Comparison of Climate Response to Anthropogenic Aerosol versus Greenhouse Gas Forcing: Distinct Patterns

Abstract Spatial patterns of climate response to changes in anthropogenic aerosols and well-mixed greenhouse gases (GHGs) are investigated using climate model simulations for the twentieth century. The climate response shows both similarities and differences in spatial pattern between aerosol and GHG runs. Common climate response between aerosol and GHG runs tends to be symmetric about the equator. This work focuses on the distinctive patterns that are unique to the anthropogenic aerosol forcing. The tropospheric cooling induced by anthropogenic aerosols is locally enhanced in the midlatitude Northern Hemisphere with a deep vertical structure around 40°N, anchoring a westerly acceleration in thermal wind balance. The aerosol-induced negative radiative forcing in the Northern Hemisphere requires a cross-equatorial Hadley circulation to compensate interhemispheric energy imbalance in the atmosphere. Associated with a southward shift of the intertropical convergence zone, this interhemispheric asymmetric mode is unique to aerosol forcing and absent in GHG runs. Comparison of key climate response pattern indices indicates that the aerosol forcing dominates the interhemispheric asymmetric climate response in historical all-forcing simulations, as well as regional precipitation change such as the drying trend over the East Asian monsoon region. While GHG forcing dominates global mean surface temperature change, its effect is on par with and often opposes the aerosol effect on precipitation, making it difficult to detect anthropogenic change in rainfall from historical observations.

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Implication of the reduction in anthropogenic aerosols due to the COVID-19 pandemic and the future recovery scenarios for radiative forcing

10.5194/egusphere-egu21-928 ◽

2021 ◽

Author(s):

Stephanie Fiedler ◽

Klaus Wyser ◽

Rogelj Joeri ◽

Twan van Noije

Keyword(s):

Radiative Forcing ◽

Internal Variability ◽

Climate Response ◽

Emission Scenarios ◽

Radiative Effects ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

Aerosol Forcing ◽

Effective Radiative Forcing ◽

Aerosol Emissions

The COVID-19 pandemic has led to unprecedented reductions in socio-economic activities. Associated decreases in anthropogenic aerosol emissions are not represented in the original CMIP6 emission scenarios. Here we estimate the implications of the pandemic for the aerosol forcing in 2020 and quantify the spread in aerosol forcing associated with the differences in the post-pandemic recovery pathways. To this end, we use new emission scenarios taking the COVID-19 crisis into account and projecting different socio-economic developments until 2050 with fossil-fuel based and green pathways (Forster et al., 2020). We use the new emission data to generate input for the anthropogenic aerosol parameterization MACv2-SP for CMIP6 models. In this presentation, we first show the results for the anthropogenic aerosol optical depth and associated effects on clouds from the new MACv2-SP data for 2020 to 2050 (Fiedler et al., in review). We then use the MACv2-SP data to provide estimates of the effective radiative effects of the anthropogenic aerosols for 2020 and 2050. Our forcing estimates are based on new atmosphere-only simulations with the CMIP6 model EC-Earth3. The model uses MACv2-SP to represent aerosol-radiation and aerosol-cloud interactions including aerosol effects on cloud lifetime. For each anthropogenic aerosol pattern, we run EC-Earth3 simulations for fifty years to substantially reduce the impact of model-internal variability on the forcing estimate. Our results highlight: (1) a change of +0.04 Wm-2 in the global mean effective radiative forcing of anthropogenic aerosols for 2020 due to the pandemic, which is small compared to the magnitude of internal variability, (2) a spread of -0.38 to -0.68 Wm-2 for the effective radiative forcing associated with anthropogenic aerosols in 2050 depending on the recovery scenario in MACv2-SP, and (3) a more negative (stronger) anthropogenic aerosol forcing for a strong green than a moderate green development in 2050 due to higher ammonium emissions in a highly decarbonized society (Fiedler et al., in review). The new MACv2-SP data are now used in climate models participating in the model intercomparison project on the climate response to the COVID-19 crisis (Covid-MIP, Jones et al., in review, Lamboll et al., in review).References:Fiedler, S., Wyser, K., Joeri, R., and van Noije, T.: Radiative effects of reduced aerosol emissions during the COVID-19 pandemic and the future recovery, in review, [preprint] https://doi.org/10.1002/essoar.10504704.1.Forster, P.M., Forster, H.I., Evans, M.J.&#160;et al.:&#160;Current and future global climate impacts resulting from COVID-19.&#160;Nat. Clim. Chang.&#160;10,&#160;913&#8211;919, 2020, https://doi.org/10.1038/s41558-020-0883-0.Jones. C., Hickman, J., Rumbold, S., et al.: The Climate Response to Emissions Reductions due to COVID-19, Geophy. Res. Lett., in review.Lamboll, R. D., Jones, C. D., Skeie, R. B., Fiedler, S., Samset, B. H., Gillett, N. P., Rogelj, J., and Forster, P. M.: Modifying emission scenario projections to account for the effects of COVID-19: protocol for Covid-MIP, in review, [preprint] https://doi.org/10.5194/gmd-2020-373.

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Reply to “Comment on ‘Rethinking the Lower Bound on Aerosol Radiative Forcing’”

Journal of Climate ◽

10.1175/jcli-d-17-0034.1 ◽

2017 ◽

Vol 30 (16) ◽

pp. 6585-6589 ◽

Cited By ~ 4

Author(s):

Bjorn Stevens ◽

Stephanie Fiedler

Keyword(s):

Twentieth Century ◽

Lower Bound ◽

Radiative Forcing ◽

Climate Models ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

Aerosol Radiative Forcing ◽

Clear Sky ◽

The North ◽

Aerosol Forcing

Kretzschmar et al., in a comment in 2017, use the spread in the output of aerosol–climate models to argue that the models refute the hypothesis (presented in a paper by Stevens in 2015) that for the mid-twentieth-century warming to be consistent with observations, then the present-day aerosol forcing, [Formula: see text] must be less negative than −1 W m−2. The main point of contention is the nature of the relationship between global SO2 emissions and [Formula: see text] In contrast to the concave (log-linear) relationship used by Stevens and in earlier studies, whereby [Formula: see text] becomes progressively less sensitive to SO2 emissions, some models suggest a convex relationship, which would imply a less negative lower bound. The model that best exemplifies this difference, and that is most clearly in conflict with the hypothesis of Stevens, does so because of an implausible aerosol response to the initial rise in anthropogenic aerosol precursor emissions in East and South Asia—already in 1975 this model’s clear-sky reflectance from anthropogenic aerosol over the North Pacific exceeds present-day estimates of the clear-sky reflectance by the total aerosol. The authors perform experiments using a new (observationally constrained) climatology of anthropogenic aerosols to further show that the effects of changing patterns of aerosol and aerosol precursor emissions during the late twentieth century have, for the same global emissions, relatively little effect on [Formula: see text] These findings suggest that the behavior Kretzschmar et al. identify as being in conflict with the lower bound in Stevens arises from an implausible relationship between SO2 emissions and [Formula: see text] and thus provides little basis for revising this lower bound.

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Anthropogenic aerosol forcing under the Shared Socioeconomic Pathways

10.5194/acp-2019-606 ◽

2019 ◽

Author(s):

Marianne T. Lund ◽

Gunnar Myhre ◽

Bjørn H. Samset

Keyword(s):

Air Pollution ◽

Radiative Forcing ◽

Global Climate ◽

Radiative Transfer Model ◽

Transfer Model ◽

Air Pollution Control ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

Aerosol Radiative Forcing ◽

Aerosol Forcing

Abstract. Emissions of anthropogenic aerosols are expected to change drastically over the coming decades, with potentially significant climate implications. Using the most recent generation of harmonized emission scenarios, the Shared Socioeconomic Pathways (SSPs) as input to a global chemistry transport and radiative transfer model, we provide estimates of the projected future global and regional burdens and radiative forcing of anthropogenic aerosols under three different levels of air pollution control: strong (SSP1), medium (SSP2) and weak (SSP3). We find that the broader range of future air pollution emission trajectories spanned by the SSPs compared to previous scenarios translates into total aerosol forcing estimates in 2100 relative to 1750 ranging from −0.04 W m−2 in SSP1-1.9 to −0.51 W m−2 in SSP3-7.0. Compared to our 1750–2015 estimate of −0.61 W m−2, this shows that depending on the success of air pollution policies over the coming decades, aerosol radiative forcing may weaken by nearly 95 % or remain close to the pre-industrial to present-day level. In all three scenarios there is a positive forcing in 2100 relative to 2015, from 0.51 W m−2 in SSP1-1.9 to 0.04 W m−2 in SSP3-7.0. Results also demonstrate significant differences across regions and scenarios, especially in South Asia and Africa. While rapid weakening of the negative aerosol forcing following effective air quality policies will unmask more of the greenhouse gas-induced global warming, slow progress on mitigating air pollution will significantly enhance the atmospheric aerosol levels and risk to human health. In either case, the resulting impacts on regional and global climate can be significant.

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Dynamics of Asian Summer Monsoon Response to Anthropogenic Aerosol Forcing

Journal of Climate ◽

10.1175/jcli-d-18-0386.1 ◽

2019 ◽

Vol 32 (3) ◽

pp. 843-858 ◽

Cited By ~ 11

Author(s):

Hai Wang ◽

Shang-Ping Xie ◽

Yu Kosaka ◽

Qinyu Liu ◽

Yan Du

Keyword(s):

Summer Monsoon ◽

Radiative Forcing ◽

Asian Summer Monsoon ◽

Atmospheric Response ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

Sst Cooling ◽

Aerosol Forcing ◽

Cooling Pattern ◽

Asian Summer

Anthropogenic aerosols partially mask the greenhouse warming and cause the reduction in Asian summer monsoon precipitation and circulation. By decomposing the atmospheric change into the direct atmospheric response to radiative forcing and sea surface temperature (SST)-mediated change, the physical mechanisms for anthropogenic-aerosol-induced changes in the East Asian summer monsoon (EASM) and South Asian summer monsoon (SASM) are diagnosed. Using coupled and atmospheric general circulation models, this study shows that the aerosol-induced troposphere cooling over Asian land regions generates anomalous sinking motion between 20° and 40°N and weakens the EASM north of 20°N without SST change. The decreased EASM precipitation and the attendant wind changes are largely due to this direct atmospheric response to radiative forcing, although the aerosol-induced North Pacific SST cooling also contributes. The SST-mediated change dominates the aerosol-induced SASM response, with contributions from both the north–south interhemispheric SST gradient and the local SST cooling pattern over the tropical Indian Ocean. Specifically, with large meridional gradient, the zonal-mean SST cooling pattern is most important for the Asian summer monsoon response to anthropogenic aerosol forcing, resulting in a reorganization of the regional meridional atmospheric overturning circulation. While uncertainty in aerosol radiative forcing has been emphasized in the literature, our results show that the intermodel spread is as large in the SST effect on summer monsoon rainfall, calling for more research into the ocean–atmosphere coupling.

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FAIR v1.1: A simple emissions-based impulse response and carbon cycle model

10.5194/gmd-2017-266 ◽

2017 ◽

Cited By ~ 3

Author(s):

Christopher J. Smith ◽

Piers M. Forster ◽

Myles Allen ◽

Nicholas Leach ◽

Richard J. Millar ◽

...

Keyword(s):

Greenhouse Gases ◽

Temperature Change ◽

Radiative Forcing ◽

Climate Model ◽

Future Climate Change ◽

Climate Response ◽

Rcp Scenarios ◽

Transient Climate Response ◽

Surface Temperature Change ◽

Future Projections

Abstract. Simple climate models can be valuable if they are able to replicate aspects of complex fully coupled earth system models. Larger ensembles can be produced, enabling a probabilistic view of future climate change. A simple emissions-based climate model, FAIR, is presented which calculates atmospheric concentrations of greenhouse gases and effective radiative forcing (ERF) from greenhouse gases, aerosols, ozone precursors and other agents. The ERFs are integrated into global mean surface temperature change. Model runs are constrained to observed temperature change from 1880 to 2016 and produce a range of future projections under the Representative Concentration Pathway (RCP) scenarios. For the historical period the ERF time series in FAIR emulates the results in the IPCC Fifth Assessment Report (AR5), whereas for RCP historical and future scenarios, the greenhouse gas concentrations in FAIR closely track the observations and projections in the RCPs. The constrained estimates of equilibrium climate sensitivity (ECS) of 2.79 (1.97 to 4.08) K, transient climate response (TCR) of 1.47 (1.03 to 2.23) K and transient climate response to cumulative CO2 emissions (TCRE) of 1.43 (1.01 to 2.16) K (1000 GtC)−1 (median and 5–95 % credible intervals) are in good agreement, with tighter uncertainty bounds, than AR5 (1.5 to 4.5 K, 1.0 to 2.5 K, and 0.8 to 2.5 K respectively). The ranges of future projections of temperature and ranges of estimates of ECS, TCR and TCRE are moderately sensitive to the historical temperature dataset used to constrain, prior distributions of ECS/TCR parameters, aerosol radiative forcing relationship and ERF from a doubling of CO2. Taking these sensitivities into account, there is no evidence to suggest that the median and credible range of observationally constrained TCR or ECS differ from climate model-derived estimates. However, the range of temperature projections under the RCP scenarios for 2081–2100 in the constrained FAIR model ensemble are lower than the emissions-based estimates reported in AR5.

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Climatic effects of 1950–2050 changes in US anthropogenic aerosols – Part 2: Climate response

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-11-24127-2011 ◽

2011 ◽

Vol 11 (8) ◽

pp. 24127-24164 ◽

Cited By ~ 3

Author(s):

E. M. Leibensperger ◽

L. J. Mickley ◽

D. J. Jacob ◽

W.-T. Chen ◽

J. H. Seinfeld ◽

...

Keyword(s):

Air Quality ◽

Radiative Forcing ◽

General Circulation ◽

Transport Model ◽

Circulation Model ◽

Climate Response ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

The Us ◽

Aerosol Sources

Abstract. We investigate the climate response to US anthropogenic aerosol sources over the 1950 to 2050 period by using the NASA GISS general circulation model (GCM) and comparing to observed US temperature trends. Time-dependent aerosol distributions are generated from the GEOS-Chem chemical transport model applied to historical emission inventories and future projections. Radiative forcing from US anthropogenic aerosols peaked in 1970–1990 and has strongly declined since due to air quality regulations. We find that the regional radiative forcing from US anthropogenic aerosols elicits a strong regional climate response, cooling the central and eastern US by 0.5–1.0 °C on average during 1970–1990, with the strongest effects on maximum daytime temperatures in summer and autumn. Aerosol cooling reflects comparable contributions from direct and indirect (cloud-mediated) radiative effects. Absorbing aerosol (mainly black carbon) has negligible warming effect. Aerosol cooling reduces surface evaporation and thus decreases precipitation along the US east coast, but also increases the southerly flow of moisture from the Gulf of Mexico resulting in increased cloud cover and precipitation in the central US. Observations over the eastern US show a lack of warming in 1960–1980 followed by very rapid warming since, which we reproduce in the GCM and attribute to trends in US anthropogenic aerosol sources. Present US aerosol concentrations are sufficiently low that future air quality improvements are projected to cause little further warming in the US (0.1 °C over 2010–2050). We find that most of the potential warming from aerosol source controls in the US has already been realized over the 1980–2010 period.

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The end of the anthropogenic aerosol era?

10.5194/egusphere-egu2020-1780 ◽

2020 ◽

Author(s):

Susanne E. Bauer ◽

Kostas Tsigaridis

Keyword(s):

Global Warming ◽

Radiative Forcing ◽

Climate Model ◽

Climate Change Impacts ◽

The United States ◽

Global Scale ◽

Cooling Effect ◽

Past Century ◽

Anthropogenic Aerosol ◽

Aerosol Forcing

The Earth&#8217;s climate is rapidly changing. Over the past century, aerosols, via their ability to absorb or scatter solar radiation and alter clouds, played an important role in counterbalancing some of the greenhouse gas (GHG) caused global warming. This, over a century-long anthropogenic aerosol cooling effect, prevented present day climate to have yet reached even higher surface air temperatures and subsequent more dramatic climate change impacts. Trends in aerosol concentrations and optical depth show that in many formerly highly polluted regions such as Europe and the United States of America aerosol precursor emissions have already decreased back to pollution levels of the 1950s. More recent polluting countries such as China may have reached a turning point in recent years as well, while India keeps still following an upward trend. Here we study aerosol trends in the CMIP6 simulations of the GISS ModelE climate model using a fully coupled atmosphere composition configuration, including interactive gas phase chemistry, and either an aerosol microphysical (MATRIX) or a mass based (OMA) aerosol module. Results show that the question if we are already at a period where aerosol radiative forcing continuously declines globally depends on the aerosol scheme used. Using the aerosol microphysical scheme, where the aerosol system reacts stronger to the trend in sulfur dioxide (SO2) emissions, global peak direct aerosol forcing was reached in the 1980&#8217;s, whereas the mass-based scheme simulates peak direct aerosol forcing around 2010. The models are tested again ice core records, satellite and surface network datasets. An evaluation with satellite data between 2001 and 2014 demonstrates that the model that better reproduces the satellite retrieved trends has reached maximal aerosol direct forcing in the 1980s, and is since on a decreasing global forcing trajectory. As a consequence, we expect that the recently observed global warming which is primarily driven by greenhouse gases has been augmented by the effect of a decreasing aerosol cooling effect on the global scale.

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Climatic effects of 1950–2050 changes in US anthropogenic aerosols – Part 2: Climate response

Atmospheric Chemistry and Physics ◽

10.5194/acp-12-3349-2012 ◽

2012 ◽

Vol 12 (7) ◽

pp. 3349-3362 ◽

Cited By ~ 100

Author(s):

E. M. Leibensperger ◽

L. J. Mickley ◽

D. J. Jacob ◽

W.-T. Chen ◽

J. H. Seinfeld ◽

...

Keyword(s):

Air Quality ◽

Radiative Forcing ◽

General Circulation ◽

Transport Model ◽

Circulation Model ◽

Climate Response ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

The Us ◽

Aerosol Sources

Abstract. We investigate the climate response to changing US anthropogenic aerosol sources over the 1950–2050 period by using the NASA GISS general circulation model (GCM) and comparing to observed US temperature trends. Time-dependent aerosol distributions are generated from the GEOS-Chem chemical transport model applied to historical emission inventories and future projections. Radiative forcing from US anthropogenic aerosols peaked in 1970–1990 and has strongly declined since due to air quality regulations. We find that the regional radiative forcing from US anthropogenic aerosols elicits a strong regional climate response, cooling the central and eastern US by 0.5–1.0 °C on average during 1970–1990, with the strongest effects on maximum daytime temperatures in summer and autumn. Aerosol cooling reflects comparable contributions from direct and indirect (cloud-mediated) radiative effects. Absorbing aerosol (mainly black carbon) has negligible warming effect. Aerosol cooling reduces surface evaporation and thus decreases precipitation along the US east coast, but also increases the southerly flow of moisture from the Gulf of Mexico resulting in increased cloud cover and precipitation in the central US. Observations over the eastern US show a lack of warming in 1960–1980 followed by very rapid warming since, which we reproduce in the GCM and attribute to trends in US anthropogenic aerosol sources. Present US aerosol concentrations are sufficiently low that future air quality improvements are projected to cause little further warming in the US (0.1 °C over 2010–2050). We find that most of the warming from aerosol source controls in the US has already been realized over the 1980–2010 period.

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The value of remote marine aerosol measurements for constraining radiative forcing uncertainty

10.5194/acp-2019-1085 ◽

2019 ◽

Author(s):

Leighton A. Regayre ◽

Julia Schmale ◽

Jill S. Johnson ◽

Christian Tatzelt ◽

Andrea Baccarini ◽

...

Keyword(s):

Southern Ocean ◽

Northern Hemisphere ◽

Radiative Forcing ◽

Climate Model ◽

Model Parameters ◽

Marine Aerosol ◽

Aerosol Cloud ◽

Aerosol Forcing ◽

Model Aerosol ◽

Aerosol Cloud Interactions

Abstract. Aerosol measurements over the Southern Ocean are used to constrain aerosol-cloud interaction radiative forcing uncertainty in a global climate model. Aerosol forcing uncertainty is quantified using one million climate model variants that sample the uncertainty in nearly 30 model parameters. Ship-based measurements of cloud condensation nuclei, particle number concentrations and sulfate mass concentrations from the Antarctic Circumnavigation Expedition: Study of Preindustrial-like Aerosols and Their Climate Effects (ACE-SPACE) are used to identify observationally implausible variants and thereby reduce the spread in the simulated forcing. Southern Ocean measurements strongly constrain natural aerosol emissions: default sea spray emissions in the model need to be increased by around a factor of 3 to be consistent with measurements. Aerosol forcing uncertainty is reduced by around 7 % using these measurements, which is comparable to the 8 % reduction achieved using an extensive set of over 9000 predominantly Northern Hemisphere measurements. The radiative forcing due to aerosol–cloud interactions (RFaci) is constrained to −2.61 to −1.10 W m−2 (95 % confidence) and the effective radiative forcing from aerosol-cloud interactions (ERFaci) is constrained to −2.43 to −0.54 W m−2. When Southern Ocean and Northern Hemisphere measurements are combined, the uncertainty in RFaci is reduced by 21 % and the strongest 20 % of forcing values are ruled out as implausible. In this combined constraint the observationally plausible RFaci is around 0.17 W m−2 weaker (less negative) with credible values ranging from −2.51 to −1.17 W m−2 and from −2.18 to −1.46 W m−2 when using one standard deviation to quantify the uncertainty. The Southern Ocean and Northern Hemisphere measurement datasets are complementary because they constrain different processes. These results highlight the value of remote marine aerosol measurements.

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Anthropogenic aerosol forcing under the Shared Socioeconomic Pathways

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-13827-2019 ◽

2019 ◽

Vol 19 (22) ◽

pp. 13827-13839 ◽

Cited By ~ 6

Author(s):

Marianne T. Lund ◽

Gunnar Myhre ◽

Bjørn H. Samset

Keyword(s):

Air Pollution ◽

Radiative Forcing ◽

Socioeconomic Development ◽

Global Climate ◽

Radiative Transfer Model ◽

Transfer Model ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

Pollution Levels ◽

Aerosol Forcing

Abstract. Emissions of anthropogenic aerosols are expected to change drastically over the coming decades, with potentially significant climate implications. Using the most recent generation of harmonized emission scenarios, the Shared Socioeconomic Pathways (SSPs) as input to a global chemistry transport and radiative transfer model, we provide estimates of the projected future global and regional burdens and radiative forcing of anthropogenic aerosols under three contrasting pathways for air pollution levels: SSP1-1.9, SSP2-4.5 and SSP3-7.0. We find that the broader range of future air pollution emission trajectories spanned by the SSPs compared to previous scenarios translates into total aerosol forcing estimates in 2100 relative to 1750 ranging from −0.04 in SSP1-1.9 to −0.51 W m−2 in SSP3-7.0. Compared to our 1750–2015 estimate of −0.55 W m−2, this shows that, depending on the success of air pollution policies and socioeconomic development over the coming decades, aerosol radiative forcing may weaken by nearly 95 % or remain close to the preindustrial to present-day level. In all three scenarios there is a positive forcing in 2100 relative to 2015, from 0.51 in SSP1-1.9 to 0.04 W m−2 in SSP3-7.0. Results also demonstrate significant differences across regions and scenarios, especially in South Asia and Africa. While rapid weakening of the negative aerosol forcing following effective air quality policies will unmask more of the greenhouse-gas-induced global warming, slow progress on mitigating air pollution will significantly enhance the atmospheric aerosol levels and risk to human health in these regions. In either case, the resulting impacts on regional and global climate can be significant.

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