Why this warmer world is not just a passing phase

In the late 1990s, three scientists published a paper charting the Earth’s temperatures over the last millennium. For the first 900 years, the trend line was the definition of boring: just little blips up and down. That changed around 1900, when the mean global temperature shot up, and kept rising.

That now-famous trend line, dubbed “the hockey stick” because of its sharp upward slope, is so vivid that it has played a key role in two decades of argument over whether the Earth’s atmosphere is warming, and whether those changes are caused by heat-trapping gases generated by human activities.