How Global Warming Is Making Hurricane Irene Worse

Hurricane Irene is bearing down on the Outer Banks of North Carolina as a Category Two storm, and is expected to track a path of destruction up the densely populated Atlantic coast, with New York City Mayor Michael Bloomberg ordering the first-evermandatory evacuation of low-lying areas of the city. As the U.S. government report “Global Climate Change Impacts in the US” summarized in 2009, warming of the oceans is causing Atlantic hurricanes to become more intense and dangerous:

The destructive potential of Atlantic hurricanes has increased since 1970, correlated with an increase in sea surface temperature. An increase in average summer wave heights along the U.S. Atlantic coastline since 1975 has been attributed to a progressive increase in hurricane power. The intensity of Atlantic hurricanes is likely to increase during this century with higher peak wind speeds, rainfall intensity, and storm surge height and strength. Even with no increase in hurricane intensity, coastal inundation and shoreline retreat would increase as sea-level rise accelerates, which is one of the most certain and most costly consequences of a warming climate.

Below, ThinkProgress Green explores in more detail how Hurricane Irene has been made more destructive by the combustion of hundreds of billions of tons of fossil fuels.

Oceanic Warming. Greenhouse pollution is causing the world’s oceans to warm. Sea surface temperatures in the region where Hurricane Irene formed and along its track are around 0.5°C warmer than they were about 30 years ago. “This rise is simulated pretty well by climate models forced by anthropogenic changes in greenhouse gases and aerosols, implicating these as the probable cause,” Dr. Doug Smith, the climate scientist who leads the Met Office Decadal Climate Prediction System tells ThinkProgress. This increased heat adds about 10 to 20 miles to the top potential speed of the hurricane’s winds. Storm surge increases proportionally to the square of the wind speed, meaning a 10 percent increase in hurricane wind speed means a 20 percent increase in storm surge. Climate scientists are debating how global warming and natural variability are interacting to change the intensity of Atlantic storms overall.

Sea Level Rise. Greenhouse pollution is causing the world’s oceans to rise, both because of warming of the ocean water and because of the melting of Greenland and Antarctica. Sea levels have been rapidly rising along the East Coast of the United States. Because of a combination of the global sea level rise and because of subsidence, Boston’s relative sea level has increased 11.8 inches since 1990, and sea level at Norfolk, VA has steadily risen 14.5 inches over the past 80 years. The one-foot rise in sea level means that damage from Hurricane Irene’s storm surge will be about 50 percent greater than it would have been otherwise.

More Atmospheric Vapor. As the world’s oceans have warmed, the amount of atmospheric water vapor has increased by about 4 percent. Rainfall rates due to hurricanes appeart to have increased by 6 to 8 percent since about 1970 in association with increased water vapor in the atmosphere and warming. “This is because of the dominant reliance of storms on the resident moisture in the atmosphere and the moisture convergence for precipitation and latent heating in storms,” Dr. Kevin Trenberth wrote in 2007. “These warm ocean temperatures will also make Irene a much wetter hurricane than is typical, since much more water vapor can evaporate into the air from record-warm ocean surfaces,” tropical meteorologist Jeff Masters explains at “The latest precipitation forecast from NOAA’s Hydrological prediction center shows that Irene could dump over eight inches of rain over coastal New England.”

Increased Extreme Precipitation. Because of greenhouse pollution, heavy rains in the United States have increased 14 percent over the 20th century, much greater than the increase in overall precipitation. This has been one of thewettest years in history for the Northeast, directly in the path of Hurricane Irene. Hurricane Irene’s wind and rain will more easily topple trees in the loose, saturated soil and flood rivers, reservoirs, and drains.

Millions of people and billions of dollars of property are at risk from this one storm, in this year of billion-dollar climate disasters. Global warming pollution is far from the only reason that Hurricane Irene shouldn’t be thought of as a “natural” disaster. Much of the devastating potential of Hurricane Irene will be a consequence of past decisions about land use, construction and coastal preservation, mitigated by the brave work of public servants under attack by Tea Party conservatives. Even as we have polluted the climate to increase the threat from Atlantic storms, we have overbuilt the increasingly vulnerable coasts. Although Irene is being described as a “once in a lifetime” threat, the weight of the evidence indicates that this storm is merely a harbinger of our dangerous future.