The Role of Anthropogenic Aerosols in Recent North Atlantic Climate Change: A Synthesis of Findings from the UK ACSIS Programme

<p>Evidence from model simulations has suggested that anthropogenic aerosols may have forced multidecadal variability in a range of North Atlantic variables including sea surface temperatures, ocean circulation, and sea ice. However, many questions remain concerning the importance of anthropogenic aerosols in driving past changes in the North Atlantic climate system. The pathways via which changes in aerosol and aerosol precursor emissions, and oxidant levels, influence climate are complex. They involve both chemical and physical processes, and likely include changes in clouds, radiation, surface temperatures, atmospheric and oceanic circulation, and Arctic sea ice. This complexity is an important factor in the large uncertainty surrounding the role of anthropogenic aerosol in North Atlantic climate change, and was one of the major motivations for the UK&#8217;s North Atlantic Climate System Integrated Study (ACSIS). ACSIS was a multidisciplinary research programme conducted over the period 2016-2022, delivered by a consortium of seven UK institutions. This presentation draws together findings from the programme to provide an overall synthesis of what was learned in ACSIS about the role of anthropogenic aerosol in North Atlantic climate change. Remaining uncertainties, the potential for observational constraints, and opportunities for future work will also be discussed.</p> <p>&#160;</p> <p>ACSIS made extensive use of simulations conducted for CMIP6, particularly historical simulations, and attribution experiments included in AerChemMIP and DAMIP. Additional sensitivity experiments with HadGEM3-GC3.1 and UKESM1 were used to quantify the effects of uncertainty in aerosol forcing in the absence of the additional uncertainty associated with model differences, to decompose the aerosol forcing, and to better illustrate the role of aerosol in recent changes.</p> <p>&#160;</p> <p>As aerosol emissions increased (1850-1985), North Atlantic CDNC increased. Emissions of ozone precursors, and resulting changes in OH, contributed to this trend. This led to downwelling surface shortwave decreases across the North Atlantic, which drove colder surface temperatures, increased sea ice extent, and increased mean sea level pressure. In contrast, the eastern subpolar gyre warmed, likely due to increased ocean heat convergence due to the increase in the AMOC.</p> <p>&#160;</p> <p>As local aerosol emissions fell (1986-2014) much of the reverse occurred. Downwelling surface shortwave increased across the North Atlantic, predominantly over land, driving warmer surface temperatures and reduced sea ice extent. The eastern subpolar gyre cooled. However, the role of aerosol in this later period is less clear due to a dominance of temperature-mediated cloud feedbacks over aerosol forcing, AMOC related feedbacks, and a changing aerosol forcing pattern.</p>