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.

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.