A Change in the Weather

It's quite clear that something is happening:

Extreme events like the Nashville flood—described by officials as a once-in-a-millennium occurrence—are happening more frequently than they used to. A month before Nashville, torrential downpours dumped 11 inches of rain on Rio de Janeiro in 24 hours, triggering mud slides that buried hundreds. About three months after Nashville, record rains in Pakistan caused flooding that affected more than 20 million people. In late 2011 floods in Thailand submerged hundreds of factories near Bangkok, creating a worldwide shortage of computer hard drives.

It’s not just heavy rains that are making headlines. During the past decade we’ve also seen severe droughts in places like Texas, Australia, and Russia, as well as in East Africa, where tens of thousands have taken refuge in camps. Deadly heat waves have hit Europe, and record numbers of tornadoes have ripped across the United States. Losses from such events helped push the cost of weather disasters in 2011 to an estimated $150 billion worldwide, a roughly 25 percent jump from the previous year. In the U.S. last year a record 14 events caused a billion dollars or more of damage each, far exceeding the previous record of nine such disasters in 2008.

So are these events evidence of a dangerous, human-caused shift in the earth's overall climate, or is this merely the consequence of bad luck? The answer, in short, is probably both:

But natural cycles can’t by themselves explain the recent streak of record-breaking disasters. Something else is happening too: The Earth is steadily getting warmer, with significantly more moisture in the atmosphere. Decades of observations from the summit of Mauna Loa in Hawaii, as well as from thousands of other weather stations, satellites, ships, buoys, deep-ocean probes, and balloons, show that a long-term buildup of greenhouse gases in the atmosphere is trapping heat and warming up the land, oceans, and atmosphere. Although some places, notably the Arctic, are warming faster than others, the average surface temperature worldwide has risen nearly one degree Fahrenheit in the past four decades. In 2010 it reached 58.12°F, tying the record set in 2005.