Rational Argument:  Since Hurricane Katrina hit the Gulf Coast of the US in August 2005, there have been many predictions that more hurricanes just like it were on the way in future years and that they will be caused by global warming.  It was also claimed often that Katrina was the result of global warming.

Liberal Argument circa 2008:  No one ever said Hurricane Katrina was caused by global warming.

Yes they did and here's one article that shows it. 

Hurricane Intensity Linked to Global Warming
August 15, 2006

A new study says climate change is affecting the intensity of Atlantic hurricanes and that hurricane damage will likely worsen in coming years due to increasing ocean temperatures. [The year 2007 produced the third-quietest hurricane season since 1996.  In 2006 not a single hurricane made landfall in the U.S.]  Unlike recent studies that have linked higher sea temperatures to an increase in the number of hurricanes, the new research shows a direct relationship between climate change and hurricane intensity.

The study, published by James Elsner of Florida State University in the August 23 issue of Geophysical Research Letters, the journal of the American Geophysical Union, found that a correlation between average air temperatures during the June through November hurricane season and sea surface temperatures that help fuel hurricanes winds. Elsner says his work "helps provide verification of a linkage between atmospheric warming caused largely by greenhouse gases and the recent upswing in frequency and intensity of Atlantic hurricanes, including Katrina and Rita."

"The large increases in powerful hurricanes over the past several decades, together with the results presented here, certainly suggest cause for concern," he said. "These results have serious implications for life and property throughout the Caribbean, Mexico, and portions of the United States."

"I infer that future hurricane hazard mitigation efforts should reflect that hurricane damage will continue to increase, in part, due to greenhouse warming," Elsner said. "This research is important to the field of hurricane science by moving the debate away from trend analyses of hurricane counts and toward a physical mechanism that can account for the various observations."

Data questioned

Nevertheless, while Elsner's findings lend support to the contention that warmer temperatures will produce stronger hurricanes, they will not settle long-standing concerns among some scientists.

While several studies published since early 2005 have linked recent climate warming to the increasing occurrence and strength of hurricanes over the past thirty years, the research has proved controversial since some scientists say the system for tracking storms is flawed. They argue that storm data from 20 years ago is not nearly as accurate as current hurricane data making it nearly impossible to accurately compare storm frequency and strength over the period.

"Before aircraft and satellite monitoring were available, the Atlantic hurricane data are likely woefully underestimated - except where a hurricane ran directly over a ship or coastal community and there were meteorological observations of pressures and/or winds recorded," Chris Landsea, a scientist as the NOAA National Hurricane Center, told mongabay.com. "Given that ship captains did their best to NOT sail into the eye of hurricanes, there is a very large underreporting bias in our databases during the late 19th and early 20th Centuries, except for hurricanes at landfall along populated coastlines. Disentangling trends due to bias in the hurricane dataset and possible global warming induced changes is then very problematic.
The new paper fails to steer around this controversy since it compares average global near-surface air temperature and Atlantic sea surface temperature with "hurricane intensities" from the National Oceanic and Atmospheric Administration's Atlantic basin hurricane database (HURDAT) over the past 50 years. Climate researchers are currently working to re-analyze existing tropical cyclone databases to address these concerns.

"What data we do have - and there certainly are biases in HURDAT that need to be addressed storm by storm - suggest that the middle of the 20th Century was about as busy as the last active 11 years have been (1995 to 2005)," Landsea added. "Disentangling trends due to bias in the hurricane dataset and possible global warming induced changes is then very problematic."

Here's another example of global warming taking the blame for hurricanes.

Global Warming Fueled Record 2005 Hurricane Season Conclude Scientists
National Center for Atmospheric Research
June 22, 2006

Global warming accounted for around half of the extra hurricane-fueling warmth in the waters of the tropical North Atlantic in 2005, while natural cycles were only a minor factor, according to a new analysis by Kevin Trenberth and Dennis Shea of the National Center for Atmospheric Research (NCAR). The study will appear in the June 27 issue of Geophysical Research Letters, published by the American Geophysical Union.

"The global warming influence provides a new background level that increases the risk of future enhancements in hurricane activity," Trenberth says. The research was supported by the National Science Foundation, NCAR's primary sponsor.

The study contradicts recent claims that natural cycles are responsible for the upturn in Atlantic hurricane activity since 1995. It also adds support to the premise that hurricane seasons will become more active as global temperatures rise. Last year produced a record 28 tropical storms and hurricanes in the Atlantic. Hurricanes Katrina, Rita, and Wilma all reached Category 5 strength.

Trenberth and Shea's research focuses on an increase in ocean temperatures. During much of last year's hurricane season, sea-surface temperatures across the tropical Atlantic between 10 and 20 degrees north, which is where many Atlantic hurricanes originate, were a record 1.7 degrees F above the 1901-1970 average. While researchers agree that the warming waters fueled hurricane intensity, they have been uncertain whether Atlantic waters have heated up because of a natural, decades-long cycle, or because of global warming.

By analyzing worldwide data on sea-surface temperatures (SSTs) since the early 20th century, Trenberth and Shea were able to calculate the causes of the increased temperatures in the tropical North Atlantic. Their calculations show that global warming explained about 0.8 degrees F of this rise. Aftereffects from the 2004-05 El Nino accounted for about 0.4 degrees F. The Atlantic multidecadal oscillation (AMO), a 60-to-80-year natural cycle in SSTs, explained less than 0.2 degrees F of the rise, according to Trenberth. The remainder is due to year-to-year variability in temperatures.

Previous studies have attributed the warming and cooling patterns of North Atlantic ocean temperatures in the 20th century—and associated hurricane activity—to the AMO. But Trenberth, suspecting that global warming was also playing a role, looked beyond the Atlantic to temperature patterns throughout Earth's tropical and midlatitude waters. He subtracted the global trend from the irregular Atlantic temperatures—in effect, separating global warming from the Atlantic natural cycle. The results show that the AMO is actually much weaker now than it was in the 1950s, when Atlantic hurricanes were also quite active. However, the AMO did contribute to the lull in hurricane activity from about 1970 to 1990 in the Atlantic.