Photo via NASA Goddard/Flickr
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Even before the ESA's satellite measurements started, data from NASA showed dramatic thinning of sea ice. Looking at the years from 2004 to 2007, NASA's satellite measurements showed an average thinning of Arctic ice of about seven inches per year. The area of multi-year sea ice—that is, ice that hasn't melted entirely in summer and then regrown in winter—declined 42 percent during that same time period. On average, multi-year sea ice is about 2.75 meters thick, with seasonal ice cover reaching 1.8 meters.The declines seem stark, but the ESA has been careful to highlight that calculating the total magnitude of decline relies on a good understanding of how ice cover varies from year to year. BBC News quotes ESA's Professor Alan O'Neill as saying that while the current data is certainly consistent with what we can expect with global warming, "we also need to understand better the natural variation that occurs in the system on perhaps decadal timescales."In other words, it's the same story as with climate change more broadly: There's long term climate change, brought about by increases in greenhouse gases from human industrial activity, mixed with other air pollution, some of which amplifies warming (black carbon) and some of which decreases warming (sulfates from coal burning, for example). That man-made climate change mixes with even longer-running natural cycles, some of which also amplify observed changes and some of which decrease them. The end result is that climate data can be relatively noisy, which makes relying on long-term trends important.