Arctic summer cloud optical properties and annual sea-ice retreat

Changes in Arctic sea-ice areal coverage have major ecological, climate, and economic implications and are driven by diverse natural and anthropogenic forcings acting over a wide range of time scales. Cloud-related variability in atmospheric radiative budgets is suspected to be particularly important in year-to-year changes in ice melt. Here, we describe a decade of pan-Arctic satellite light detection and ranging observations indicating that shortwave transmittance differences of low-altitude clouds play a key role in ice retreat through summer. These summer cloud transmittance differences result in surface solar radiation budget changes comparable to melt energy requirements for observed year-to-year changes in September sea-ice extent. As a working hypothesis, we propose that variations in summer cloud optical properties reflect a combination of positive- and negative-feedback processes involving aerosol cloud condensation nuclei. Our findings suggest a delicate atmosphere-biosphere-cryosphere balance where minor perturbations in Arctic cloud properties can have major implications on September sea-ice extent.