"The last couple of years have certainly felt unusually hot in many parts of the U.S., but are they really all that unusual?"

"Many people wonder whether a warming climate is turning up the temperature or whether it's all just part of the normal variation in the weather. Among scientists, there's a growing view that these latest heat waves are indeed a result of climate change."

"NASA climate scientist James Hansen has looked at the past century's temperatures all over the world. He has measured hot spells with what you might call a "unit of weirdness" — a standard deviation. It's a measure of abnormality."

"One standard deviation from what's normal might be throwing snake eyes three times in a row. The more snake eyes you roll in a row, the more standard deviations away from normal you are."