Cause and Effect

How much did humans contribute to this year’s extreme weather events?

As hurricane Irene blasted up the eastern seaboard in August, it left record damage in its wake. Rivers in 10 states reached all-time highs, and communities suffered devastating floods. In all, it made 2011 the worst year, in terms of economic damage, from weather/climate disasters in the past 31 years. Yet Irene’s severity wasn’t exactly unexpected. Intense Atlantic hurricanes have been growing in number since the 1970s. Heat waves have become more common since the 1950s and droughts more widespread. And this summer was the second warmest in recorded history, after only 1936.

The public has begun to suspect that the severe weather is due to human-caused climate change and to wonder whether it will be the norm in a warmer world. Rising atmospheric greenhouse-gas concentrations trap heat in the air, causing surface temperatures to rise. A warmer atmosphere holds more water vapor, likely leading to heavier rain events and worse flooding and drought in some areas. That all sounds like what we saw in 2011, but climatologists caution that it’s too soon to tell exactly what role climate change played.

It’s hard because “we don’t understand very well the potential role of natural variations in these severe weather events,” says Tom Knutson, a NOAA meteorologist who studies climate change’s effects and hurricanes. Further complicating the issue: Weather records vary widely. “We can’t make concrete conclusions yet,” he says. “There are too many unknowns.”

Neville Nicholls, a climate scientist at Australia’s Monash University who’s helping spearhead the International Panel on Climate Change’s latest science chapter on extremes, agrees. While “almost certainly global warming is exacerbating at least some weather and climate extremes,” he says, “we don’t have conclusive proof, at least as yet,” to definitively link our emissions with the extreme weather. Those sentiments are reflected in an IPCC report (PDF) on extreme weather released on November 18. Researchers concluded that there’s “high confidence” that the increase in maximum and minimum daily temperatures across the globe is due to humans pumping more greenhouse gases into the atmosphere, as are more intense and longer droughts in some regions. (The IPCC’s next all-encompassing report is expected in 2013/2014.)

Still, the recent IPCC report and other research underscores that while no single weather event can be pegged to global climate trends, there’s evidence that some events will become more frequent. In 2010 Knutson and colleagues announced findings that indicate category 4 and 5 Atlantic hurricanes could double in number by century’s end, and that rainfall rates near these storms’ cores could rise 20 percent. The IPCC also projects rampant heat waves, droughts, and flooding this century, all likely exacerbated by human interference with the climate.

With evidence mounting and the consequences of inaction so catastrophic, Congress and the White House continue to dither. And at the climate talks underway in Durban, South Africa—attended by delegates from more than 190 countries—it doesn’t appear that there will be any breakthroughs regarding an international agreement to reign in greenhouse gas emissions. “We need to invest in solutions that will help lower emissions and transition us to cleaner forms of energy to head off some of the most severe consequences,” says Rachel Cleetus, a Union of Concerned Scientists senior climate economist. While new federal laws aren’t on the table now, she’s optimistic that this year’s storms will raise awareness. “Maybe the next time we have an opportunity to enact climate legislation, there will be a groundswell of support that makes politicians vote the right way.”