evidence: Over the last 40 years, annual Arctic sea ice measurements show ice shrinking by 12.6 percent each decade, a pace of decline that’s unmatched by any point in at least the last 1,500 years. Scientists measure Arctic sea ice every year in September, when satellite imaging shows ice coverage at its lowest. Since 1979, when the U.S. government started taking those measurements, Arctic ice has declined by more than 2 million square kilometers.1 "It’s a pretty large chunk of ice that is lost," says Gianluca Meneghello, a research scientist who focuses on Arctic oceanography in MIT’s department of Earth, Atmospheric and Planetary Sciences. "It’s one of the strongest signals of climate change." [...] Arctic ice fluctuates a lot over geologic time, and there have been periodic examples of a completely ice-free Arctic over the last 350,000 years. But at least some amount of year-round Arctic ice cover developed around 5,000 years ago and has been with us ever since. The spread of today’s Arctic ice is not only small compared to earlier points in human history—it’s also shrinking faster than at any point in at least the last 1,500 years, the period for which we have the most detailed data on historical ice cover.2 Based on current climate models, scientists expect that Arctic ice will disappear completely over the summer in the coming decades.3