Hurricane season is heating up

Hurricane Irene may herald the peak of tropical cyclone season, which often surges in late summer. But with much of the Atlantic Ocean simmering in near-record heat this year, could global warming be playing a role, too?

Fri, Aug 26 2011 at 4:42 PM EST

COAST UNCLEAR: This map shows Hurricane Irene off the U.S. East Coast on Aug. 26, trailed by Tropical Depression 10. (Image: National Hurricane Center)

As Hurricane Irene slams into the Eastern U.S. this weekend, forecasters say it could be one of the region's worst hurricanes in years, if not decades. It's forecast to sweep the coast from North Carolina to New England, possibly remaining a hurricane most of the way to Canada. According to the National Hurricane Center, it might even still be a tropical storm when it approaches Greenland and the Arctic Circle next week.

The Atlantic hurricane season normally peaks from late August to early September, and Irene may be a sign the 2011 season is shifting to high gear. Not only is Irene the Atlantic's strongest cyclone so far this year, but also its first hurricane — and experts expect more to be hot on her heels in the coming weeks. (Update, 8/27: Irene is alone in the Atlantic for now; the NHC reports Tropical Depression 10 is breaking apart.)

Tropical cyclone activity in the Atlantic Basin typically peaks on Sept. 10.

Of course, those heels are also hotter than they used to be. Hurricane season peaks in late summer because that's when seawater is at its warmest, and warm seawater is fuel for hurricanes. But parts of the Atlantic, and other oceans around the world, are warmer than usual this year — the first seven months of 2011 saw Earth's 11th warmest January-July surface temperatures since 1880, and the combined land-ocean temperature for July was the fifth highest on record. This follows 2010's warmest year on record (it tied 2005, which also happened to be the most active Atlantic hurricane season on record). The U.S. felt few tropical storms in 2010, but it was an active season; Americans just got lucky that most cyclones stayed out at sea.

Climate scientists have predicted for years that global warming will continue raising global sea-surface temperatures far into the future, which could theoretically fuel more strong, long-lived storms. These hurricanes could also feasibly travel farther from the tropics than they have in the past, as higher-latitude regions lose their cool-water deterrent.

And as environmentalist Bill McKibben writes in the Daily Beast, Irene is just the kind of hurricane one might expect to become more common due to global warming. It's rare for such a large, powerful hurricane to maintain its strength beyond the Carolinas, meteorologist Jeff Masters tells McKibben, because wind shear and/or cool seawater normally wear down any tropical storms that make it that far. Wind shear could still disrupt Irene, but as McKibben writes, "ocean temperatures won't." Sea-surface temperatures in Irene's path are now about 1 to 3 degrees Fahrenheit above average, "which will make it easier for Irene to maintain its strength much farther to the north than a hurricane usually can," Masters explains.

Warm water evaporates more easily, letting it boost a variety of weather problems in addition to hurricanes. More evaporation means more moisture in the atmosphere, leading to wilder storms interspersed with longer, drier droughts. And McKibben points out that while Irene is the year's first Atlantic hurricane, 2011 is already a record year for billion-dollar disasters: From blizzards and tornadoes to droughts and wildfires, the U.S. has suffered for months on end — as have other countries, from flooding in Pakistan and Australia to droughts in Somalia and Russia.

It's entirely plausible that rising global temperatures have played a role, and according to a report from the U.S. Global Change Research Program, "It is very likely that the human-induced increase in greenhouse gases has contributed to the increase in sea surface temperatures in the hurricane formation regions. Over the past 50 years there has been a strong statistical connection between tropical Atlantic sea surface temperatures and Atlantic hurricane activity ... This evidence suggests a human contribution to recent hurricane activity."

But McKibben goes a step further, flatly stating that "Irene has a middle name, and it's global warming." There's a good chance he's right, but the problem is no one can know if he is — not even him. There's no proof of a link between broad, manmade climate changes and one specific weather event, at least not yet. The GCRP cautions that "a confident assessment of human influence on hurricanes will require further studies." Plenty of sound theories and historical data suggest McKibben is right, and he's certainly not the first to make the connection — a group of Mississippians even filed a lawsuit after Hurricane Katrina, blaming the storm on energy companies' emissions. (The U.S. Supreme Court declined to hear the case in January.)

Ultimately, linking any storms to climate change may not be helpful until there's hard data to back it up. The temptation is understandable, since climate legislation appears DOA in Congress and the U.N. has proved unable to multilaterally curb emissions. But it risks eroding public confidence in climate science, since it toes the slippery, anecdotal slopes of climate skeptics who claim snowstorms disprove climate change. In the meantime, the efforts of McKibben and others might be better spent on things like protecting climate-science funding, which could eventually help us know if global warming really is responsible for monsters like Katrina and Irene.

(That said, the smart money is still on McKibben's argument, even without definitive data to prove him right. Below are five graphics that support his claims: The first shows sea-surface temperature anomalies from July 2011, the second is a timeline of land-ocean surface temperatures from 1880 to 2006, and the final three are timelines of named storms, hurricanes and major hurricanes in the Atlantic from 1850 to 2010.)