May 18, 2012

CO2 Not to Blame for Southwest Droughts?

Filed under: Floods, Precipitation

In our last WCR, we discussed a series of articles that found that higher resolution climate models—models which include a better representation of the complex terrain features of the Southwest—produce less drought stress on the Southwestern U.S. in their projections of future climate change from greenhouse gas emissions than do coarser resolution general circulation models.

Now comes along a new paper published in Nature magazine by Robert Allen and colleagues which suggests that the drying trend which remains is being caused more by black carbon aerosols and tropospheric ozone than by greenhouse gas emissions.

Recall that increased aridity in the Southwest is one of “robust” climate change projections for the U.S., at least according to the Intergovernmental Panel on Climate Change (IPCC), the U.S. Global Change Research Program (USGCRP), and the U.S. Environmental Protection Agency (EPA). The primary driver for the increased aridity is the projected northward expansion of the tropical belt component of the atmospheric circulation. This expansion of the tropics leads to a drying of subtropical regions such as the American Southwest as the preferred track for mid-latitude storm systems is shifted poleward.

Observations over the past 30 years or so show that the tropics have broadened by about 2 to 5 degrees latitude (a couple hundred miles) lending support to the climate model projections that the tropics will expand due to the continued build-up of atmospheric carbon dioxide. However, the observed rate of expansion far exceeds that projected to result from GHG increases. According to Allen et al. this may be the result of “relatively short observational record, the large natural variability of some expansion metrics, or model deficiencies.” They set out to see if they could help identify the cause (spoiler alert: it turns out to be model deficiencies).

Their hunch was that other atmospheric warming agents (besides greenhouse gases)—specifically black carbon (soot) and tropospheric (low-level) ozone—may play a significant role.

First, they examined the projections from a suite of climate models and found that models which included time varying black carbon and tropospheric ozone concentrations produced a greater degree of expansion of the tropics than did models which lacked temporal changes in black carbon and tropospheric ozone. This result suggested that they were on the right track.

To investigate further, Allen et al., used a single climate model (the Community Atmospheric Model version 3 (CCM3) of the National Center for Atmospheric Research (NCAR)) and ran it multiple times, each run differing by the inclusion of different climate forcing agents. By comparing the change in the tropics across the model runs, the researchers could better identify which factors were responsible and in what magnitude for the expansion.

What they found was that black carbon and tropospheric ozone act to heat the lower atmosphere in the mid-latitudes of the Northern Hemisphere (where the emissions occur) relative to the other latitude bands which drives the north-south temperature gradient poleward and pulls the northern boundary of the tropics northwards. Together, black carbon and tropospheric ozone produced about twice the northward expansion of the Northern Hemisphere tropics than did greenhouse gases.

According to Allen et al.:

“Our analysis strongly suggests that recent Northern Hemisphere tropical expansion is driven mainly by black carbon and tropospheric ozone, with greenhouse gases playing a smaller part.”

Admittedly, the observed rate of expansion was still greater than the modeled rate—even when trends in black carbon and tropospheric ozone were included. Allen et al. state:

“Compared to observations, the magnitude of the simulated change is underestimated. This could be related to … caveats with the observations, model deficiencies, or deficient black carbon aerosol forcing.”

The authors argue that there is a good likelihood that black carbon emissions have been underestimated—especially those arising from Southeast Asia. And that if emissions were higher than have been estimated, then the discrepancy between modeled tropical expansion and observations would decline.

The bottom line is that the primary influences on a major component of the earth’s atmospheric circulation and thus general weather patterns turns out to be, on further examination, not atmospheric greenhouse gas concentration changes, but rather black carbon (soot) and tropospheric ozone. And one impact from the forced atmospheric circulation changes is a tendency for more aridity across the Southwestern U.S.

So, the EPA can try all they want to reduce U.S. greenhouse gas emissions, but no matter how successful they are, they will have little impact, if any, on the future of drought in the Southwestern U.S., as drought there is a complex interaction between natural variability and human climate alterations—of which, as shown by Allen et al., greenhouse gases play only a minor role.

What we’ve seen in our last two World Climate Report articles is that new research published in the peer-reviewed scientific literature shows that not only do higher resolution climate models produce less tendency for drought conditions across the Southwestern U.S., but the drought conditions that may develop have little to do with human-caused greenhouse gas emissions.

So much for the “robust” signal that human greenhouse gases will lead to more drought in the Southwest.

It seems like a reassessment of this projection is in order—by the IPCC, the USGCRP, and the EPA.


Allen, R.J., S.C. Sherwood, J.R. Norris, and C.S. Zender, 2012. Recent Northern Hemisphere tropical expansion primarily driven by black carbon and tropospheric ozone. Nature, 485, 350-355.

Gao, Y., J. Vano, C. Zhu, and D. P. Lettenmaier, 2011. Evaluating climate change over the Colorado River basin using regional climate models. Journal of Geophysical Research, 116, D13104, doi:10.1029/2010JD015278.

Gao, Y., et al., 2012. Moisture flux convergence in regional and global climate models: Implications for drought in the southwestern United States under climate change. Geophysical Research Letters, 39, L09711, doi:10.1029/2012GL051560

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