evidence: Not that long ago, in the early 1990s, scientists were not able to determine whether polar land ice was growing, shrinking, or in balance. Satellite and airborne missions, complemented by field measurements, have not only answered that question, but also provided the means for scientists to determine the mechanisms that are contributing to the growth and shrinkage of polar ice. [...] What scientists do know is that warming Arctic temperatures – and a darkening surface of the Greenland ice sheet – are causing so much summer melting that it is now the dominant factor in Greenland’s contribution to sea level rise. Greenland’s summer melt season now lasts 70 days longer than it did in the early 1970s. Every summer, warmer air temperatures cause melt over about half of the surface of the ice sheet – although recently, 2012 saw an extreme event where 97 percent of the ice sheet experienced melt at its top layer.