Ten-year anniversary: Was Hurricane Katrina "caused" by global warming?

In the aftermath of Hurricane Katrina, ten years ago, the question emerged of whether global warming "caused" this event. Climate Central recently looked back on this question and what we have learned.

The process of determining what caused an extreme climate event is known as attribution, and its a tricky game. The first step is to ask the right question. Increasingly, climate science is recognizing that based on what we know about climate change, the question of whether an event was "caused by" climate change is not answerable.

Why? The atmosphere is warmer and wetter now, due to human-caused global warming. It is more energetic and more able to extract moisture from soils, plants, and open waters. Similarly, ocean surface temperatures are warmer, providing more energy to the overlying atmosphere. How could a weather event develop in such a context and NOT be influenced by these changes? A more appropriate question might be "How much did global warming contribute to this event?". Increasingly, this is where attribution studies are heading. 

What have we learned regarding Katrina? A few highlights:

• Warmer ocean intensified storm by adding vapor and energy
• Higher sea level increased the storm surge by 15-60%
• Subsidence of the land adds to damage from storm surge
• Land lowered by pumping of groundwater and oil
• More generally, studies suggest that in a warmer world
• Hurricane intensity will increase
• Numbers of hurricanes may be stable (?)
• Tracks will shift northwards
• Odds of truly major events (e.g. 1/10,000) will increase