This study examines the Arctic surface air temperature response to regional aerosol emissions reductions using three fully coupled chemistry-climate models: National Center for Atmospheric Research-Community Earth System Model version 1, Geophysical Fluid Dynamics Laboratory-Coupled Climate Model version 3 (GFDL-CM3) and Goddard Institute for Space Studies-ModelE version 2. Each of these models was used to perform a series of aerosol perturbation experiments, in which emissions of different aerosol types (sulfate, black carbon (BC), and organic carbon) in different northern mid-latitude source regions, and of biomass burning aerosol over South America and Africa, were substantially reduced or eliminated. We find that the Arctic warms in nearly every experiment, the only exceptions being the U.S. and Europe BC experiments in GFDL-CM3 in which there is a weak and insignificant cooling. The Arctic warming is generally larger than the global mean warming (i.e. Arctic amplification occurs), particularly during non-summer months. The models agree that changes in the poleward atmospheric moisture transport are the most important factor explaining the spread in Arctic warming across experiments: the largest warming tends to coincide with the largest increases in moisture transport into the Arctic. In contrast, there is an inconsistent relationship (correlation) across experiments between the local radiative forcing over the Arctic and the simulated Arctic warming, with this relationship being positive in one model (GFDL-CM3) and negative in the other two. Our results thus highlight the prominent role of poleward energy transport in driving Arctic warming and amplification, and suggest that the relative importance of poleward energy transport and local forcing/feedbacks is likely to be model dependent.

We analyse an ensemble of statistically downscaled Global Climate Models (GCMs) to investigate future water availability in the Upper Indus Basin (UIB) of Pakistan for the time horizons when the global and/or regional warming levels cross Paris Agreement (PA) targets. The GCMs data is obtained from the 5th Phase of Coupled Model Inter-Comparison Project under two Representative Concentration Pathways (RCP4.5 and RCP8.5). Based on the five best performing GCMs, we note that global 1.5 degrees C and 2.0 degrees C warming thresholds are projected in 2026 and 2047 under RCP4.5 and 2022 and 3036 under RCP8.5 respectively while these thresholds are reached much earlier over Pakistan i.e. 2016 and 2030 under RCP4.5 and 2012 and 2025 under RCP8.5 respectively. Interestingly, the GCMs with the earliest emergence at the global scale are not necessarily the ones with the earliest emergence over Pakistan, highlighting spatial non-linearity in GCMs response. The emergence of 2.0 degrees C warming at global scale across 5 GCMs ranges from 2031 (CCSM4) to 2049 (NorESM) under RCP8.5. Precipitation generally exhibits a progressive increasing trend with stronger changes at higher warming or radiative forcing levels. Hydrological simulations representing the historical, 1.5 degrees C and 2.0 degrees C global and region warming time horizons indicate a robust but seasonally varying increase in the inflows. The highest inflows in the baseline and future are witnessed in July. However, the highest future increase in inflows is projected in October under RCP4.5 (37.99% and 65.11% at 1.5 degrees C and 2.0 degrees C) and in April under RCP8.5 (37% and 62.05% at 1.5 degrees C and 2.0 degrees C). These hydrological changes are driven by increases in the snow and glacial melt contribution, which are more pronounced at 2.0 degrees C warming level. These findings should help for effective water management in Pakistan over the coming decades. (c) 2021 Elsevier B.V. All rights reserved.

There has been growing interest in the potential of short-lived climate forcer (SLCF) mitigation to reduce near-term global warming. Black carbon (BC), organic carbon (OC), and sulfur dioxide (SO2) are SLCFs which change the Earth's radiative balance directly by affecting radiation, and indirectly by altering cloud properties. We used the ECHAM-HAMMOZ aerosol-climate model to study the radiative forcings due to mitigating the anthropogenic emissions of BC, OC, and SO2 from Chile and Mexico. Limiting our analysis to areas where these emissions had notable effects on both aerosol and clouds, we found that the total radiative forcings of anthropogenic aerosol emissions are different for Chile and Mexico. This was explained by differences in aerosol emissions, orography, and meteorology in these two countries. Especially the radiative forcing for Chilean emissions was influenced by the persistent stratocumulus cloud deck west of Chile. To reduce the uncertainty of our radiative forcing calculations, we nudged the wind and surface pressure toward pre-generated fields. As nudging affects the calculated effective radiative forcing (ERF), we here used the identifier ERFNDG. Our results indicate that the removal of OC and SO2 emissions caused a positive ERFNDG while the removal of BC emissions caused a positive ERFNDG for Chile, but a negative ERFNDG for Mexico. When accounting for co-emission of other aerosol compounds, reducing BC emissions led to positive ERFNDG in both countries. Compared to China, the removal of anthropogenic SO2 emissions in Chile and Mexico caused a much larger global average ERFNDG per emitted unit mass of SO2.

To date, the treatment of permafrost in global climate models has been simplified due to the prevailing uncertainties in the processes involving frozen ground. In this study, we improved the modeling of permafrost processes in a state-of-the-art climate model by taking into account some of the relevant physical properties of soil such as changes in the thermophysical properties due to soil freezing. As a result, the improved version of the global land surface model was able to reproduce a more realistic permafrost distribution at the southern limit of the permafrost area by increasing the freezing of soil moisture in winter. The improved modeling of permafrost processes also had a significant effect on future projections. Using the conventional formulation, the predicted cumulative reduction of the permafrost area by year 2100 was approximately 60% (40-80% range of uncertainty from a multi-model ensemble) in the RCP8.5 scenario, while with the improved formulation, the reduction was approximately 35% (20-50%). Our results indicate that the improved treatment of permafrost processes in global climate models is important to ensuring more reliable future projections.

Impacts of absorbing and scattering aerosols on global energy balance are investigated with a global climate model. A series of sensitivity experiments perturbing emissions of black carbon and sulfate aerosols individually is conducted with the model to explore how components of global energy budget change in response to the instantaneous radiative forcing due to the two types of aerosols. It is demonstrated how differing vertical structures of the instantaneous radiative forcing between the two aerosols induce distinctively different proportions of fast and slow climate responses through different energy redistribution into atmosphere and surface. These characteristics are quantified in the form of the whole picture of global energy budget perturbations normalized by the top-of-atmosphere instantaneous radiative forcing. The energy budget perturbation per unit instantaneous forcing thus quantified reveals relative magnitudes of changes to different component fluxes in restoring atmospheric and surface energy balances through fast and slow responses. The normalized picture then directly links the initial forcing to the eventual climate responses, thereby explaining how starkly different responses of the global-mean temperature and precipitation are induced by the two types of aerosols. The study underscores a critical need for better quantifications of the forcings' vertical structure and atmospheric rapid adjustment for reliable estimates of climatic impact of absorbing and scattering aerosols. In particular, cloud responses through the indirect and semidirect effects and the sensible heat decrease in response to stabilized atmosphere due to the black carbon heating are identified as key uncertain components in the global energy budget perturbation. Plain Language Summary The minute particles suspended in the atmosphere, called aerosols, have warming or cooling impacts on climate depending on their color that determines their ability to scatter or absorb the sunlight. The black aerosols, like black carbon, enhance the heating on atmosphere and reduce the sunlight reaching the surface through absorbing the sunlight, while the white aerosols, like sulfate, directly cool the surface with little influence on atmosphere through scattering the sunlight. This study analyzes simulations with a global climate model to quantify how the two types of aerosols with such different characteristics modulate the Earth's energy budget differently to induce distinctively different responses of the global-mean temperature and precipitation. The results explain why the global temperature response to perturbations of black carbon tends to be muted in contrast to the pronounced response to perturbations of sulfate. The energy budget picture also illustrates how increased black carbon can increase and decrease the global precipitation through two competing pathways to result a net decrease while increased sulfate monotonically decreases the global precipitation. The findings of this study provide a theoretical basis for better quantifying the climate change driven by future emission changes of different types of aerosols.

A series of 60-year numerical experiments starting from 1851 was conducted using a global climate model coupled with an aerosol-cloud-radiation model to investigate the response of the Asian summer monsoon to variations in the secondary organic aerosol (SOA) flux induced by two different estimations of biogenic volatile organic compound (BVOC) emissions. One estimation was obtained from a pre-existing archive and the other was generated by a next-generation model (the Model of Emissions of Gases and Aerosols from Nature, MEGAN). The use of MEGAN resulted in an overall increase of the SOA production through a higher rate of gasto-particle conversion of BVOCs. Consequently, the atmospheric loading of organic carbon (OC) increased due to the contribution of SOA to OC aerosol. The increase of atmospheric OC aerosols was prominent in particular in the Indian subcontinent and Indochina Peninsula (IP) during the pre- and early-monsoon periods because the terrestrial biosphere is the major source of BVOC emissions and the atmospheric aerosol concentration diminishes rapidly with the arrival of monsoon rainfall. As the number of atmospheric OC particles increased, the number concentrations of cloud droplets increased, but their size decreased. These changes represent a combination of aerosol-cloud interactions that were favorable to rainfall suppression. However, the modeled precipitation was slightly enhanced in May over the oceans that surround the Indian subcontinent and IP. Further analysis revealed that a compensating updraft in the surrounding oceans was induced by the thermally-driven downdraft in the IP, which was a result of surface cooling associated with direct OC aerosol radiative forcing, and was able to surpass the aerosolcloud interactions. The co-existence of oceanic ascending motion with the maximum convective available potential energy was also found to be crucial for rainfall formation. Although the model produced statistically significant rainfall changes with locally organized patterns, the suggested pathways should be considered guardedly because in the simulation results, 1) the BVOC-induced aerosol direct effect was marginal; 2) cloud-aerosol interactions were modeldependent; and 3) Asian summer monsoons were biased to a nonnegligible extent.

Black carbon (BC) in the atmosphere contributes to the human health effects of particulate matter and contributes to radiative forcing of climate. The lifetime of BC, particularly the smaller particle sizes (PM2.5) which can be transported over long distances, is therefore an important factor in determining the range of such effects, and the spatial footprint of emission controls. Theory and models suggest that the typical lifetime of BC is around one week. The frequency distributions of measurements of a range of hydrocarbons at a remote rural site in southern Scotland (Auchencorth Moss) between 2007 and 2010 have been used to quantify the relationship between atmospheric lifetime and the geometric standard deviation of observed concentration. The analysis relies on an assumed common major emission source for hydrocarbons and BC, namely diesel-engined vehicles. The logarithm of the standard deviation of the log-transformed concentration data is linearly related to hydrocarbon lifetime, and the same statistic for BC can be used to assess the lifetime of BC relative to the hydrocarbons. Annual average data show BC lifetimes in the range 4-12 days, for an assumed OH concentration of 7 x 10(5) cm(-3). At this site there is little difference in BC lifetime between winter and summer, despite a 3-fold difference in relative hydrocarbon lifetimes. This observation confirms the role of wet deposition as an important removal process for BC, as there is no difference in precipitation between winter and summer at this site. BC lifetime was significantly greater in 2010, which had 23% less rainfall than the preceding 3 years. (C) 2012 Elsevier Ltd. All rights reserved.