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.

Australian wildfires burning from December 2019 to January 2020 injected approximately 0.9 Tg of smoke into the stratosphere; this is the largest amount observed in the satellite era. A comparison of numerical simulations to satellite observations of the plume rise suggests that the smoke mass contained 2.5% black carbon. Model calculations project a 1 K warming in the stratosphere of the Southern Hemisphere midlatitudes for more than 6 months following the injection of black-carbon containing smoke. The 2020 average global mean clear sky effective radiative forcing at top of atmosphere is estimated to be -0.03 W m(-2) with a surface value of -0.32 W m(-2). Assuming that smoke particles coat with sulfuric acid in the stratosphere and have similar heterogeneous reaction rates as sulfate aerosol, we estimate a smoke-induced chemical decrease in total column ozone of 10-20 Dobson units from August to December in mid-high southern latitudes.

The VAMPERS (Vrije Universiteit Amsterdam Permafrost Snow Model) has been coupled within iLOVECLIM, an earth system model. This advancement allows the thermal coupling between permafrost and climate to be examined from a millennial timescale using equilibrium experiments during the Last Glacial Maximum (21 ka) and transient experiments for the subsequent deglaciation period (21-11 ka). It appears that the role of permafrost during both stable and transitional (glacial-interglacial) climate periods is seasonal, resulting in cooler summers and warmer winters by approximately +/- 2 degrees C maximum. This conclusion reinforces the importance of including the active layer within climate models. In addition, the coupling of VAMPERS also yields a simulation of transient permafrost conditions, not only for estimating areal changes in extent but also total permafrost gain/loss.

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.

Heterogeneous Holocene climate evolutions in the Northern Hemisphere high latitudes are primarily determined by orbital-scale insolation variations and melting ice sheets. Previous inter-model comparisons have revealed that multi-simulation consistencies vary spatially. We, therefore, compared multiple model results with proxy-based reconstructions in Fennoscandia, Greenland, north Canada, Alaska and Siberia. Our model-data comparisons reveal that data and models generally agree in Fennoscandia, Greenland and Canada, with the early-Holocene warming and subsequent gradual decrease to 0 ka BP (hereinafter referred as ka). In Fennoscandia, simulations and pollen data suggest a 2 degrees C warming by 8 ka, but this is less expressed in chironomid data. In Canada, a strong early-Holocene warming is suggested by both the simulations and pollen results. In Greenland, the magnitude of early-Holocene warming ranges from 6 degrees C in simulations to 8 degrees C in delta O-18-based temperatures. Simulated and reconstructed temperatures are mismatched in Alaska. Pollen data suggest strong early Holocene warming, while the simulations indicate constant Holocene cooling, and chironomid data show a stable trend. Meanwhile, a high frequency of Alaskan peatland initiation before 9 ka can reflect a either high temperature, high soil moisture or large seasonality. In high-latitude Siberia, although simulations and proxy data depict high Holocene temperatures, these signals are noisy owing to a large spread in the simulations and between pollen and chironomid results. On the whole, the Holocene climate evolutions in most regions (Fennoscandia, Greenland and Canada) are well established and understood, but important questions regarding the Holocene temperature trend and mechanisms remain for Alaska and Siberia. (C) 2017 Elsevier Ltd. All rights reserved.