Recent precipitation declines over the poleward and western margins of subtropical dry zones from North America to Australia are consistent with a reorganization of atmospheric dynamics attributable to global warming. But are these droughts attributable to anthropogenic forcings like increased GHG, aerosols and ozone, or are they within the realm of natural variability? The answer is not always clear. In the case of California, climate scientists have hesitated to say the drought is beyond the realm of natural variability. In South Australia, however, a recent study in Nature Geoscience attributes anomalously dry conditions to a global increase in GHG concentration. While ozone depletion over Antarctica was also found to contribute to the drying, neither aerosols nor natural variability were determined to be significant factors.
To reach these conclusions the authors ran many simulations of the 20th century using comprehensive GCMs in various climate scenarios including changes in individual forcings (GHG, ozone, aerosols) and allowing natural variability to play by itself. Running a large ensemble of simulations for each scenario provides specific probability distribution. Comparing given mean values of precipitation for the past 20 years to the probability of individual climate scenarios informs us of the probability of the relative contribution of GHG, ozone or aerosols.
This study reminds us that climate predictions are inherently probabilistic, and of the complexity in attributing even decadal weather patterns to the influence of global warming.