scholarly journals Contrasting Climate Responses to the Scattering and Absorbing Features of Anthropogenic Aerosol Forcings

2014 ◽  
Vol 27 (14) ◽  
pp. 5329-5345 ◽  
Author(s):  
Ilissa B. Ocko ◽  
V. Ramaswamy ◽  
Yi Ming
Keyword(s):  
Regional Climate ◽  
Ocean Model ◽  
Anthropogenic Aerosols ◽  
Anthropogenic Aerosol ◽  
Climate Conditions ◽  
Radiative Forcings ◽  
Aerosol Forcing ◽  
The Individual ◽  
Induced Changes ◽  
Climate Responses

Abstract Anthropogenic aerosols comprise optically scattering and absorbing particles, with the principal concentrations being in the Northern Hemisphere, yielding negative and positive global mean radiative forcings, respectively. Aerosols also influence cloud albedo, yielding additional negative radiative forcings. Climate responses to a comprehensive set of isolated aerosol forcing simulations are investigated in a coupled atmosphere–ocean framework, forced by preindustrial to present-day aerosol-induced radiative perturbations. Atmospheric and oceanic climate responses (including precipitation, atmospheric circulation, atmospheric and oceanic heat transport, sea surface temperature, and salinity) to negative and positive particulate forcings are consistently anticorrelated. The striking effects include distinct patterns of changes north and south of the equator that are governed by the sign of the aerosol forcing and its initiation of an interhemispheric forcing asymmetry. The presence of opposing signs of the forcings between the aerosol scatterers and absorbers, and the resulting contrast in climate responses, thus dilutes the individual effects of aerosol types on influencing global and regional climate conditions. The aerosol-induced changes in the variables also have a distinct fingerprint when compared to the responses of the more globally uniform and interhemispherically symmetric well-mixed greenhouse gas forcing. The significance of employing a full ocean model is demonstrated in this study by the ability to partition how individual aerosols influence atmospheric and oceanic conditions separately.

scholarly journals Aerosol radiative effects with MACv2

2019 ◽  
Author(s):  
Stefan Kinne
Keyword(s):  
Anthropogenic Sources ◽  
Transfer Model ◽  
Strong Impact ◽  
Radiative Effects ◽  
Anthropogenic Aerosols ◽  
Anthropogenic Aerosol ◽  
Aerosol Forcing ◽  
Aerosol Radiative Effects ◽  
The Impact ◽  
Aerosol Radiative

Abstract. onthly global maps for aerosol properties of the MACv2 climatology are applied in an off-line radiative transfer model to determine aerosol radiative effects. For details beyond global averages in most cases global maps are presented to visualize regional and seasonal details. Aside from the direct radiative (aerosol presence) effect, including those for aerosol components as extracted from MACv2 aerosol optics, also the major aerosol indirect radiative effect is covered. Hereby, the impact of smaller drops in water clouds due to added anthropogenic aerosol was simulated by applying a satellite retrieval based fit from locally associations between aerosol and drop concentrations over oceans. Present-day anthropogenic aerosols of MACv2 – on a global average basis – reduce the radiative net-fluxes at the top of the atmosphere (TOA) by −1.0 W/m2 and at the surface by −2.1 W/m2. Direct cooling contributions are only about half of indirect contributions (−.35 vs −.65) at TOA, but about twice at the surface (−1.45 vs −.65), as solar absorption of the direct effect warms the atmosphere by +1.1 W/m2. Natural aerosols are on average less absorbing (for a relatively larger solar TOA cooling) and larger in size (now contributing with IR greenhouse warming). Thus, average TOA direct forcing efficiencies for total and anthropogenic aerosol happen to be similar: −11 W/m2/AOD at all-sky and −24 W/m2/AOD at clear-sky conditions. The present-day direct impact by all soot (BC) is globally averaged +0.55W/m2 and at least half of it should be attributed to anthropogenic sources. Hereby any accuracy of anthropogenic impacts, not just for soot, suffers from the limited access to a pre-industrial reference. Anthropogenic uncertainty has a particular strong impact on aerosol indirect effects, which dominate the (TOA) forcing. Accounting for uncertainties in the anthropogenic definition, present-day aerosol forcing is estimated to stay within the −0.7 to −1.6 W/m2 range, with a best estimate at −1 W/m2. Calculations with model predicted temporal changes to anthropogenic AOD indicate that qualitatively the anthropogenic aerosol forcing has not changed much over the last decades and is not likely to increase over the next decades, despite strong regional shifts. These regional shifts explain most solar insolation (brightening or dimming) trends that have been observed by ground-based radiation data.

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

2017 ◽  
Vol 30 (16) ◽  
pp. 6585-6589 ◽  
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.

scholarly journals Nonlinear Climate Responses to Increasing CO2 and Anthropogenic Aerosols Simulated by CESM1

2019 ◽  
Vol 33 (1) ◽  
pp. 281-301 ◽  
Author(s):  
Jiechun Deng ◽  
Aiguo Dai ◽  
Haiming Xu
Keyword(s):  
Sea Ice ◽  
Regional Climate ◽  
Surface Air Temperature ◽  
Nonlinear Effects ◽  
Arctic Sea Ice ◽  
The Arctic ◽  
Anthropogenic Aerosols ◽  
Aerosol Loading ◽  
Temperature And Precipitation ◽  
The Individual

Abstract Atmospheric CO2 and anthropogenic aerosols (AA) have increased simultaneously. Because of their opposite radiative effects, these increases may offset each other, which may lead to some nonlinear effects. Here the seasonal and regional characteristics of this nonlinear effect from the CO2 and AA forcings are investigated using the fully coupled Community Earth System Model. Results show that nonlinear effects are small in the global mean of the top-of-the-atmosphere radiative fluxes, surface air temperature, and precipitation. However, significant nonlinear effects exist over the Arctic and other extratropical regions during certain seasons. When both forcings are included, Arctic sea ice in September–November decreases less than the linear combination of the responses to the individual forcings due to a higher sea ice sensitivity to the CO2-induced warming than the sensitivity to the AA-induced cooling. This leads to less Arctic warming in the combined-forcing experiment due to reduced energy release from the Arctic Ocean to the atmosphere. Some nonlinear effects on precipitation in June–August are found over East Asia, with the northward-shifted East Asian summer rain belt to oppose the CO2 effect. In December–February, the aerosol loading over Europe in the combined-forcing experiment is higher than that due to the AA forcing, resulting from CO2-induced circulation changes. The changed aerosol loading results in regional thermal responses due to aerosol direct and indirect effects, weakening the combined changes of temperature and circulation. This study highlights the need to consider nonlinear effects from historical CO2 and AA forcings in seasonal and regional climate attribution analyses.

scholarly journals Anthropogenic aerosol forcing of the AMOC and the associated mechanisms in CMIP6 models

2020 ◽  
Author(s):  
Taufiq Hassan ◽  
Robert J. Allen ◽  
Wei Liu ◽  
Cynthia Randles
Keyword(s):  
Climate Variability ◽  
Atmospheric Circulation ◽  
20Th Century ◽  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Solar Variability ◽  
Anthropogenic Aerosols ◽  
Anthropogenic Aerosol ◽  
Aerosol Forcing ◽  
Volcanic Aerosols

Abstract. By regulating the global transport of heat, freshwater and carbon, the Atlantic Meridional Overturning Circulation (AMOC) serves as an important component of the climate system. During the late 20th and early 21st centuries, indirect observations and models suggest a weakening of the AMOC. Direct AMOC observations also suggest a weakening during the early 21st century, but with substantial interannual variability. Long-term weakening of the AMOC has been associated with increasing greenhouse gases (GHGs), but some modeling studies suggest the build up of anthropogenic aerosols (AAs) may have offset part of the GHG-induced weakening. Here, we quantify 1900–2020 AMOC variations and assess the driving mechanisms in state-of-the-art climate models from the Coupled Model Intercomparison Project phase 6 (CMIP6). The CMIP6 all forcing (GHGs, anthropogenic and volcanic aerosols, solar variability, and land use/land change) multi-model mean shows negligible AMOC changes up to ~1950, followed by robust AMOC strengthening during the second half of the 20th century (~1950–1990), and weakening afterwards (1990–2020). These multi-decadal AMOC variations are related to changes in North Atlantic atmospheric circulation, including an altered sea level pressure gradient, storm track activity, surface winds and heat fluxes, which drive changes in the subpolar North Atlantic surface density flux. Similar to previous studies, CMIP6 GHG simulations yield robust AMOC weakening, particularly during the second half of the 20th century. Changes in natural forcings, including solar variability and volcanic aerosols, yield negligible AMOC changes. In contrast, CMIP6 AA simulations yield robust AMOC strengthening (weakening) in response to increasing (decreasing) anthropogenic aerosols. Moreover, the CMIP6 all-forcing AMOC variations and atmospheric circulation responses also occur in the CMIP6 AA simulations, which suggests these are largely driven by changes in anthropogenic aerosol emissions. Although aspects of the CMIP6 all-forcing multi-model mean response resembles observations, notable differences exist. This includes CMIP6 AMOC strengthening from ~1950–1990, when the indirect estimates suggest AMOC weakening. The CMIP6 multi-model mean also underestimates the observed increase in North Atlantic ocean heat content. And although the CMIP6 North Atlantic atmospheric circulation responses–particularly the overall patterns–are similar to observations, the simulated responses are weaker than those observed, implying they are only partially externally forced. The possible causes of these differences include internal climate variability, observational uncertainties and model shortcomings–including excessive aerosol forcing. A handful of CMIP6 realizations yield AMOC evolution since 1900 similar to the indirect observations, implying the inferred AMOC weakening from 1950–1990 (and even from 1930–1990) may have a significant contribution from internal (i.e., unforced) climate variability. Nonetheless, CMIP6 models yield robust, externally forced AMOC changes, the bulk of which are due to anthropogenic aerosols.

scholarly journals Anthropogenic aerosol forcing under the Shared Socioeconomic Pathways

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.

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

2016 ◽  
Vol 29 (14) ◽  
pp. 5175-5188 ◽  
Author(s):  
Hai Wang ◽  
Shang-Ping Xie ◽  
Qinyu Liu
Keyword(s):  
Northern Hemisphere ◽  
Radiative Forcing ◽  
Climate Model ◽  
East Asian Monsoon ◽  
Climate Response ◽  
Anthropogenic Aerosols ◽  
Anthropogenic Aerosol ◽  
Asian Monsoon Region ◽  
Surface Temperature Change ◽  
Aerosol Forcing

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.

scholarly journals Dynamics of Asian Summer Monsoon Response to Anthropogenic Aerosol Forcing

2019 ◽  
Vol 32 (3) ◽  
pp. 843-858 ◽  
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.

Aerosol Direct Radiative and Cloud Adjustment Effects on Surface Climate over Eastern China: Analyses of WRF Model Simulations

2019 ◽  
Vol 32 (4) ◽  
pp. 1293-1306 ◽  
Author(s):  
Yangyang Song ◽  
Guoxing Chen ◽  
Wei-Chyung Wang
Keyword(s):  
Eastern China ◽  
Wrf Model ◽  
Heat Fluxes ◽  
Climate Forcing ◽  
Surface Cooling ◽  
Anthropogenic Aerosols ◽  
Temperature Changes ◽  
Radiative Forcings ◽  
Aerosol Forcing ◽  
Surface Climate

The WRF-simulated changes in clouds and climate due to the increased anthropogenic aerosols for the summers of 2002–08 (vs the 1970s) over eastern China were used to offline calculate the radiative forcings associated with aerosol–radiation (AR) and aerosol–cloud–radiation (ACR) interactions, which subsequently facilitated the interpretation of surface temperature changes. During this period, the increases of aerosol optical depth (ΔAOD) averaged over eastern China range from 0.18 in 2004 to 0.26 in 2007 as compared to corresponding cases in the 1970s, and the multiyear means (standard deviations) of AR and ACR forcings at the surface are −6.7 (0.58) and −3.5 (0.63) W m−2, respectively, indicating the importance of cloud changes in affecting both the aerosol climate forcing and its interannual variation. The simulated mean surface cooling is 0.35°C, dominated by AR and ACR with a positive (cooling) feedback associated with changes in meteorology (~10%), and two negative (warming) feedbacks associated with decreases in latent (~70%) and sensible (~20%) heat fluxes. More detailed spatial characteristics were analyzed using ensemble simulations for the year 2008. Three regions—Jing-Jin-Ji (ΔAOD ~ 0.63), Sichuan basin (ΔAOD ~ 0.31), and middle Yangtze River valley (ΔAOD ~ 0.26)—at different climate regimes were selected to investigate the relative roles of AR and ACR. While the AR forcing is closely related to ΔAOD values, the ACR forcing presents different regional characteristics owing to cloud changes. In addition, the surface heat flux feedbacks are also different between regions. The study thus illustrates that ACR forcing is useful as a diagnostic parameter to unravel the complexity of climate change to aerosol forcing over eastern China.

scholarly journals Role of climate model dynamics in estimated climate responses to anthropogenic aerosols

2019 ◽  
Vol 19 (15) ◽  
pp. 9969-9987 ◽  
Author(s):  
Kalle Nordling ◽  
Hannele Korhonen ◽  
Petri Räisänen ◽  
Muzaffer Ege Alper ◽  
Petteri Uotila ◽  
...  
Keyword(s):  
Climate Models ◽  
Regional Climate ◽  
Climate Impacts ◽  
The Arctic ◽  
Anthropogenic Aerosols ◽  
Temperature And Precipitation ◽  
Model Aerosol ◽  
Regional Temperature ◽  
Temperature Responses ◽  
Climate Responses

Abstract. Significant discrepancies remain in estimates of climate impacts of anthropogenic aerosols between different general circulation models (GCMs). Here, we demonstrate that eliminating differences in model aerosol or radiative forcing fields results in close agreement in simulated globally averaged temperature and precipitation responses in the studied GCMs. However, it does not erase the differences in regional responses. We carry out experiments of equilibrium climate response to modern-day anthropogenic aerosols using an identical representation of anthropogenic aerosol optical properties and the first indirect effect of aerosols, MACv2-SP (a simple plume implementation of the second version of the Max Planck Institute Aerosol CLimatology), in two independent climate models (NorESM, Norwegian Earth System Model, and ECHAM6). We find consistent global average temperature responses of −0.48 (±0.02) and −0.50 (±0.03) K and precipitation responses of −1.69 (±0.04) % and −1.79 (±0.05) % in NorESM1 and ECHAM6, respectively, compared to modern-day equilibrium climate without anthropogenic aerosols. However, significant differences remain between the two GCMs' regional temperature responses around the Arctic circle and the Equator and precipitation responses in the tropics. The scatter in the simulated globally averaged responses is small in magnitude when compared against literature data from modern GCMs using model intrinsic aerosols but same aerosol emissions −(0.5–1.1) K and −(1.5–3.1) % for temperature and precipitation, respectively). The Pearson correlation of regional temperature (precipitation) response in these literature model experiments with intrinsic aerosols is 0.79 (0.34). The corresponding correlation coefficient for NorESM1 and ECHAM6 runs with identical aerosols is 0.78 (0.41). The lack of improvement in correlation coefficients between models with identical aerosols and models with intrinsic aerosols implies that the spatial distribution of regional climate responses is not improved via homogenizing the aerosol descriptions in the models. Rather, differences in the atmospheric dynamic and snow/sea ice cover responses dominate the differences in regional climate responses. Hence, even if we would have perfect aerosol descriptions inside the global climate models, uncertainty arising from the differences in circulation responses between the models would likely still result in a significant uncertainty in regional climate responses.

scholarly journals Comparison of Anthropogenic Aerosol Climate Effects among Three Climate Models with Reduced Complexity

Atmosphere ◽  
2019 ◽  
Vol 10 (8) ◽  
pp. 456 ◽  
Author(s):  
Xiangjun Shi ◽  
Wentao Zhang ◽  
Jiaojiao Liu
Keyword(s):  
Radiative Forcing ◽  
Climate Models ◽  
Reliable Estimate ◽  
Anthropogenic Aerosol ◽  
Climate Effects ◽  
Cloud Forcing ◽  
Radiative Forcings ◽  
Ensemble Simulations ◽  
Aerosol Forcing ◽  
The Difference

The same prescribed anthropogenic aerosol forcing was implemented into three climate models. The atmosphere components of these participating climate models were the GAMIL, ECHAM, and CAM models. Ensemble simulations were carried out to obtain a reliable estimate of anthropogenic aerosol effective radiative forcing (ERF). The ensemble mean ERFs from these three participating models with this aerosol forcing were −0.27, −0.63, and −0.54 W∙m−2. The model diversity in ERF is clearly reduced as compared with those based on the models’ own default approaches (−1.98, −0.21, and −2.22 W∙m−2). This is consistent with the design of this aerosol forcing. The modeled ERF can be decomposed into two basic components, i.e., the instantaneous radiative forcing (RF) from aerosol–radiation interactions (RFari) and the aerosol-induced changes in cloud forcing (△Fcloud*). For the three participating models, the model diversity in RFari (−0.21, −0.33, and −0.29 W∙m−2) could be constrained by reducing the differences in natural aerosol radiative forcings. However, it was difficult to figure out the reason for the model diversity in △Fcloud* (−0.05, −0.28, and −0.24 W∙m−2), which was the dominant source of the model diversity in ERF. The variability of modeled ERF was also studied. Ensemble simulations showed that the modeled RFs were very stable. The rapid adjustments (ERF − RF) had an important role to play in the quantification of the perturbation of ERF. Fortunately, the contribution from the rapid adjustments to the mean ERF was very small. This study also showed that we should pay attention to the difference between the aerosol climate effects we want and the aerosol climate effects we calculate.

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