Satellite imagery only goes back so far. To measure the rate of ice loss across the Himalayan glaciers, researchers turned to recently declassified spy satellite photos from 1975. The photos were used to create a digital elevation model (above) which was compared with more recent data. They concluded that the rate of ice loss was accelerating: it was twice as much from 2000 to 2016 than it was from 1975 to 2000. Columbia University, Science News. [Geography Realm]