Unmasking the Effects of Aerosols on Greenhouse Warming Over Europe

Abstract

Unmasking the Effects of Aerosols on Greenhouse Warming Over EuropeP. Glantz1, O. G. Fawole2, J. Ström1, M. Wild3 and K. J. Noone11Department of Environmental Science, Stockholm University, Stockholm, Sweden2Dept. of Physics and Engineering Physics, Obafemi Awolowo University, Ile-Ife, Nigeria3Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland This work with corresponding publication in Journal of Geophysical Research: Atmospheres, 127, e2021JD035889. https://doi.org/10.1029/2021JD035889 is supported by FORMAS, grant 2018-01291. Aerosol optical thickness (AOT) has decreased substantially in Europe in the summer half year (April–September) since 1980, with almost a 50% reduction in Central and Eastern Europe, according to Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) reanalysis. At the same time, strong positive trends in ERA5 reanalysis surface solar radiation downward for all-sky and clear-sky conditions (SSRD and SSRDc, respectively) and temperature at 2 m are found for Europe in summer during the period 1979–2020. The Global Energy Balance Archive (GEBA) observations show as well strong increases in SSRD during the latest four decades. Estimations of changes in SSRDc, using the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model, show similarly strong increases when fed by MERRA-2 AOT. The estimates of warming in this study, caused by increases in SSRD and SSRDc, are based on energy budget approximations and the Stefan Boltzmann law. The increases in near surface temperature, estimated both for clear-sky and all-sky conditions, are up to about 1°C for Central and Eastern Europe. The total warming over large parts of this region for clear-sky conditions is however nearly double the global mean temperature increase of 1.1°C, while somewhat less for all-sky conditions. Although the largest effects from aerosols on the radiation balance occurred in the 1980s and 1990s, the total warming has continued to increase more or less at the same rates during the latest four decades over large parts of Europe, considering both all-sky and clear-sky situations. Thus, decline in aerosols can certainly not explain all warming observed and particularly not considering the southern Iberian Peninsula where the aerosol effects on warming are weaker compared to countries further north. The largest increases in sensible heat flux at the expense of latent heat flux have occurred in Iberian Peninsula, which is probably a result of drier surface conditions. This means a positive feedback associated with reduced evaporate cooling and warming of the lowest air layers. Decline in water vapor in combination with the warming may have contributed to decreased cloud cover, which is found for large parts of Europe in the summer half year during the latest four decades. Anthropogenic aerosols over large parts of Europe have thus temporarily masked, until around 1980, parts of rapid warming from increases in greenhouse gases. CO2 from fossil fuels is of particularly serious concern, since it can continue to affect climate for thousand years.