Climate science at a glance
- Flood elevations during Katrina would have been 15 to 60 percent lower around 1900 than the conditions observed in 2005. Between 1900 and 2005,  in New Orleans rose 0.75 meters, 0.57 of which was due to land subsidence.
- Scientists estimate a doubling of Katrina magnitude events associated with the warming over the 20th century.
Meet Hurricane Katrina—the costliest hurricane in US history
In the US, hurricanes accounted for seven out of ten of the costliest natural disasters from 1980-2014, and Hurricane Katrina—which devastated the US Gulf coast in August 2005—tops the list. Katrina cost $152.5 billion in damages, caused more than 1,800 deaths and displaced 1.2 million people.
Research has shown that climate change  and storm surge associated with hurricanes, and there is strong evidence that climate change also increases hurricane intensity and wind speed.
Sea level rise gave Hurricane Katrina a boost
Sea level rise and increasing storm surge risk are the clearest links between climate change and more destructive coastal storms. Rising sea levels slowly increase the chance that a storm will cause surges, even if other aspects of the storm—like wind speed—remain stable. This is because the storm, riding along higher waters, can penetrate further inland once it makes landfall. Hurricane Katrina produced the highest storm surge ever recorded on the U.S. coast—an astonishing 27.8 feet at Pass Christian, Mississippi. From eastern Louisiana to Alabama, 90 miles of coastline received a storm surge characteristic of a Category 3 hurricane.
A 2013 study finds that, in New Orleans, sea level rise dominates changes in flooding that are due to storm-surge by increasing mean sea level and by leading to decreased wetland area. By comparing conditions on the Gulf Coast in 2005 to the climate and sea level conditions of 1900, the same study finds that Hurricane Katrina's impact would have been significantly less damaging in 1900, with a storm surge anywhere from 15 to 60 percent lower.
Hurricane heat engines fueled by warm tropical waters
The intensity, , and duration of North Atlantic hurricanes, as well as the frequency of the strongest hurricanes, have all increased since the early 1980s, according to the US National Climate Assessment. These increases are due in part to warmer sea surface temperatures in the areas where Atlantic hurricanes form and travel.
By 2100, computer models consistently show that greenhouse warming will increase the average global intensity of hurricanes by 2 to 11 percent. However, there are many environmental factors that contribute to development of a tropical cyclone—including ocean water temperatures, atmospheric temperatures, air moisture levels, distance from the equator, and wind speeds and directions—making it difficult for scientists to truly predict how global warming will affect hurricanes in the long-term.