Robust and Perfectible Constraints on Human-Induced Arctic Amplification

The Arctic near-surface warming is much faster than its global counterpart. Yet, this Arctic amplification occurs a rate that is season, model and forcing-dependent. The present study aims at using temperature observations and reanalyses to constrain the projections of Arctic climate during the November-to-March season. Results show that the recently observed four-fold warming ratio is not entirely due to a human influence, and will decrease with increasing radiative forcings. Global versus regional temperature observations lead to complementary constraints on the projections. When Arctic amplification is defined as the additional polar warming relative to global warming, model uncertainties are narrowed by 30% after constraint. Similar results are obtained for projected changes in the Arctic sea ice extent (40%) and when using sea ice concentration and polar temperature observations to constrain the projected polar warming (37%), thereby confirming the key role of sea ice as a positive but model-dependent surface feedback. Arctic amplification of global warming by a factor of four is not entirely caused by human influences and is projected to decrease as radiative forcing increases, according to constraints from observations and reanalyses, applied to climate model projections for the winter.