To create the map, the researchers used satellite data collected from 2000 to 2013 to look at plants on a global scale. They calculated the sensitivity index by looking at how the satellite-measured vegetation ground cover compared with three factors, air temperature, water availability, and degree of cloud cover, and how all four factors had changed over that time period. They did this for every 2-square-mile block on the Earth's land surface. Some areas, like Antarctica or the Sahara desert, were classed as barren or ice-covered, but the rest of the land was graded from most to least sensitive to the effects of changes in climate over the past 14 years.