Arizona gets most of its rain from thunderstorms during the summer, a period called the North American monsoon (see Arizona Monsoon for background and the anatomy of thunderstorms). By government decree, the monsoon season lasts from June 15 through September 30. In actuality, rains usually start in early July following the rain-dance ceremony of the Tohono O’odham people. In 2017, there were unusually heavy rains in July and below normal rain in August and September.
Researchers from Princeton University, using a new precipitation model, claim that global warming will decrease the rain of the monsoon. From the abstract of their paper published in Nature:
Future changes in the North American monsoon, a circulation system that brings abundant summer rains to vast areas of the North American Southwest, could have significant consequences for regional water resources. How this monsoon will change with increasing greenhouse gases, however, remains unclear, not least because coarse horizontal resolution and systematic sea-surface temperature biases limit the reliability of its numerical model simulations. Here we investigate the monsoon response to increased atmospheric carbon dioxide (CO2) concentrations using a 50-km-resolution global climate model which features a realistic representation of the monsoon climatology and its synoptic-scale variability. It is found that the monsoon response to CO2 doubling is sensitive to sea-surface temperature biases. When minimizing these biases, the model projects a robust reduction in monsoonal precipitation over the southwestern United States, contrasting with previous multi-model assessments.
Let’s see how this model premise has worked so far:
The graph below, from NOAA data, shows that year-to-year precipitation varies quit a bit. The overall trend is for increasing precipitation with global warming, not a decrease.
This new model, as all climate models, assumes that carbon dioxide is the major forcing of global temperature, an assumption for which there is no physical evidence.