With its wreath of sea ice shrinking ever smaller over the last half-century, the Arctic has served as global warming's canary in the coal mine. By 2050 to 2100, according to climate model predictions, Arctic summers will be ice-free for the first time in about a million years. But new research reveals the ice has been vanishing about 3 times faster than the models have predicted, shifting the inevitable meltdown about 30 years ahead of schedule.
Global climate forecasts from the Intergovernmental Panel on Climate Change (IPCC) are based on computer models that use mathematical equations to describe key aspects of the physical world such as greenhouse gas levels. To simulate past climates and project future trends, the models start at a given year in the past, say 1800, and run forward in time, allowing all the parts of the faux world to interact. Although the simulations include real-world observations, this information doesn’t necessarily capture fluctuations on small spatial-scales in factors such as ocean heat and ice thickness. Together, these changes may significantly diminish sea ice. That's why climatologist Julienne Stroeve of the National Snow and Ice Data Center at the University of Colorado, Boulder, and her colleagues wanted to double-check the IPCC projections on melting with records of what actually happened to the ice.
Stroeve's team compared results from the IPCC's 18 climate models with data from aircraft and ship reports and satellite measurements. The team found that, on average, the IPCC models simulated ice losses in September (when ice retreats to its annual minimum) at 2.5% per decade from 1953 to 2006. In contrast, the real-world observations show September ice actually diminished by about 7.8% per decade during that period. This suggests current model projections are overly conservative, and summer sea ice may disappear considerably earlier than thought, the authors conclude online 1 May in Geophysical Research Letters.
Arctic modeler Wieslaw Maslowski of the Naval Postgraduate School in Monterey, California, says that one reason global climate models lowball the rate of melting is that they underestimate the amount of heat transported into the Arctic from the Atlantic Ocean and Bering Sea, which affects the rate of sea ice melting.