Is the Southwest United States drying up? Is the “current drought” going to be exacerbated by global warming? If this article from earlier this year is to believed it is.

Scientific American: Dust Bowl 2.0: Is the Southwest Drying Up?

A research team, led by a group at Columbia University’s Lamont-Doherty Earth Observatory (LDEO) in Palisades, N.Y., reveal in this week’s Science that southwestern North America will likely be saddled with increasingly arid conditions during the next century. This drying effect, the researchers say, is directly related to man-made climate change and will demand new methods for managing water resources in the region. They based their findings on 19 climate models, all of which contributed to the Intergovernmental Panel on Climate Change’s (IPCC) Fourth Assessment Report released in Paris in early February.

However, there is no “current drought” in the desert Southwest. If we first look at the 3 month SPI, we can see that the Southwest is for the most part above average in terms of SPI. This means that compared to normal, over the last three months there has been more precipitation.

But let’s look at a longer timescale. Here I’ve shown the 48 month (4 year) SPI. This shows that in the Southwest, all regions are either at the climatological average or slightly above in terms of rainfall.

But that’s not all. There is also something called the SW Monsoon Region rainfall index from Earth Systems Research Laboratory, formerly the Climate Diagnostics Center. It is described as “Area averaged precipitation for Arizona and New Mexico: Calculated using NCDC’s climate division dataset. Monthly precipitation values for each of the climate divisions in Arizona and New Mexico are are averaged to produce a single monthly value.”

First the time series of accumulated precipitation in the months of July, August, and September is plotted. That is the black line in the plot below. The solid blue line is a moving Gaussian average with a FWHM of slightly more than 1 month. This just smooths out the precipitation, so we can view the long-term trends. The dashed blue line is a linear fit to the precipitation data. As can be seen, the trend is upward; lately the Southwest is recieving more precipitation. There is an obvious jump around 1999, and a less obvious jump at 2006. This monsoon has been above average for rainfall, so the monsoon precipitation if calculated for 2007 would be near the value 0f 2006. It is not shown because the September data was not yet in the file from ESRL.

We can also plot the total yearly precipitation values. The lines are the same as above, and there is the same overall pattern.

From this data, I am not sure how one concludes that there has been a decrease in the rainfall in the Southwest. The SPI shows that we are not in a drought - at least for the last 4 years. The precipitation has not decreased.

There have been model studies which show that precipitation will decrease in the Southwest due to global warming, such as Seager et.al. [2007], but as yet, these models do not seem to reproduce the conditions seen in the Southwest. The Seager paper does not even mention the word “monsoon” which is responsible for approximately half of the precipitation in the Tucson area. As one moves southward from Tucson, the percent precipitation from the monsoon increases, and as one moves northward, the percent precipitation from the monsoon decreases.

References:
Seager, R., M. Ting, I. Held, Y. Kushnir, J. Lu, G. Vecchi, H.-P. Huang, N. Harnik, A. Leetmaa, N.-C. Lau, C. Li, J. Velez, and N. Naik, 2007, A collection of 19 climate models predict that southwestern North America will dry significantly in the coming century, a transition that may already be under way, Science 316 (5828), 1181. DOI: 10.1126/science.1139601

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