Climate Models Reveal How Human Activity May Be Locking the Southwest into Permanent Drought
Until recently, climate models have not offered a clear picture of how rainfall patterns are likely to change in the near future as greenhouse gas emissions from vehicles, power plants and industries continue to heat up the planet. The models can diverge sharply in where, when and how precipitation will change. Even forecasts that average the results of several models differ when it comes to changes in rainfall patterns.
The techniques we deployed are helping to sharpen that picture for North America and across the tropics.
We looked back at the pattern of PDO phase changes over the past century using an exceptionally large ensemble of climate simulations. The massive number of simulations, more than 500, allowed us to isolate the human influences. This showed that the shifts in the PDO were driven by an interplay of increasing warming from greenhouse gas emissions and cooling from sun-blocking particles called aerosols that are associated with industrial pollution.
From the 1950s through the 1980s, we found that increasing aerosol emissions from rapid industrialization following World War II drove a positive trend in the PDO, making the Southwest rainier and less parched.
After the 1980s, we found that the combination of a sharp rise in greenhouse gas emissions from industries, power plants and vehicles and a reduction in aerosols as countries cleaned up their air pollution shifted the PDO into the negative, drought-generating trend that continues today.
This finding represents a paradigm shift in our scientific understanding of the PDO and a warning for the future. The current negative phase can no longer be seen as just a roll of the climate dice – it has been loaded by humans.
Our conclusion that global warming can drive the PDO into its negative, drought-inducing phase is also supported by geological records of past megadroughts. Around 6,000 years ago, during a period of high temperatures, evidence shows the emergence of a similar temperature pattern in the North Pacific and widespread drought across the Southwest.
Tropical Drought Risks Underestimated
The past is also providing clues to future rainfall changes in the tropics and the risk of droughts in locations such as the Amazon.
One particularly instructive example comes from approximately 17,000 years ago. Geological evidence shows that there was a period of widespread rainfall shifts across the tropics coinciding with a major slowdown of ocean currents in the Atlantic.
These ocean currents, which play a crucial role in regulating global climate, naturally weakened or partially collapsed then, and they are expected to slow further this century at the current pace of global warming.
A recent study of that period, using computer models to analyze geologic evidence of earth’s climate history, found much stronger drying in the Amazon basin than previously understood. It also shows similar patterns of aridification in Central America, West Africa and Indonesia.
The results suggest that rainfall could decline precipitously again. Even a modest slowdown of a major Atlantic Ocean current could dry out rainforests, threaten vulnerable ecosystems and upend livelihoods across the tropics.
What Comes Next
Drought is a growing problem, increasingly driven by human influence. Confronting it will require rethinking water management, agricultural policy and adaptation strategies. Doing that well depends on predicting drought with far greater confidence.
Climate research shows that better predictions are possible by using computer models in new ways and rigorously validating their performance against evidence from past climate shifts. The picture that emerges is sobering, revealing a much higher risk of drought across the world.
Pedro DiNezio is Associate Professor of Atmospheric and Ocean Sciences, University of Colorado Boulder. Timothy Shanahan in Associate Professor of Geological Science, The University of Texas at Austin. This article is published courtesy of The Conversation.
