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CGD Climate Highlights: Climate Change

Why do we believe the climate is changing?

Changes in Sea Ice

Changes in the ice cover result in positive feedback mechanisms that amplify climate perturbations. Because ice is highly reflective, it bounces a considerable amount of sunlight back to space. When sea ice retreats, the underlying dark ocean surface is exposed. This absorbs more sunlight, which acts to further warm the system. Because of these properties, it is expected that the largest response to global warming will occur in the polar regions.

Observations from the Arctic indicate a retreat of the sea ice cover is underway. Satellite data from 1979 reveal a decrease of approximately 8% per decade in the September ice cover. Since 2002, there have been consecutive low ice years in the Arctic summers. This is in contrast to the earlier record when an anomalously low ice year was typically followed by a recovery to normal or higher than normal conditions within 1-2 years. If the current rate of ice retreat continues, September ice-free Arctic conditions would prevail by approximately 2060. Such conditions are unlikely to have occurred in the last one million years. Data from upward looking sonar measurements also indicate a reduction in sea ice thickness, with an approximately 30% decrease in ice volume over 30-40 years.

These reductions in sea ice thickness and extent are part of a larger, coordinated change in the Arctic climate system. The Arctic land temperatures are warming at about twice the rate of the global average, permafrost in the region is warming and degrading, glaciers are melting, changes in vegetation are occurring, river runoff is increasing, and the Arctic ocean is warming. These observations are compelling and make a strong case that the Arctic climate is changing. The observed changes are consistent with climate model simulations that project continued sea ice loss, amplified warming in the Arctic, and the possibility of seasonally ice-free summers within this century.