<p>We investigate how a regionally confined radiative forcing of South and East Asian aerosols translate into local and remote surface temperature responses across the globe. To do so, we carry out equilibrium climate simulations with and without modern day South and East Asian anthropogenic aerosols in two climate models with independent development histories (ECHAM6.1 and NorESM1). &#160;We run the models with the same anthropogenic aerosol representations via MACv2-SP (a simple plume implementation of the 2<sup>nd</sup> version of the Max Planck Institute Aerosol Climatology). This leads to a near identical change in instantaneous direct and indirect aerosol forcing due to removal of Asian aerosols in the two models. We then robustly decompose and compare the energetic pathways that give rise to the global and regional surface temperature effects in the models by a novel temperature response decomposition method, which translated the changes in atmospheric and surface energy fluxes into surface temperature responses by using a concept of planetary emissivity. &#160;</p><p>We find that the removal of South and East Asian anthropogenic aerosols leads to strong local warming &#160;response from increased clear-sky shortwave radiation over the region, combined with opposing warming and cooling responses due to changes in cloud longwave and shortwave radiation. However, the local warming response is strongly modulated by the changes in horizontal atmospheric energy transport. Atmospheric energy transport and changes in clear-sky longwave radiation redistribute the surface temperature responses efficiently across the Northern hemisphere, and to a lesser extent also over the Southern hemisphere. The model-mean global surface temperature response to Asian anthropogenic aerosol removal is 0.26&#177;0.04 &#176;C (0.22&#177;0.03 for ECHAM6.1 and 0.30&#177;0.03 &#176;C for NorESM1) of warming. Model-to-model differences in global surface temperature response mainly arise from differences in longwave cloud (0.01&#177;0.01 for ECHAM6.1 and 0.05&#177;0.01 &#176;C for NorESM1) and shortwave cloud (0.03&#177;0.03 for ECHAM6.1 and 0.07&#177;0.02 &#176;C for NorESM1) responses. The differences in cloud responses between the models also dominate the differences in regional temperature responses. In both models, the Northern hemispheric surface warming amplifies towards the Arctic, where the total temperature response is highly seasonal and modulated by seasonal changes in oceanic heat exchange and clear-sky longwave radiation.</p><p>We estimate that under a strong Asian aerosol mitigation policy tied with strong greenhouse gas mitigation (Shared Socioeconomic Pathway 1-1.9) the Asian aerosol reductions can add around 8 years&#8217; worth of current day global warming during the next few decades.</p>
<div> <div> <div> <p>There is large uncertainty in future aerosol emissions scenarios explored in the Shared Socioeconomic Pathways (SSPs), with plausible pathways spanning a range of possibilities from large global reductions in emissions to 2050 to moderate global increases over the same period. Diversity in emissions across the pathways is particularly large over Asia. CMIP6 models indicate that rapid anthropogenic aerosol and precursor emission reductions between the present day and the 2050s lead to enhanced increases in global and Asian summer monsoon precipitation relative to scenarios with weak air quality policies. However, the effects of aerosol reductions don&#8217;t persist in precipitation to the end of the 21st century, when response to greenhouse gases dominates differences across the SSPs. The relative magnitude and spatial distribution of aerosol changes is particularly important for South Asian summer monsoon precipitation changes. Precipitation increases here are initially suppressed in SSPs 2-4.5 and 5-8.5 relative to SSP 1-1.9 and 3-7.0 when the impact of East Asian emission decreases is counteracted by that due to continued increases in South Asian emissions.</p> </div> </div> </div>
Abstract. South and East Asian anthropogenic aerosols mostly reside in an air mass extending from the Indian Ocean to the North Pacific. Yet the surface temperature effects of Asian aerosols spread across the whole globe. Here, we remove Asian anthropogenic aerosols from two independent climate models (ECHAM6.1 and NorESM1) using the same representation of aerosols via MACv2-SP (a simple plume implementation of the second version of the Max Planck Institute Aerosol Climatology). We then robustly decompose the global distribution of surface temperature responses into contributions from atmospheric energy flux changes. We find that the horizontal atmospheric energy transport strongly moderates the surface temperature response over the regions where Asian aerosols reside. Atmospheric energy transport and changes in clear-sky longwave radiation redistribute the temperature effects efficiently across the Northern Hemisphere and to a lesser extent also over the Southern Hemisphere. The model-mean global surface temperature response to Asian anthropogenic aerosol removal is 0.26±0.04 ∘C (0.22±0.03 for ECHAM6.1 and 0.30±0.03 ∘C for NorESM1) of warming. Model-to-model differences in global surface temperature response mainly arise from differences in longwave cloud (0.01±0.01 for ECHAM6.1 and 0.05±0.01 ∘C for NorESM1) and shortwave cloud (0.03±0.03 for ECHAM6.1 and 0.07±0.02 ∘C for NorESM1) responses. The differences in cloud responses between the models also dominate the differences in regional temperature responses. In both models, the northern-hemispheric surface warming amplifies towards the Arctic, where the total temperature response is highly seasonal and weakest during the Arctic summer. We estimate that under a strong Asian aerosol mitigation policy tied with strong climate mitigation (Shared Socioeconomic Pathway 1-1.9) the Asian aerosol reductions can add around 8 years' worth of current-day global warming during the next few decades.
Anthropogenic aerosols alter the climate by scattering and absorbing the incoming solar radiation and by modifying clouds’ optical properties, causing a global cooling or warming effect. Anthropogenic aerosols are partly co-emitted with anthropogenic greenhouse gases, and future climate mitigation actions lead to the decline of anthropogenic aerosols’ cooling effect. However, the exact cooling effect is still uncertain. Part of this uncertainty is related to the structural differences of current climate models. This work evaluates the present-day anthropogenic aerosol temperature and precipitation effect and factors affecting the model difference. The key objectives of this thesis were: 1) What are the climate effects of present-day anthropogenic aerosols?, 2) What mechanisms drive the model-to-model differences?, and 3) How do future reductions affect local and global climates? The global models ECHAM6 and NorESM1 were used to evaluate the present-day climate effects with theidentical anthropogenic aerosol scheme MACv2-SP. Results reveal that an identical anthropogenic aerosol description does not reduce the uncertainty related to anthropogenic aerosol climate effects, and the difference in the estimated difference is due to model dynamics and oceans. The key mechanism driving the difference in the models was evaluated using data from the Precipitation Driven Model Intercomparison Project (PRMIP). Similar mechanisms drive the model-to-model difference for greenhouse gases and aerosols, where the key drivers are the differences in water vapor, the vertical temperature structure of the atmosphere, and sea ice and snow cover changes. However, on a regional scale, the key drivers differ. Future anthropogenic aerosol effects were evaluated using new CMIP6 data. This work shows the importance of anthropogenic aerosols for current and future climate change. For amore accurate assessment of climate impacts of anthropogenic aerosols, one needs to also consider remote effects of the local aerosols. The Arctic regions are particularly sensitive to midlatitude aerosols, such as Asian aerosols, which are expected to decline in the next decades. To gain a more accurate estimation of anthropogenic aerosols, it is not sufficient to only focus on composition and geographical distribution of aerosols, as the dynamic response of climate is also important. On global temperature results did not indicate clear aerosols signal, however future temperature development over the Asian regions is modulated by the future Asian aerosol emissions.
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.
Abstract. There is large uncertainty in future aerosol emissions scenarios explored in the Shared Socioeconomic Pathways (SSPs), with plausible pathways spanning a range of possibilities from large global reductions in emissions to 2050 to moderate global increases over the same period. Diversity in emissions across the pathways is particularly large over Asia. Rapid anthropogenic aerosol and precursor emission reductions between the present day and the 2050s lead to enhanced increases in global and Asian summer monsoon precipitation relative to scenarios with weak air quality policies. However, the effects of aerosol reductions don't persist in precipitation to the end of the 21st century, when response to greenhouse gases dominates differences across the SSPs. The relative magnitude and spatial distribution of aerosol changes is particularly important for South Asian summer monsoon precipitation changes. Precipitation increases here are initially suppressed in SSPs 2–4.5 and 5–8.5 relative to SSP 1–1.9 and 3–7.0 when the impact of East Asian emission decreases is counteracted by that due to continued increases in South Asian emissions.
Abstract. As global warming progresses, weather conditions like daily temperature and precipitation are changing due to changes in their means and distributions of day-to-day variability. In this study, we show that changes in variability have a stronger influence on the number of extreme precipitation days than the change in the mean state in many locations. We analyze daily precipitation and maximum temperatures at four levels of global warming and under different emission scenarios for the Northern Hemisphere (NH) summer (June – August). Our analysis is based on initial condition large ensemble simulations from three fully coupled Earth System Models (MPI-ESM1-2-LR, CanESM5, and ACCESS-ESM1-5) contributing to the Climate Model Inter-comparison Project phase 6 (CMIP6). We also use information from the Precipitation Driver Response Model Intercomparison Project (PDRMIP) to discern the influence of different climate drivers (notably aerosols and greenhouse gases). We decompose the total changes in daily NH summer precipitation and daily maximum temperature into mean and variability components (standard deviation and skewness). Our results show that in many locations, variability exerts a stronger influence than mean changes on daily precipitation. Changes in the widths and shapes of precipitation distributions are especially dominating over mean changes in Asia, the Arctic and Sub-Saharan Africa. In contrast, temperature changes are primarily driven by changes in the mean state. For the near future (2020–2040), we find that reductions in aerosol emissions would increase the likelihood of extreme summertime precipitation only over Asia. This study emphasizes the importance of incorporating daily variability changes into climate change impact assessments and advocates that future emulator and impact model development should focus on improving the representation of daily variability.
Abstract. Here we present for the first time a proof of concept for an emulation-based method that uses a large-eddy simulations (LES) to present sub-grid cloud processes in a general circulation model (GCM). We focus on two key variables affecting the properties of shallow marine clouds: updraft velocity and precipitation formation. The LES is able to describe these processes with high resolution accounting for the realistic variability in cloud properties. We show that the selected emulation method is 5 able to represent the LES outcome with relatively good accuracy and that the updraft velocity and precipitation emulators can be coupled with the GCM practically without increasing the computational costs. We also show that the emulators influence the climate simulated by the GCM, but do not consistently improve or worsen the agreement with observations on cloud related properties. Although especially the updraft velocity at cloud base is better captured. A more quantitative evaluation of the emulator impacts against observations would, however, have required model re-tuning, which is a significant task and thus could 10 not be included in this proof-of-concept study. All in all, the approach introduced here is a promising candidate for representing detailed cloud and aerosol related sub-grid processes in GCMs. Further development work together with increasing computing capacity can be expected to improve the accuracy and the applicability of the approach in climate simulations.
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