Droughts have intensified in already dry regions around the world, including in the Southwest United States and in Australia throughout the first decade of this century. The severity of these droughts has been attributed to global warming and climate change, which climate models predict should make traditional weather patterns more extreme, so dry regions will get drier.
However, there’s an inherent difficulty in determining the causes behind contemporary droughts because of their naturally long timescales. During the medieval 12th and 13th centuries, North America experienced ‘megadroughts’ spanning 1000 years! With such long timescales, it’s hard to know whether the intense dry spells we see now in California and nearby states are due to natural climatic variability or spurred on by anthropogenic carbon emissions. Is man-made global warming to blame?
A recent paper in Science Advances provides a comprehensive answer to this question. The group from NASA Goddard Institute for Space Studies, led by Benjamin Cook, compared 17 different general circulation models that predict soil moisture in the drought-affected regions of the Southwest and Midwest United States through the end of the 21st century. Soil moisture is traditionally used as a proxy for drought conditions, as it takes into account a balance of precipitation (added moisture) and evapotranspiration (moisture taken away).
So what did the models find in terms of projected moisture? All 17 of them predicted the same worsened drought conditions! It is quite rare in climate modeling research to find such consistent results across models, as they all represent wind, humidity, and feedback loops a little differently. Below is the model-averaged results for The Palmer Drought Severity Index (PDSI), a normalized index for moisture availability, as well as predicted values of soil moisture 30 cm and 2 m below the surface:Brown colors indicate model predictions of regions with decreased soil moisture by the end of the 21st century. In particular, the Southwest and Mexico appear particularly vulnerable. Another way to look at this is soil moisture averaged over either the Southwest or Midwest as a function of year: Clearly, in both regions, moisture drops dramatically by the mid 21st century (the different colors correspond to different measurements – PDSI or soil moisture – but they all show the same trend.)
So the models all agree that the droughts will continue, but are we causing this? Or is it just the variability of the Earth’s climate, and our modern civilization hasn’t been around long enough to witness its entire period of change?
Enter tree rings, a natural time capsule that lets us peer back into the past to understand its drought conditions. The thickness of tree rings decreases when it’s dry, so scientists can study the changes in thickness to assess the severity of droughts in past centuries. Using this information, they can create a quantitative measure of the PDSI for past droughts, and then compare to the PDSI predicted by the models. This gives a direct comparison between droughts far in the past and predicted future droughts in the 21st century to see if they will be worse. (The PDSI measurements based on tree rings were compared to PDSI from climate models over a 60-year interval from 1931-1990 and shown to be statistically indistinguishable. This is crucial to confirming that they both ways of assessing PDSI are reliably measuring the same thing and can be compared across time periods.)
Unfortunately, the tree rings give some bad news. By the end of the 21st century, the average drought conditions in the Southwest US will exceed even the worst conditions during the megadroughts in the Medieval period. Thus, based on the best comparison we can make to previous known drought conditions, the predicted severity is unprecedented and seems likely associated with anthropogenic causes.
These results strongly suggest that anthropogenic emissions are leading to drought intensities beyond the natural cycles seen in previous paleoclimate records. Up to this point, the scientific community has not agreed on this (and more evidence should continue to be collected to replicate these findings). Even the Intergovernmental Panel on Climate Change expressed doubt in their most recent report about how accurate drought projections could be and how global warming is affecting them. This is the first study to indicate with such certainty that these droughts will worsen due to our emissions – another piece of evidence that reducing carbon emissions is key not only to help future generations, but also to alleviate those in drought-affected regions who will continue to suffer from lack of water.
Benjamin I. Cook, Toby R. Ault, Jason E. Smerdon (2015). Unprecedented 21st century drought risk in the American Southwest and Central Plains Science Advances