improved quantification of the QBO effect on climate sensitivity helps to better understand mechanisms of the stratospheric contribution to natural and externally forced climate variability.
Shine, K P., M S Bourqui, Piers M Forster, S H E Hare, U Langematz, P Braesicke, V Grewe, M Ponater, C Schnadt, C A Smith, J D Haigh, John Austin, Neal Butchart, Drew Shindell, W J Randel, T Nagashima, R W Portmann, S Solomon, D J Seidel, John R Lanzante, Stephen A Klein, V Ramaswamy, and M Daniel Schwarzkopf, 2003: A comparison of model-simulated trends in stratospheric temperatures. Quarterly Journal of the Royal Meteorological Society, 129(590), DOI:10.1256/qj.02.186. Abstract
Estimates of annual-mean stratospheric temperature trends over the past twenty years, from a wide variety of models, are compared both with each other and with the observed cooling seen in trend analyses using radiosonde and satellite observations. The modelled temperature trends are driven by changes in ozone (either imposed from observations or calculated by the model), carbon dioxide and other relatively well-mixed greenhouse gases, and stratospheric water vapour.
The comparison shows that whilst models generally simulate similar patterns in the vertical profile of annual-and global-mean temperature trends, there is a significant divergence in the size of the modelled trends, even when similar trace gas perturbations are imposed. Coupled-chemistry models are in as good agreement as models using imposed observed ozone trends, despite the extra degree of freedom that the coupled models possess.
The modelled annual- and global-mean cooling of the upper stratosphere (near 1 hPa) is dominated by ozone and carbon dioxide changes, and is in reasonable agreement with observations. At about 5 hPa, the mean cooling from the models is systematically greater than that seen in the satellite data; however, for some models, depending on the size of the temperature trend due to stratospheric water vapour changes, the uncertainty estimates of the model and observations just overlap. Near 10 hPa there is good agreement with observations. In the lower stratosphere (20-70 hPa), ozone appears to be the dominant contributor to the observed cooling, although it does not, on its own, seem to explain the entire cooling.
Annual- and zonal-mean temperature trends at 100 hPa and 50 hPa are also examined. At 100 hPa, the modelled cooling due to ozone depletion alone is in reasonable agreement with the observed cooling at all latitudes. At 50 hPa, however, the observed cooling at midlatitudes of the northern hemisphere significantly exceeds the modelled cooling due to ozone depletion alone. There is an indication of a similar effect in high northern latitudes, but the greater variability in both models and observations precludes a firm conclusion.
The discrepancies between modelled and observed temperature trends in the lower stratosphere are reduced if the cooling effects of increased stratospheric water vapour concentration are included, and could be largely removed if certain assumptions were made regarding the size and distribution of the water vapour increase. However, given the uncertainties in the geographical extent of water vapour changes in the lower stratosphere, and the time period over which such changes have been sustained, other reasons for the discrepancy between modelled and observed temperature trends cannot be ruled out.
Ramaswamy, V, M E Gelman, M Daniel Schwarzkopf, and J-J R Lin, 2002: An update of stratospheric temperature trends. SPARC Newsletter, 18, 7-9. PDF
The effects of changes in ozone and well-mixed greenhouse gases upon the annual-mean stratospheric temperatures are investigated using a general circulation model and compared with the observed (1979–2000) trends. In the global-mean lower stratosphere (50–100 hPa), ozone changes exert the most important influence upon the cooling trend. In the upper stratosphere, where both ozone and greenhouse gas changes influence the temperature trends, the amount of cooling is sensitive to the background ozone climatology. Taking into account the uncertainties in the observed temperature trend estimates and the dynamical variability of the model, the simulated results are in reasonable quantitative agreement with the vertical profile of the observed global-and-annual-mean stratospheric cooling, and with the observed lower stratospheric zonal-and-annual-mean cooling. This affirms the major role of these species in the temperature trend of the stratosphere over the past two decades.
Monthly and seasonal stratospheric zonal-mean temperature trends arising from recent changes in stratospheric ozone and well-mixed greenhouse gases (WMGGs) are simulated using a general circulation model and compared with observed (1979–2000) trends. The combined effect of these gases yields statistically significant cooling trends over the entire globally averaged stratosphere in all months. In the Arctic (60°N–90°N), statistically significant trends occur only in summer and extend through the entire stratosphere. In the Antarctic (90°S–65°S), the simulations reproduce the observed seasonal pattern of the lower stratosphere temperature trend. Seasonal trends at 50 hPa are consistent with observed trends at all latitudes, considering model dynamical variability and observational uncertainty. The lack of robustness in simulated and observed Arctic winter trends indicates the futility of attributing these trends to trace gas concentration changes. Such attribution arguments may be made with greater confidence regarding middle and high latitude Northern Hemisphere summer temperature trends.
Stenchikov, Georgiy, A Robock, V Ramaswamy, M Daniel Schwarzkopf, Kevin P Hamilton, and S Ramachandran, 2002: Arctic oscillation response to the 1991 Mount Pinatubo eruption: effects of volcanic aerosols and ozone depletion. Journal of Geophysical Research, 107(D24), 4803, DOI:10.1029/2002JD002090. Abstract
Observations show that strong equatorial volcanic eruptions have been followed by a pronounced positive phase of the Arctic Oscillation (AO) for one or two Northern Hemisphere winters. It has been previously assumed that this effect is forced by strengthening of the equator-to-pole temperature gradient in the lower stratosphere, caused by aerosol radiative heating in the tropics. To understand atmospheric processes that cause the AO response, we studied the impact of the 1991 Mount Pinatubo eruption, which produced the largest global volcanic aerosol cloud in the twentieth century. A series of control and perturbation experiments were conducted with the GFDL SKYHI general circulation model to examine the evolution of the circulation in the 2 years following the Pinatubo eruption. In one set of perturbation experiments, the full radiative effects of the observed Pinatubo aerosol cloud were included, while in another only the effects of the aerosols in reducing the solar flux in the troposphere were included, and the aerosol heating effects in the stratosphere were suppressed. A third set of perturbation experiments imposed the stratospheric ozone losses observed in the post-Pinatubo period. We conducted ensembles of four to eight realizations for each case. Forced by aerosols, SKYHI produces a statistically significant positive phase of the AO in winter, as observed. Ozone depletion causes a positive phase of the AO in late winter and early spring by cooling the lower stratosphere in high latitudes, strengthening the polar night jet, and delaying the final warming. A positive phase of the AO was also produced in the experiment with only the tropospheric effect of aerosols, showing that aerosol heating in the lower tropical stratosphere is not necessary to force positive AO response, as was previously assumed. Aerosol-induced tropospheric cooling in the subtropics decreases the meridional temperature gradient in the winter troposphere between 30°N and 60°N. The corresponding reduction of mean zonal energy and amplitudes of planetary waves in the troposphere decreases wave activity flux into the lower stratosphere. The resulting strengthening of the polar vortex forces a positive phase of the AO. We suggest that this mechanism can also contribute to the observed long-term AO trend being caused by greenhouse gas increases because they also weaken the tropospheric meridional temperature gradient due to polar amplification of warming.
Ramaswamy, V, and M Daniel Schwarzkopf, et al., 2001: Radiative forcing of climate change In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, UK, Cambridge University Press, 350-416.
Soden, Brian J., S Tjernkes, J Schmetz, R Saunders, J Bates, B Ellingson, R Engelen, and M Daniel Schwarzkopf, et al., 2000: An intercomparison of radiation codes for retrieving upper-tropospheric humidity in the 6.3-:m band: A report from the first GVaP Workshop. Bulletin of the American Meteorological Society, 81(4), 797-808. Abstract PDF
An intercomparison of radiation codes used in retrieving upper-tropospheric humidity (UTH) from observations in the <2 (6.3 :m) water vapor absorption band was performed. This intercomparison is one part of a coordinated effort within the Global Energy and Water Cycle Experiment Water Vapor Project to assess our ability to monitor the distribution and variations of upper-tropospheric moisture from spaceborne sensors. A total of 23 different codes, ranging from detailed line-by-line (LBL) models, to coarser-resolution narrowband (NB) models, to highly parameterized single-band (SB) models participated in the study. Forward calculations were performed using a carefully selected set of temperature and moisture profiles chosen to be representative of a wide range of atmospheric conditions. The LBL model calculations exhibited the greatest consistency with each other, typically agreeing to within 0.5 K in terms of the equivalent blackbody brightness temperature (Tb). The majority of NB and SB models agreed to within ±1 K of the LBL models, although a few older models exhibited systematic Tb biases in excess of 2 K. A discussion of the discrepancies between various models, their association with differences in model physics (e.g., continuum absorption), and their implications for UTH retrieval and radiance assimilation is presented.
Schwarzkopf, M D., and V Ramaswamy, 1999: Radiative effects of CH4, N2O, halocarbons and the foreign-broadened H2O continuum: A GCM experiment. Journal of Geophysical Research, 104(D8), 9467-9488. Abstract PDF
The simplified exchange approximation (SEA) method for calculation of infrared radiative transfer, used for general circulation model (GCM) climate simulations at the Geophysical Fluid Dynamics Laboratory (GFDL) and other institutions, has been updated to permit inclusion of the effects of methane (CH4), nitrous oxide (N2O), halocarbons, and water-vapor-air molecular broadening (foreign broadening). The effects of CH4 and N2O are incorporated by interpolation of line-by-line (LBL) transmissivity calculations evaluated at standard species concentrations; halocarbon effects are calculated from transmissivities computed using recently measured frequency-dependent absorption coefficients. The effects of foreign broadening are included by adoption of the "CKD" formalisim for the water vapor continuum [Clough et al., 1989]. For a standard midlatitude summer profile, the change in the net infrared flux at the model tropopause due to the inclusion of present-day concentrations of CH4 and N2O is evaluated to within ~5% of corresponding LBL results; the change in net flux at the tropopause upon inclusion of 1 ppbv of CFC-11, CFC-12, CFC-113, and HCFC-22 is within ~10% of the LBL results. Tropospheric heating rate changes resulting from the introduction of trace species (CH4, N2O, and halocarbons) are calculated to within ~0.03 K/d of the LBL results. Introduction of the CKD water vapor continuum causes LBL-computed heating rates to decrease by up to ~0.4 K/d in the upper troposphere and to increase by up to ~0.25 K/d in the midtroposphere; the SEA method gives changes within ~0.05 K/d of the LBL values. The revised SEA formulation has been incorporated into the GFDL "SKYHI" GCM. Two simulations (using fixed sea surface temperatures and prescribed clouds) have been performed to determine the changes to the model climate from that of a control calculation upon inclusion of (1) the trace species and (2) the foreign-broadened water vapor continuum. When the trace species are added, statistically significant warming (~1 K) occurs in the annual-mean tropical upper troposphere, while cooling (~1.5 K) is noted in the upper stratosphere and stratopause region. The changes are generally similar to annual-mean equilibrium calculations made using a radiative-convective model assuming fixed dynamical heating. The effects of the CKD water vapor continuum include cooling (~1 K) in the annual-mean troposphere above ~6 km, with significant warming in the lower troposophere. When effects of both trace gases and the CKD continuum are included, the annual-mean temperature increases below ~5 km and cools between 5 and 10 km, indicating that continuum effects dominate in determining temperature changes in the lower and middle troposphere. Above, trace gas effects dominate, resulting in warming in the tropical upper troposphere and cooling in most of the middle atmosphere. Clear-sky outgoing longwave irradiances have been computed for observed European Centre for Medium-Range Weather Forecasting atmospheric profiles using three versions of the SEA formulation, including the effects of (1) water vapor, carbon dioxide, and ozone; (2) the above species plus present-day concentrations of the new trace species; (3) all of the above species plus the CKD H2O continuum. Results for all three cases are within ~10 W/m2 of corresponding Earth Radiation Budget Experiment clear-sky irradiance measurements. The combined effect of trace gases and the CKD continuum result in a decrease of ~8 W/m2 in the computed irradiances.
Haywood, Jim M., M Daniel Schwarzkopf, and V Ramaswamy, 1998: Estimates of radiative forcing due to modeled increases in tropospheric ozone. Journal of Geophysical Research, 103(D14), 16,999-17,007. Abstract PDF
The GFDL R30 general circulation model (GCM) and a fixed dynamical heating model (FDHM) are used to assess the instantaneous and adjusted radiative forcing due to changes in troposopheric ozone caused by anthropogenic activity. Ozone perturbations from the GFDL global chemical transport model are applied to the GCM, and the instantaneous solar and terrestrial radiative forcings are calculated excluding and including clouds. The FDHM is used to calculate the adjusted radiative forcing at the tropopause. The net global annual mean adjusted radiative forcing, including clouds, ranges from +0.29 to +0.35 W m-2 with ~80% of this forcing being in the terrestrial spectrum. If stratospheric adjustment is ignored, the forcing increases by ~10%, and if clouds are ignored, the radiative forcing increases by a further 20-30%. These results are in reasonable agreement with earlier studies and suggest that changes in tropospheric ozone due to anthropogenic emissions exert a global mean radiative forcing that is of similar magnitude but of opposite sign to the direct forcing of sulfate aerosols.
Ramaswamy, V, and M Daniel Schwarzkopf, 1997: Stratospheric temperature trends: observations and model simulations In Stratospheric Processes and Their Role in Climate (SPARC), of the First SPARC General Assembly, WMO/TD-No. 814, WCRP-99, Geneva, Switzerland, World Meteorological Organization, 149-152.
Schwarzkopf, M D., and V Ramaswamy, 1997: Stratospheric climatic effects due to CH4, N2O, CFCs and the H2O infrared continuum: A GCM experiment In IRS '96: Current Problems in Atmospheric Radiation, Proceedings of the International Radiation Symposium, Fairbanks, Alaska, 19-24 August 1996. Hampton, Deepak Publishing, 336-339. Abstract
e GFDL longwave radiation parameterization has been modified to employ the CKD 2.1 formulation of the water vapor continuum. A general circulation model (GCM) experiment using the GFDL "SKYHI" model has been performed using the revised algorithm. The calculation also includes the radiative effects of CH4, N2O and CFCs. The model-simulated radiative heating rates, fluxes and associated temperture changes are compared to those obtained using the Roberts H2O continuum formulation.
Ramaswamy, V, M Daniel Schwarzkopf, and W J Randel, 1996: Fingerprint of ozone depletion in the spatial and temporal pattern of recent lower-stratospheric cooling. Nature, 382, 616-618. Abstract PDF
Observations of air temperatures in the lower stratosphere from 1979 to 1990 reveal a cooling trend that varies both spatially and seasonally. The possible causes of this cooling include changes in concentrations of ozone or of other greenhouse gases, and entirely natural variability, but the relative contributions of such causes are poorly constrained. Here we incorporate the observed decreases in stratospheric ozone concentrations over the same period into a general circulation model of the atmosphere, to investigate the role of the ozone losses in affecting patterns of temperature change. We find that the simulated latitudinal pattern of lower-stratospheric cooling for a given month through the decade corresponds well with the pattern of the observed decadal temperature changes. This result confirms the expectation, from simpler model studies, that the observed ozone depletion exerts a spatially and seasonally varying fingerprint in the decadal cooling of the lower stratosphere, with the influence of increases in concentrations of other greenhouse gases being relatively small. As anthropogenic halocarbon chemicals are important causes of stratospheric ozone depletion, our study suggests a human influence on the patterns of temperature change in the lower stratosphere over this 11-year period.
The observed spatial patterns of temperature change in the free atmosphere from 1963 to 1987 are similar to those predicted by state-of-the-art climate models incorporating various combinations of changes in carbon dioxide, anthropogenic sulphate aerosol and stratospheric ozone concentrations. The degree of pattern similarity between models and observations increases through this period. It is likely that this trend is partially due to human activities, although many uncertainties remain, particularly relating to estimates of natural variability.
Santer, B D., Abraham H Oort, V Ramaswamy, M Daniel Schwarzkopf, and Ronald J Stouffer, et al., 1995: A Search for Human Influences on the Thermal Structure of the Atmosphere, Program for Climate Model Diagnosis and Intercomparison, PCMDI Report No. 27, UCRL-ID-121956: Lawrence Livermore, CA, 26 pp. Abstract
Recent studies have shown that patterns of near-surface temperature change due to combined forcing by CO and anthropogenic sulfate aerosols are easier to identify in the observations than signals due to changes in CO alone (Santer et al., 1995; Mitchell et al., 1995a). Here we extend this work to the vertical structure of atmospheric temperature changes, and additionally consider the possible effects of stratospheric ozone reduction. We compare modelled and observed patterns over the lower troposphere to the lower stratosphere (850 to 50 hPa) and over the low- to mid-troposphere (850 to 500 hPa). In both regions there are strong similarities between observed changes and model-predicted signals. Over 850 to 50 hPa similarities are evident both in CO-only signals and in signals that incorporate the added effects of sulfate aerosols and stratospheric ozone reduction. These similarities are due largely to a common pattern of stratospheric cooling and tropospheric warming in the observations and model experiments. Including the effects of stratospheric ozone reduction results in a more realistic height for the transition between stratospheric cooling and results in a more realistic height for the transition between stratospheric cooling and tropospheric warming. In the low- to mid-troposphere the observations are in better agreement with the temperature-change patterns due to combined forcing than with the CO-only pattern. This is the result of hemispheric-scale temperature-change contrasts that are common to the observations and the combined forcing signal but absent in the CO-only case. The levels of model-versus-observed pattern similarity in both atmospheric regions increase over the period 1963 to 1987. If model estimates of natural internal variability are realistic, it is likely that these trends in pattern similarity are partially due to human activities.
Shine, K P., V Ramaswamy, and M Daniel Schwarzkopf, et al., 1995: Radiative forcing due to changes in ozone: A comparison of different codes In Atmospheric Ozone as a Climate Gas, NATO ASI Series I, Vol. 32, Heidelberg, Germany, Springer-Verlag, 373-396. Abstract
The radiative forcing due to changes in ozone in the troposphere and stratosphere is calculated using a number of different radiative transfer codes and the results are compared. The calculations use a tightly specified set of input parameters. The 14 um band of ozone is shown to make a significant contribution to the forcing for changes in stratospheric ozone, although, because of line overlap, it is of less importance for tropospheric ozone changes. The main cause of the spread in results is differences in the solar forcings; these differences are believed to reflect simplifications used in parameterizations rather than the actual uncertainty in modelling solar irradiances.
Schwarzkopf, M D., and V Ramaswamy, 1993: Radiative forcing due to ozone in the 1980s: Dependence on altitude of ozone change. Geophysical Research Letters, 20(2), 205-208. Abstract PDF
The radiative forcing of the surface-troposphere system caused by the changes in ozone in the 1980s is sensitive to the altitude profile of these changes. In the tropics, inclusion of lower stratospheric ozone depletions observed by SAGE results in a substantial negative radiative ozone forcing. In mid-latitudes, the magnitude of the negative stratospheric ozone forcing diminishes as the altitude of ozone depletion is raised above the tropopause. By contrast, the radiative forcing corresponding to the decadal tropospheric ozone increases observed at certain Northern Hemisphere mid-latitude locations is strongly positive. The magnitude and sign of the total (tropospheric + stratospheric) ozone forcing in Northern Hemisphere mid-latitudes is criticaly dependent on the vertical profile of the tropospheric ozone increases and the lower stratospheric losses near the tropopause.
Observations from satellite and ground-based instruments indicate that between 1979 and 1990 there have been statistically significant losses of ozone in the lower stratosphere of the middle to high latitudes in both hemispheres. Here we determine the radiative forcing of the surface-troposphere system due to the observed decadal ozone losses, and compare it with that due to the increased concentrations of the other main radiatively active gases (CO2, CH4, N2O and chlorofluorocarbons) over the same time period. Our results indicate that a significant negative radiative forcing results from ozone losses in middle to high latitudes caused by the CFCs and other gases. As the anthropogenic emissions of CFCs and other halocarbons are thought to be largely responsible for the observed ozone depletions, our results suggest that the net decadal contribution of CFCs to the greenhouse climate forcing is substantially less than previously estimated.
Feigelson, E M., and M Daniel Schwarzkopf, et al., 1991: Calculation of longwave radiation fluxes in atmospheres. Journal of Geophysical Research, 96(D5), 8985-9001. Abstract PDF
A technique for the computation of longwave radiative quantities using the line-by-line approach has been developed in the Soviet Union. The method has been applied to obtain fluxes and cooling rates for standard atmospheric profiles used in the Intercomparison of Radiation Codes used in Climate Models (ICRCCM) sponsored by the World Meteorological Organization. The sensitivity of the result to changes in the vertical quadrature scheme, the angular integration, and the spectral line shape is evaluated. Fluxes and cooling rates in the troposphere are in general agreement with those obtained with different line-by-line models. Results from parameterized models, including a wideband statistical model and one employing the integral transmission function, have been compared to the line-by-line results. Flux errors in the simplified schemes are of the order of 10 W/m2. The sensitivity of these models to changes in atmospheric profiles, or to an increase in CO2 amount, is similar to that of the line-by-line calculations.
Fels, S, J T Kiehl, A A Lacis, and M Daniel Schwarzkopf, 1991: Infrared cooling rate calculations in operational general circulation models: Comparisons with benchmark computations. Journal of Geophysical Research, 96(D5), 9105-9120. Abstract PDF
As part of the Intercomparison of Radiation Codes in Climate Models (ICRCCM) project, careful comparisons of the performance of a large number of radiation codes were carried out, and the results compared with those of benchmark calculations. In this paper, we document the performance of a number of parameterized models which have been heavily used in climate and numerical prediction research at three institutions: Geophysical Fluid Dynmaics Laboratory (GFDL), National Center for Atmospheric Research (NCAR), and Goddard Institute for Space Studies (GISS).
Schwarzkopf, M D., and S Fels, 1991: The simplified exchange method revisited: An accurate, rapid method for computation of infrared cooling rates and fluxes. Journal of Geophysical Research, 96(D5), 9075-9096. Abstract PDF
The performance and construction of a new algorithm for the calculation of infrared cooling rates and fluxes in terrestrial general circulation models are described in detail. The computational method, which is suitable for use in models of both the troposphere and the middle atmosphere, incorporates effects now known to be important, such as an extended water vapor epsilon-type continuum, careful treatment of water vapor lines, of water-carbon dioxide overlap, and of Voigt line shape. The competing requirements of accuracy and speed are both satisfied by extensive use of a generalization of the simplified exchange approximation of Fels and Schwarzkopf (1975). Cooling rates and fluxes are validated by comparison with benchmark line-by-line calculations on standard atmospheric profiles obtained for the Intercomparison of Radiation Codes Used in Climate Models (ICRCCM). Results indicate that the new algorithm is substantially more accurate than any previously used at the Geophysical Fluid Dynamics Laboratory.
Schwarzkopf, M D., and S Fels, 1989: GFDL radiation codes: The next generation In IRS '88: Current Problems in Atmospheric Radiation, A. Deepak Publishing, 433-435.
Crisp, D, S Fels, and M Daniel Schwarzkopf, 1986: Approximate methods for finding CO2 15-µm band transmission in planetary atmospheres. Journal of Geophysical Research, 91(D11), 11,851-11,866. Abstract PDF
The CO2 15-µm band provides an important source of thermal opacity in the atmospheres of Venus, Earth, and Mars. Efficient and accurate methods for finding the transmission in this band are therefore needed before complete, self-consistent physical models of these atmospheres can be developed. In this paper we describe a hierarchy of such methods. The most versatile and accurate of these is an "exact" line-by-line model (Fels and Schwarzkopf, 1981). Other methods described here employ simplifying assumptions about the structure of the 15-µm band which significantly improve their efficiency. Because such approximations can reduce the accuracy of a model, as well as its computational expense, we established the range of validity of these simpler models by comparing their results to those generated by the line-by-line model. Pressures and absorber amounts like those encountered in the atmospheres of Venus, Earth, and Mars were used in these tests. Physical band models based on the Goody (1952) random model compose the first class of approximate methods. These narrow-band models include a general random model and other more efficient techniques that employ the Malkmus (1967) line-strength distribution. Two simple strategies for including Voigt and Doppler line-shape effects are tested. We show that the accuracy of these models at low pressures is very sensitive to the line-strength distribution as well as the line shape. The second class of approximate methods is represented by an exponential wideband model. This physical band model is much more efficient than those described above, since it can be used to find transmission functions for broad sections of the CO2 15-µm band in a single step. When combined with a simple Voigt parameterization, this method produces results almost as accurate as those obtained from the more expensive narrow-band random models. The final class of approximate methods tested here includes the empirical logarithmic wideband models that have been used extensively in climate-modeling studies (Kiehl and Ramanathan, 1983; Pollack, et al., 1981). These methods are very efficient, but their range of validity is more limited than that of the other methods tested here. These methods should therefore be used with caution.
Schwarzkopf, M D., and S Fels, 1985: Improvements to the algorithm for computing CO2 transmissivities and cooling rates. Journal of Geophysical Research, 90(C10), 10,541-10,550. Abstract PDF
A new interpolation algorithm is derived for obtaining CO2 15-μm transmissivities at any pressure from tables of transmission functions at standard pressures. The new method is a revision of the Fels-Schwarzkopf (1981) technique. Improvements to the standard transmissivity tables are also discussed. An extension of these methods to calculate transmissivities at CO2 concentrations other than those used for the tables is described.
Fels, S, and M Daniel Schwarzkopf, 1981: An efficient, accurate algorithm for calculating an efficient, accurate algorithm for calculating CO2 15 um band cooling rates. Journal of Geophysical Research, 86(C2), 1205-1232. Abstract PDF
A fast, accurate method for obtaining atmospheric carbon dioxide transmission functions for the 15-Mum band is presented. Tables of transmissivities for m band is presented. Tables of transmissivities for CO2 mixing ratios of 330 and 660 ppmv on standard pressure grids comprising geopotential heights ranging from 0 to 80 km and using standard temperatures are included. An algorithm for interpolating from these values to any desired temperature profile and to any other pressure grid is detailed.
Fels, S, Jerry D Mahlman, M Daniel Schwarzkopf, and R W Sinclair, 1980: Stratospheric sensitivity to perturbations in ozone and carbon dioxide: Radiative and dynamical response. Journal of the Atmospheric Sciences, 37(10), 2265-2297. Abstract PDF
We have attempted to assess the stratospheric effects of two different perturbations: 1) a uniform 50% reduction in ozone; and 2) a uniform doubling of carbon dioxide. The primary studies employ an annual mean insolation version of the recently developed GFDL 40-level general circulation model (GCM). Supporting the auxiliary calculations using purely radiative models are also presented. One of these, in which the thermal sensitivity is computed using the assumption that heating by dynamical processes is unaffected by changed composition, gives results which generally are in excellent agreement with those from the GCM. Exceptions to this occur in the ozone reduction experiment at the tropical tropopause and the tropical mesosphere.
The predicted response to the ozone reduction is largest at 50 km in the tropics, where the temperature decreases by 25 K; at the tropical tropopause, the decrease is 5 K. The carbon dioxide increase results in a 10 K decrease at 50 km, decreasing to zero at the tropopause. The temperature change in the CO2 experiment is remarkably uniform in latitude.
Fels, S, and M Daniel Schwarzkopf, 1975: The simplified exchange approximation: a new method for radiative transfer calculations. Journal of the Atmospheric Sciences, 32(7), 1475-1488. Abstract
A new scheme for the efficient calculation of longwave radiative heating rates is proposed. Its speed and accuracy make it attractive for use in large atmospheric circulation models.
The approximation suggested isA new scheme for the efficient calculation of longwave radiative heating rates is proposed. Its speed and accuracy make it attractive for use in large atmospheric circulation models.
The approximation suggested is q ~ qe - qe CTS + qCTS,CTS, where q is the heating rate, qe an "emissivity" heating rate calculated using the strong-line approximation and neglecting variation of line intensity with temperature, qe CTS the heating rate calculated using the cool-to-space approximation and the emissivity assumption, and qCTS the heating rate calculated by the cool-to-space approximation.
Tests using a variety of soundings indicate that for both clear sky and cloudy cases the new approximation is substantially more accurate than either the emissivity or the cool-to-space approximations alone. Deviations from exact calculations are generally under 0.05 K day-1. Errors in the calculated flux at the surface are also shown to be small especially with the inclusion of a "heat from ground" term in the approximation.
Some alternate schemes using similar approximations are presented and their utility discussed.
