Paleoclimatologist weighs in on 'hot drought' as a lead author on IPCC climate report

The Intergovernmental Panel on Climate Change, or IPCC, has released the first of three climate reports expected in the coming months, and a University of Arizona faculty member is a lead author, continuing the university's tradition of contributing to the global, informational resource.

Department of Geosciences associate professor Jessica Tierney is one of about 20 U.S. authors on the IPCC Working Group 1 contribution to the Sixth Assessment Report, which provides the latest assessment of scientific knowledge about the warming of the planet as well as projections for future warming and its impacts on the climate system. Tierney helped write much of the Working Group 1 report, including the sections on drought and aridity in the water cycle chapter. The other two reports will come out in December and March.

Tierney was nominated and selected from a pool of scientists to work on the report. The scientists work as a team to write chapters that then go through a long and multilevel review process in which climate scientists, the public and government officials provide feedback.

"The University of Arizona has a tradition of someone involved in the reports consistently over the years," Tierney said.

In 2018, Diana Liverman, who recently retired as director of the School of Geography, Development and Environment, contributed to the IPCC's "Summary for Policymakers of the Special Report on Global Warming of 1.5 degrees Celsius." Former UArizona faculty member Jonathan Overpeck also contributed to past reports.

One of the major updates in the latest report has to do with the human influence on drought and aridity, which are particularly relevant for the American Southwest, Tierney said.

In 2013, the IPCC reported with low confidence that any changes in droughts could be attributed to human influence. According to the new report, there is now medium to high confidence that recent droughts and drying trends can be attributed to humans in some regions.

"For example, the 2012-2014 California drought has been studied intensely by climate scientists who determined that it was the worst one in 1,200 years," Tierney said. "Anywhere from about 10 to 30% of that drought was caused by humans."

'Hot drought' becoming more common

The California drought and many others are being driven by what is now being called "hot drought," Tierney said.

"You have to have low precipitation for drought, but what's making droughts really bad now is that it is hotter due to global warming," Tierney said. "When it's hot, the atmosphere has a higher demand for moisture. To meet that demand, it evaporates moisture from the soil or through plants. And when you lose all moisture in the land surface, it only makes the drought worse."

To determine how much of the drought was human-caused, scientists used what's called detection attribution. They compared preindustrial climate models of a climate not yet perturbed by humans to climate models that reflect current conditions. The difference in outcomes can be attributed to human influence.

"The U.S. Southwest and California are the case study for this," Tierney said, noting that the American Southwest has been in the grip of a megadrought for the last 20 years. It is the second driest megadrought in 1,200 years.

"But we also observe hot drought in the Mediterranean," Tierney said. "These are two hot spots that we've seen really bad droughts that we can attribute to humans."

Hot droughts will become more common in more places around the world, according to the Working Group 1 report. Places expected to be more prone to hot drought include Central America, the Amazon, Chile, Southwestern Australia and South Africa.

Another issue affecting water in the American Southwest is a decline in streamflow. For example, Colorado River water comes from snowpack, but as the world warms and less precipitation falls as snow, the snowpack declines. According to the new IPCC report, between 30 and 50% of the recent decline in streamflow in the Colorado River Basin can be attributed to humans.

The rest of the report covers topics such as the changing state of the climate system, human influence on the climate, and scenario-based projections of future climate, as well as ocean, cryosphere, and sea level change and more.