Global Climate Change and the Southwest

I was up in Phoenix last night to discuss climate change with a crowd of interested people up at the Arizona Historical Society Museum. It’s a nice place for a talk and discussion, and at the end I had several requests to post a copy of my talk. I thought I’d do that here, and also take a couple minutes to reflect on some of the things we talked about in a nice long Q&A session.

There were some excellent questions, including ones about what’s the role of the Arctic Oscillation in our recent weather/climate, geothermal heating of the earth system, the role of clouds and how we can use remote sensing from space to constrain our understanding of how the Earth’s climate will change in the future.

Another question brought up the issue of the expanding tropics and how that relates to the ongoing drying of the Southwest and other regions of the globe. I didn’t have the figure handy last night, but I’ve added it to the talk posted above – taken from the March, 2008 issue of the "Southwest Climate Outlook". The graphic shows a classic view of how the Earth’s Hadley circulation serves to move heat poleward from the thermal equator, with warm, moist air rising at the equator and moving poleward at high altutudes before descending over the Earth’s huge subtropical dry zones. These dry zones include the Sahara Desert and the U.S. Southwest in the Northern Hemisphere, as well as symmetrical dry regions south of the equator in South America and Australia. And, they are dry because of this descending air.

The big issue is that this tropical circulation seems to be expanding poleward in both hemispheres as climate models suggest it should be. This spells potential trouble for the Southwest as you can read more about in the March issue of the “Southwest Climate Outlook”.