Climate Change May Not Cause Hurricanes, But It Can Make Them Much Stronger
The perhaps inevitable question: So what's the connection between climate change and Hurricane Irene? The stock answer is that it's hard if not impossible to blame any extreme weather event and climate change, but it does contribute to making hurricanes, heat waves, droughts, even blizzards (for a while at least) more likely.
As for hurricanes, one of the factors contributing to their strength is warm ocean water--hurricanes need ocean surface temperatures to exceed 79°F. Something which is increasing, as most TreeHugger readers will probably know.
Union of Concerned Scientists explains how this figures into hurricane formation:
Two factors that contribute to more intense tropical cyclones-ocean heat content and water vapor-have both increased over the past several decades--ocean heat content and water vapor--have both increased over the past several decades. This is primarily due to human activities such as the burning of fossil fuels and the clearing of forests, which have significantly elevated carbon dioxide (CO2) levels in the atmosphere. CO2 and other heat-trapping gases act like an insulating blanket that warms the land and ocean and increases evaporation.
The world's oceans have absorbed about 20 times as much heat as the atmosphere over the past half-century, leading to higher temperatures not only in surface waters (e.g., depths of less than 100 feet) but also down to substantial depths, with the most severe warming occurring in the first 1,500 feet below the surface. As this warming occurs, the oceans expand and raise sea level. This expansion, combined with the inflow of water from melting land ice, has raised global sea level more than one inch over the last decade. In addition, observations of atmospheric humidity over the oceans show that water vapor content has increased four percent since 1970; because warm air holds more water vapor than cold air, these findings correlate with an increase in air temperature.
More basically, climate change doesn't cause hurricanes per se, but rather compounds their intensity, creating the conditions where storms with faster wind speeds and more rain are likely (more: NRDC).
If you want to know more in-depth about this, Union of Concerned Scientists has a good PDF on the subject: Hurricanes in a Warming World
More on Hurricanes
18 Resources For Preparing and Responding to Hurricanes
Hurricane Irene Heading to East Coast & New York (LIVE-ish Blog)