CL3.1.2

The climatic implications of regional aerosol and precursor emissions reductions implemented to protect human health are poorly understood. However, quantitative estimates of climate responses to emission perturbations are needed by the climate assessment and impacts community. To address this need, we investigate the global and regional mean climate response to regional changes in aerosol emissions using three coupled chemistry-climate models: NOAA GFDL-CM3, NCAR-CESM1, and NASA GISS-E2. Our approach contrasts a long present-day control simulation from each model (up to 400 years with perpetual year 2000 or 2005 emissions) with fourteen individual aerosol emissions perturbation simulations (160-240 years each). We perturb emissions of sulfur dioxide (SO₂) and/or carbonaceous aerosol within six world regions and assess the statistical significance of temperature and precipitation responses relative to internal variability determined by the control simulation and across the models. Using the three models and their statistical significance as an indicator of robustness of climate responses to aerosols, we develop emulators of the climate response to changes in aerosol emissions. Emulators are defined as the change in a climatic variable (e.g. temperature) in a region i normalized by the change in emissions and/or radiative forcing for species S in region j, i.e. dT_i/dE_j,S, where T is temperature and E is emissions. In all models, the emulators for global mean surface temperature response (perturbation minus control) to aerosol is positive (warming). Results also indicate that the Arctic is the most sensitive region to nonlocal aerosol emissions or forcing, as the emulators are largest for the Arctic. Emulator calculations indicate a robust regional response to aerosol emissions or forcing within the northern hemisphere mid-latitudes, regardless of where the aerosol forcing is located longitudinally. We assess the utility of our emulators by applying them to an ensemble of historical and future CESM simulations in which anthropogenic aerosol emissions are removed to isolate the climate response to aerosols. We find good agreement between the ensemble mean temperature response to aerosols as simulated by CESM and the reconstructed temperature response from the emissions-based emulators and the emissions input to CESM.  This work is a first step towards providing statistical relationships between the changes in regional aerosol emissions and the statistically significant changes in climate that can be attributed to them. Such relationships would allow for the generation of regional climate change scenarios without having to simulate computationally demanding chemistry-climate models.

How to cite: Westervelt, D., Fiore, A., Shindell, D., and Lamarque, J.-F.: Developing emulators of regional climate responses to regional aerosol perturbations using three coupled chemistry-climate models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6885, https://doi.org/10.5194/egusphere-egu21-6885, 2021.

The global mean surface temperature went through a cooling period during the mid-20th century despite the continuous increase in greenhouse gas concentration. This generates a renewed interest to look at the multi-decadal climate variabilities across the 20th century, which are often believed to be related to the internal variabilities caused by ocean-atmosphere interaction.

At the same time, an obvious interhemispheric tropospheric temperature trend asymmetry is found in both reanalysis datasets and model simulations during this time. Considering the rapid increase of industrial activities in North America and Europe, it generates another possibility that anthropogenic emissions play a role during this period. And if anthropogenic emissions do have significant effects, then the relative contributions of anthropogenic emissions and internal variabilities to the mid-20th-century cooling is worth understanding because of the increasing importance of human activities to the natural environment.

To test this hypothesis, we did a detailed analysis on the global temperature trend and the interhemispheric temperature trend asymmetry from the surface to the mid-troposphere based on Coupled Model Intercomparison Project phase 5 (CMIP5) multi-model ensemble and multiple reanalysis datasets. Our results show that the anthropogenic aerosol emissions contribute to global cooling and particularly asymmetry during the mid-20th century, and the fingerprint of anthropogenic emissions is more obvious in the mid-troposphere compared with the surface.

By different attribution methods (such as multi-linear regression and pattern correlation), we quantified the relative contributions of Anthropogenic Emissions and Internal variabilities based on single forcing simulations of seven CMIP5 models. We conclude that a superposition of Internal Variabilities originating from the Atlantic Ocean and anthropogenic aerosol emissions overwhelms the warming influence of GHGs and lead to the mid-20th century cooling period.

How to cite: Diao, C. and Xu, Y.: Attribution of the relative contributions of anthropogenic aerosols and decadal variability to the mid-20th century global “cooling”: a focus on tropospheric temperature latitudinal gradient, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3314, https://doi.org/10.5194/egusphere-egu21-3314, 2021.

Modern climate models vary in their temperature responses to different climate forcers (such as CO2, methane, sulfate aerosols and black carbon). Here we study the reasons for model discrepancies  between different forcers by analyzing Precipitation Driver Response Model Intercomparison Project (PDRMIP) data. PDRMIP contains four different experiments in addition to the present-day base case: 1) fivefold sulfur concentrations, 2) tenfold black carbon concentrations, 3) twofold CO2 concentrations, and 4) threefold methane concentrations We use a set of modern climate models from PRDMIP dataset to decompose the temperature responses to various energy budget terms, the longwave and shortwave, cloudy and clear sky components, surface terms and horizontal energy transport. This study allows us to better understand the key processes responsible for climate model discrepancies in estimates of anthropogenic climate change impacts. Preliminary results show that magnitude of the temperature response of each forcer is similar, and mechanisms causing temperature changes are similar between different forcers. Somewhat surprisingly most of the model spread originates from changes in long wave radiations. Here we investigate global and regional responses and model spread for different climate forcers.

How to cite: Nordling, K., Merikanto, J., Räisänen, J., Samset, B., and Korhonen, H.: Regional and global temperature response, in PDRMIP data from a energy balance perspective, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10722, https://doi.org/10.5194/egusphere-egu21-10722, 2021.

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).  We run the models with the same anthropogenic aerosol representations via MACv2-SP (a simple plume implementation of the 2^nd 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.  

We find that the removal of South and East Asian anthropogenic aerosols leads to strong local warming  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±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 modulated by seasonal changes in oceanic heat exchange and clear-sky longwave radiation.

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’ worth of current day global warming during the next few decades.

How to cite: Merikanto, J., Nordling, K., Räisänen, P., Räisänen, J., O'Donnell, D., Partanen, A.-I., and Korhonen, H.: Decomposing local and remote surface temperature impacts of Asian aerosols, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13320, https://doi.org/10.5194/egusphere-egu21-13320, 2021.

How to cite: Kitabayashi, S. and Takahashi, H. G.: Climate response to anthropogenic aerosols and related SST variabilities including ENSO in the Asian monsoon region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14099, https://doi.org/10.5194/egusphere-egu21-14099, 2021.

The occurrence of severe haze events remains a serious problem in Beijing. Previous studies suggested that the frequency of weather patterns conducive to haze may increase with global warming. The new Shared Socioeconomic Pathways (SSPs) cover a wide range of uncertainties in aerosol and greenhouse gases emissions. Global and Chinese aerosol emissions are projected to decrease in most SSPs, while increases in greenhouse gases and global warming will continue for the rest of the century. The future, therefore, remains unclear.

We quantified the air pollution over Beijing and associated weather patterns using multiple indices calculated from the SSPs

We show that the occurrence of weather patterns conducive to the formation of haze significantly increases by the end of the century due to increases in greenhouse gases. Aerosol reductions also cause an increase in their occurrence, but reduce the severity of haze, and overall reducing aerosol emissions will be beneficial.

How to cite: Guo, L., Wilcox, L., Bollasina, M., Turnock, S., Lund, M., and Zhang, L.: Future changes in Beijing haze events under different shared socioeconomic pathways, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5787, https://doi.org/10.5194/egusphere-egu21-5787, 2021.

    Future projection of diffusion conditions associated with extreme haze events over eastern China is of great importance to government emission regulations and public human health. Here, the diffusion conditions and their changes under future warming scenarios are examined. The relative strength of haze events in the Northern China Plain region increase from 150% during 2006–15 to 190% during 2090–99 under RCP8.5 scenarios, induced by a stronger and longer-lasting anticyclone anomaly in eastern China. The strengthened anticyclone anomaly is mainly induced by increased northern wave train convergence emanating from the Barents–Kara Sea, and the longer duration of the anticyclone anomaly is mainly induced by stronger local feedback that can extract more energy from the basic state to maintain the anticyclone anomaly in eastern China. Aerosol reduction is found to play a dominant role in strengthening the upstream wave train near the Barents–Kara Sea and the downstream anticyclone in eastern China, while the effects from increased greenhouse gases are small. The results of this study indicate that future aerosol emissions reduction can induce deteriorating diffusion conditions, suggesting more stringent regulations on aerosol emissions in China are needed to meet air quality standards.

How to cite: Feng, W., Wang, M., Zhang, Y., Dai, X., Liu, X., and Xu, Y.: Intraseasonal variation and future projection of atmospheric diffusion conditions conducive to extreme haze formation over eastern China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-450, https://doi.org/10.5194/egusphere-egu21-450, 2021.

Understanding the tropical Pacific response to global warming remains a challenging problem due to discrepancies between models and observations, as well as a large intermodel spread in future projections. Here, we assess the recent and future evolution of the equatorial Pacific east-west temperature gradient, and the Walker circulation within the CMIP6 dataset. Using 40 models, we compare simulated tropical climate change across a wide range of experiments with varying CO₂ and aerosol forcing. In abrupt CO₂-increase scenarios, many models generate an initial strengthening of the east-west gradient resembling an ocean thermostat (OT), characterized by lack of warming in the central Pacific, followed by a small weakening; other models generate an immediate weakening that becomes progressively larger establishing a pronounced eastern equatorial Pacific (EP) warming pattern. The initial response in these CO₂-only experiments is a very good predictor for the future EP pattern simulated in future warming scenarios, but not in historical simulations showing no multi-model trend. The likely explanation is that recent CO₂-driven changes in the tropical Pacific, which are relatively small compared to future projections, are masked by aerosol effects. In future warming scenarios, however, the EP warming pattern emerges within 20-40 years as greenhouse gases overcome aerosol forcing. These findings highlight the need to understand the largely overlooked, but possibly significant role of aerosols in delaying sea surface warming in the tropical Pacific, and the implications for predicting future climate change across the tropics.

How to cite: Heede, U. K. and Fedorov, A. V.: Eastern equatorial Pacific warming delayed by aerosols and thermostat response to CO2, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13946, https://doi.org/10.5194/egusphere-egu21-13946, 2021.

Sahel rainfall experienced significant multidecadal variability over the twentieth century. Previous work have proposed several drivers to explain the severe drought and the subsequent recovery of Sahel rainfall in the past century, including anthropogenic aerosols, GHGs, and internal variabilities. However, the attribution remained ambiguous. Sahel summertime monsoon has close teleconnections with North Atlantic sea surface temperature (NASST) variability, which has been proven to be affected by aerosols. Therefore, changes in regional aerosols emission can potentially drive multidecadal Sahel rainfall variability.

Here we use ensembles of state-of-the-art global climate models (the CESM-large ensemble and CMIP6 models) and observational datasets to demonstrate that anthropogenic aerosols have significant impacts on twentieth-century Sahel rainfall multidecadal variability through modifying NASST. Aerosol-induced multidecadal variations of downward solar fluxes over the North Atlantic Ocean cause NASST variability during the 20^th century, altering the strength of the Hadley cell and the ITCZ position, therefore, dynamically linking aerosol effects to Sahel rainfall variability. While the observed linear trend of NASST might still be affected by a mix of various external and internal drivers, our results suggest that NASST variability is most likely caused by aerosol-induced changes in radiative fluxes rather than changes in ocean circulations, and that anthropogenic aerosols can explain most of the detrended Sahel rainfall variability. CMIP6 models further suggest that aerosol-cloud interactions contributed more to the variability than aerosol-radiation interactions. These findings highlight the importance of accurate representation of regional aerosol radiative effects for the simulation of Sahel rainfall variability.

How to cite: Zhang, S., Stier, P., Dagan, G., and Wang, M.: Anthropogenic aerosols modulated twentieth-century Sahel rainfall variability via impacts on North Atlantic sea surface temperature, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8164, https://doi.org/10.5194/egusphere-egu21-8164, 2021.

The development of robust emission metrics to guide climate policy is more complicated for short-lived climate forcers like black carbon (BC) than for long-lived greenhouse gases like CO₂. The challenge is that for short-lived climate forcers, the atmospheric concentrations, the radiative forcing (RF), and ultimately, effects on climate, depend on the location and timing of the emissions.  In the present work, the impact of emission location and season on the RF resulting from emissions of BC is studied using the NorESM1 climate model. NorESM1 is run in a configuration in which the distribution of aerosols is simulated using a state-of-the-art aerosol scheme, but the interactive aerosols are not allowed to influence the simulated meteorological conditions. Consequently, the patterns of weather are repeated identically irrespective of the assumed aerosol emissions. This allows for an essentially noise-free evaluation of the radiative forcing associated with changes in aerosol emissions, irrespective of the magnitude and spatiotemporal extent of the emission changes.

We employ the model to systematically evaluate the radiative forcing efficiency (i.e., global-mean RF divided by the emissions) of BC emissions, for various assumptions about the latitude, longitude and season of the emissions. The BC direct effect and the effect of BC on snow albedo are considered. Preliminary results from tests focusing on BC emissions in the subarctic region (60-70°N) indicate the RF efficiency depends strongly both on the timing and longitude of the emissions. The RF efficiency of emissions in spring and summer is much larger than that of emissions in fall and winter, mainly due to the stronger insolation. Furthermore, emissions in the Siberian and North American sectors have higher RF efficiency than emissions in the Atlantic and European sectors. This is largely because emissions from subarctic Siberia and North America preferentially increase the atmospheric BC burden and BC deposition in regions with seasonal snow cover persisting into late spring / early summer. This acts to increase both the BC direct RF and the RF due to BC in snow. Furthermore, long atmospheric residence times act to increase the direct RF associated with Siberian BC emissions in summer.

An implication is that the use of large-scale mean (e.g., subarctic average) emission metrics may mispresent the role of BC emissions from smaller regions like individual countries.

How to cite: Räisänen, P., Partanen, A.-I., Makkonen, R., Merikanto, J., Savolahti, M., Kirkevåg, A., Sand, M., and Seland, Ø.: In terms of radiative forcing, not all BC emissions are equal, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12804, https://doi.org/10.5194/egusphere-egu21-12804, 2021.

Many nations responded to the COVID-19 pandemic by restricting travel and other activities during 2020, resulting in temporarily reduced emissions of CO2, other greenhouse gases and ozone and aerosol precursors. We perform a coordinated Intercomparison, CovidMIP, of Earth System model simulations to assess the impact on climate of these emissions reductions. Eleven models performed multiple initial-condition ensembles to produce over 280 simulations spanning both initial condition and model structural uncertainty. We find model consensus on reduced aerosol amounts (particularly over East Asia) and associated increases in surface shortwave radiation levels. However, any impact on near-surface temperature or rainfall during 2020-2024 is extremely small and is not detectable in this initial analysis. Regional analyses on a finer scale, and closer attention to extremes (especially linked to changes in atmospheric composition and air quality) are required to test the impact of COVID-19-related emission reductions on near-term climate.

This first-look at results has focussed on surface climate, but future analysis will include attribution of drivers of climate signals; longer term implications of emissions reductions and options for economic recovery; quantifying changes in extremes; influence on atmospheric circulation and the carbon cycle.

How to cite: Jones, C. and the Covid-MIP analysis: The Climate Response to Emissions Reductions due to COVID-19, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1266, https://doi.org/10.5194/egusphere-egu21-1266, 2021.