Abstract
State-of-the-art climate models simulate a large spread in the projected
decline of Arctic sea-ice area (SIA) over the 21st century. Here we
diagnose causes of this intermodel spread using a simple model that
approximates future SIA based on present SIA and the sensitivity of SIA
to Arctic temperatures. This model accounts for 70-95% of the
intermodel variance, with the majority of the spread arising from
present-day biases. The remaining spread arises from intermodel
differences in Arctic warming, with some contribution from differences
in the local sea-ice sensitivity. Using observations to constrain the
projections moves the probability of an ice-free Arctic forward by 10-35
years when compared to unconstrained projections. Under a high-emissions
scenario, an ice-free Arctic will likely (>66%
probability) occur between 2036-2056 in September and 2050-2068 from
July-October. Under a medium-emissions scenario, the ‘likely’ date
occurs between 2040-2062 in September and much later in the 21st century
from July-October.