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.