But it's a dry heat: Climate change and the aridification of North America

May 19, 2020

Discussions of drought often center on the lack of precipitation. But among climate scientists, the focus is shifting to include the growing role that warming temperatures are playing as potent drivers of greater aridity and drought intensification.

Increasing aridity is already a clear trend across the western United States, where anthropogenic climate warming is contributing to declining river flows, drier soils, widespread tree death, stressed agricultural crops, catastrophic wildfires and protracted droughts, according to the authors of a Commentary article published online May 19 in Proceedings of the National Academy of Sciences.

At the same time, human-caused warming is also driving increased aridity eastward across North America, with no end in sight, according to climate scientists Jonathan Overpeck of the University of Michigan and Bradley Udall of Colorado State University.

"The impact of warming on the West's river flows, soils, and forests is now unequivocal," write Overpeck, dean of the U-M School for Environment and Sustainability, and Udall, senior water and climate scientist at Colorado State. "There is a clear longer-term trend toward greater aridification, a trend that only climate action can stop."

The Commentary article responds to a PNAS paper, published May 11 by Justin Martin of the U.S. Geological Survey and his colleagues, that showed how warming is causing streamflow declines in the northern Rocky Mountains, including the nation's largest river basin, the Missouri.

The Martin et al. study used tree-ring records to analyze the 2000-2010 Upper Missouri River Basin drought and concluded that "recent warming aligns with increasing drought severities that rival or exceed any estimated over the last 12 centuries."

The study details the mechanisms of temperature-driven streamflow declines, and it "places more focus on how anthropogenic climate warming is progressively increasing the risk of hot drought and more arid conditions across an expanding swath of the United States," according to Overpeck and Udall.

The Martin et al. study also highlights the way temperature-driven aridity in the West is typically framed in terms of episodic drought. Many water and land managers, as well as the general public, implicitly assume that when returning rains and snowfall break a long drought, arid conditions will also fade away.

But that's a faulty assumption, one that ignores mounting evidence all around us, according to Overpeck and Udall.

"Anthropogenic climate change calls this assumption into question because we now know with high confidence that continued emissions of greenhouse gases into the atmosphere guarantees continued warming, and that this continued warming makes more widespread, prolonged and severe dry spells and droughts almost a sure bet," they write. "Greater aridity is redefining the West in many ways, and the costs to human and natural systems will only increase as we let the warming continue."

Anticipated impacts in the Upper Missouri River Basin mirror changes already occurring in the Southwest, where the trend toward warming-driven aridification is clearest.

Rivers in the Southwest provide the only large, sustainable water supply to more than 40 million people, yet flows have declined significantly since the late 20th century. Declining flows in the region's two most important rivers, the Colorado and the Rio Grande, have been attributed in part to increasing temperatures caused by human activities, most notably the burning of fossil fuels.

Multiple processes tied to warming are likely implicated in the observed aridification of the West, according to Overpeck and Udall. For starters, warmer air can hold more water vapor, and this thirsty air draws moisture from water bodies and land surfaces through evaporation and evapotranspiration--further drying soils, stressing plants and reducing streamflow.

But the atmosphere's increased capacity to hold water vapor also boosts the potential for precipitation; rain and snow amounts are, in fact, rising in many regions of the United States outside the Southwest. However, the frequency and intensity of dry spells and droughts are expected to increase across much of the continent in coming decades, even if average annual precipitation levels rise, according to Overpeck and Udall.

"Perhaps most troubling is the growing co-occurrence of hot and dry summer conditions, and the likely expansion, absent climate change action, of these hot-dry extremes all the way to the East Coast of North America, north deep into Canada, and south into Mexico," they write.

"Other parts of North America likely won't see the widespread aridification and decadal to multi-decadal droughts of the West, but will nonetheless continue to see more frequent and severe arid events--extreme dry spells, flash droughts and interannual droughts will become part of the new normal," according to Overpeck and Udall.

"Unfortunately, climate change and this aridification are likely irreversible on human time scales, so the sooner emissions of greenhouse gases to the atmosphere are halted, the sooner the aridification of North America will stop getting worse."
Study: Climate change and the aridification of North America

University of Michigan

Related Climate Change Articles from Brightsurf:

Are climate scientists being too cautious when linking extreme weather to climate change?
Climate science has focused on avoiding false alarms when linking extreme events to climate change.

Mysterious climate change
New research findings underline the crucial role that sea ice throughout the Southern Ocean played for atmospheric CO2 in times of rapid climate change in the past.

Mapping the path of climate change
Predicting a major transition, such as climate change, is extremely difficult, but the probabilistic framework developed by the authors is the first step in identifying the path between a shift in two environmental states.

Small change for climate change: Time to increase research funding to save the world
A new study shows that there is a huge disproportion in the level of funding for social science research into the greatest challenge in combating global warming -- how to get individuals and societies to overcome ingrained human habits to make the changes necessary to mitigate climate change.

Sub-national 'climate clubs' could offer key to combating climate change
'Climate clubs' offering membership for sub-national states, in addition to just countries, could speed up progress towards a globally harmonized climate change policy, which in turn offers a way to achieve stronger climate policies in all countries.

Review of Chinese atmospheric science research over the past 70 years: Climate and climate change
Over the past 70 years since the foundation of the People's Republic of China, Chinese scientists have made great contributions to various fields in the research of atmospheric sciences, which attracted worldwide attention.

A CERN for climate change
In a Perspective article appearing in this week's Proceedings of the National Academy of Sciences, Tim Palmer (Oxford University), and Bjorn Stevens (Max Planck Society), critically reflect on the present state of Earth system modelling.

Fairy-wrens change breeding habits to cope with climate change
Warmer temperatures linked to climate change are having a big impact on the breeding habits of one of Australia's most recognisable bird species, according to researchers at The Australian National University (ANU).

Believing in climate change doesn't mean you are preparing for climate change, study finds
Notre Dame researchers found that although coastal homeowners may perceive a worsening of climate change-related hazards, these attitudes are largely unrelated to a homeowner's expectations of actual home damage.

Older forests resist change -- climate change, that is
Older forests in eastern North America are less vulnerable to climate change than younger forests, particularly for carbon storage, timber production, and biodiversity, new research finds.

Read More: Climate Change News and Climate Change Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.