Assessing the role of absorbed solar radiation, clouds, and boreal fire aerosols in the slowdown of Arctic warming and sea ice loss.

Since the 1970s, the Arctic has warmed about four times faster than the global average and has experienced substantial sea ice loss. However, since 2005, Arctic warming and sea ice loss have slowed significantly, despite an acceleration in global-mean temperature rise. At the same time, Arctic summer cloudiness and airborne particles have increased, potentially slowing warming by reflecting more sunlight back to space. This project will explore how changes in winds, cloud and aerosols can account for recent temperature and sea ice changes. Changes in the Arctic environment over the coming decades are important for national security because of the region’s socio-economic value and the influence of Arctic climate and sea ice conditions on human activities. The proposed work will improve understanding of the drivers of recent Arctic trends, helping to inform projections of future change and support decision-making by Arctic stakeholders. Arctic amplification—the ratio of Arctic warming to global warming—was approximately 4 times greater over 1980–2020, while Arctic sea ice cover experienced a 20% loss over that period. Over the past two decades, however, warming and sea ice loss rates have slowed down significantly compared to earlier decades. Calculated over 2005-2024, Arctic and global warming rates were approximately equal (i.e., Arctic amplification was near 1), while Arctic sea ice cover trends have been near-zero. In addition, Arctic summer cloud cover and aerosol optical depth have increased, and the net absorbed solar radiation (ASR) of the Arctic has been smaller than expected given the changes in surface albedo. The contribution of internal variability versus external radiative forcing to the changes in trends is unknown, as is the role played by other important Arctic climate features such as cloud and aerosol properties and poleward heat transport. This project will explore these issues by addressing two main topics, (i) the contribution of changes in atmospheric circulation to the recent trends in Arctic temperature and sea ice, and (ii) the role of clouds and boreal wildfires in recent ASR trends. Topic (i) will combine regression models, circulation-nudged Earth system model simulations, and machine-learning methods to quantify the contribution of observed circulation variability to recent trends. Topic (ii) will investigate recent Arctic ASR trends using an isotropic radiation model, explore the meteorological factors controlling Arctic summer cloud cover, and use Earth system model simulations with and without observed boreal wildfires to explore changes in summer aerosols over the Arctic. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.