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Of particular concern are the profound changes happening in the Greenland ice sheet: It appears that the massive amount of freshwater from melting Greenland glaciers has now begun to slow the ocean's circulating currents.
Monday's study, published in the journal Nature Climate Change, is as frightening as it is significant. Among its authors are some of the biggest names in climate science: Jason Box, a glaciologist at the Geological Survey of Denmark and Greenland, whose ongoing "Dark Snow" project is measuring the rapid melting of ice in Greenland; and Michael Mann, a meteorologist at Penn State University, whose famous 1999 "hockey stick" graph showed the sharp influence of human greenhouse gas emissions in context of 1,000 years of temperature data from ice cores and tree rings. Mann's graph was so powerful it became a lightning rod of climate denial.
Fresh water is less dense than saltwater. So when glacial melt from Greenland enters the ocean, it resists the natural sinking motion at the northern edge of the Gulf Stream and slows down the Atlantic's deep current—creating a ripple effect across the entire planet.
The study uses a library of ice cores, tree rings, coral, and sediments to generate a new reconstruction of the historical strength of the Atlantic's circulation based on temperature changes. The team found recent changes in ocean circulation are "unprecedented" since at least the year 900 A.D., about as far back as these proxy data can reliably go. According to the paper, the probability of a similar circulation slowdown caused by natural variability alone (with no influence from human-caused climate change) was less than 0.5 percent.
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