The IPCC summary sums up a review of climate science research done in the last 6 or 7 years.
Below is a statement pulled from that summary that references impacts of climate change in the Southwest, United States.
It says "Regional to global-scale projected decreases in soil moisture and increased agricultural drought are likely (medium confidence) in presently dry regions by the end of this century under the RCP8.5 scenario. Soil moisture drying in the Mediterranean, Southwest US and southern African regions is consistent with projected changes in Hadley circulation and increased surface temperatures, so there is high confidence in likely surface drying in these regions by the end of this century under the RCP8.5 scenario."
It basically says drought conditions are expected to worsen in the Southwest, as well as in the Mediterranean and southern Africa throughout this century.
When the IPCC mentions the "RCP8.5 scenario" it is referencing a projected concentration of greenhouse gasses in the atmosphere over a period of time.
RCP stands for 'Representative Concentration Pathways'.
There are 4 different scenarios used for research in the latest IPCC review of climate change science.
The RCP 2.6 shows how greenhouse gas concentrations change if emissions are drastically reduced by mid-century. Van Vuuren et al. (2007)
The RCP 4.5 shows a stabilization of greenhouse gas emissions before 2100, while RCP 6.0 shows a stabilization after 2100. Clarke et al. (2007) Wise et al (2009) Fujino et al. (2006) Hijioka et al. (2008)
Both the 4.5 and 6.0 scenarios employ various technologies and strategies for curbing emissions.
RCP 8.5 is by far the most dramatic of the four models showing a fast increase of emissions for much of this century. Riahi et al. (2007)
At a climate confernce this weekend I was able to ask how these scenarios relate to the real world.
Ben Santer, a climate scientist from the Lawrence Livermore National Laboratory in California, said the 8.5 scenario is what is happening right now.
This line is shows fast emissions have gone up from 2005 and how that projects into the future.
Santer said emissions in the last 10 years have gone up faster than expected, mainly because of rapid development in China.
One or more of these scenarios are plugged into complex climate computer models.
Researchers then analyze the results, which show a forecast for future climate.
Many of these computer models are run out to year 2100.
The statement at the top of this article also references Hadley Cell circulation, which is a big concern for drought in this area of the world.
The Hadley Cell is the large scale movement of air that beings with rising air near the equator.
That air forms the Trade Winds blowing north and then the air sinks back towards land at the subtropics around 30° latitude.
Sinking air squashes storm chances.
Therefore it is no surprise some of the world's great deserts are located near 30° latitude.
The concern is that the Hadley Cell may expand with warmer global temperatures.
If this happens the dry areas currently at 30° latitude could expand northward.
Here in the United States, that would impact many people living in states on or near the Mexico border.
Currently in Arizona, a decade long drought continues.
Some part of the state has been in Dry or Drought conditions since mid-January of 2002.
In Tucson only one year since the 2001 has received above average rainfall.
It is too early to tell if the drought is linked to an expansion of the Hadley Cell or not.
The Southwest U.S. and northern Mexico have experience extended periods of drought, known as Megadroughts, in the past.