Hansen and colleagues looked at statistics and linked this increase in extreme heat waves to climate change.
These "extremely hot" temperatures covered less than one percent of the Northern Hemisphere land surface during the time period 1951 to 1980. Since 2006 these extreme temperatures have covered about 10 percent of this land area. These regions of "extremely hot" temperatures are shown on the map as brown in the YouTube video below.
The YouTube video shows how temperatures by region differed from the 1951-1980 seasonal average for June, July and August. White areas are considered "normal" temperatures, while blues and purples represent colder than usual temperatures. The range of hotter than normal temperatures is defined by the scientists as "hot" (orange), "very hot" (red) and "extremely hot" (brown).
Notice how the areas covered by "extremely hot" temperatures increases from the 1980s to the present. The massive heat waves of Western Europe in 2003, Russia in 2010 and Texas, Oklahoma and Mexico in 2011 particularly stand out.