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Abstract It has been suggested that the Atlantic meridional overturning circulation (AMOC) in many CMIP6 models is overly sensitive to anthropogenic aerosol forcing, and it has been proposed that this is due to the inclusion of aerosol indirect effects for the first time in many CMIP6 models. We analyze the AMOC response in a newly released ensemble of simulations performed with CESM2 forced by the CMIP5 input data sets (CESM2‐CMIP5). This AMOC response is then compared to the CMIP5‐generation CESM1 large ensemble (CESM1‐LE) and the CMIP6‐generation CESM2 large ensemble (CESM2‐LE). A key conclusion, only made possible by this experimental setup, is that changes in aerosol‐indirect effects cannot explain differences in AMOC response between CESM1‐LE and CESM2‐LE. Instead, we hypothesize that the difference is due to increased interannual variability of anthropogenic emissions. This forcing variability may act through a nonlinear relationship between the surface heat budget of the North Atlantic and the AMOC. 

Abstract In the tropics, the absorbed solar radiation is larger than the outgoing longwave radiation, while the opposite is true at high latitudes. This basic fact implies a poleward energy transport (PET) in both hemispheres, which is accomplished by the atmosphere and oceans. The magnitude of PET is determined by the top of atmosphere gradient in the net radiation flux, and small changes to this quantity must change the total PET in the absence of changes in heat uptake. We analyze a large ensemble of 50 historic climate simulations from the CESM LENS2 project and find a significant PET anomaly in the latter half of the twentieth century. The temporal evolution of this anomaly—with a rapid increase after 1950, a peak near 1975, and a rapid decrease in the 1990s—mirrors the historic trend of sulfur dioxide (SO₂, a significant aerosol predecessor) emissions from Europe and North America. This anomaly also appears in an analysis of the PET calculated from ERA5 reanalyses and from the CESM2 Single Forcing Large Ensemble. Consistent with previous studies, we find that historic SO₂emissions from Europe and North America brightened clouds, which reflected additional solar radiation back to space in the midlatitudes: this shortwave anomaly increased the meridional gradient in the net TOA radiation flux and induced an anomalous northward energy transport. Finally, our results suggest that cryosphere processes become an additional important factor in setting the PET anomaly during the first years of the twenty-first century by contributing to the difference in absorbed solar radiation between hemispheres alongside cloud radiative effects. significance statementIn this study, we analyze a large group of climate model simulations from 1850 to 2014 and find that this historical pollution changed the way that heat was transported from the tropics to Earth’s poles. We also find this change in heat transport when we analyzed an atmospheric reanalysis, which is a historical dataset that combines many meteorological observations into a best estimate of the past climate state. This extra reflection of sunlight from polluted clouds cooled the Northern Hemisphere, and we hypothesize that this cooling caused more heat transport out of the tropics. Last, we find that similar pollution emitted from China and India in more recent decades has not led to a change in Earth’s heat transport because of counteracting changes in snow and ice in the Northern Hemisphere.