Commentary: Climate change and the aridification of North America — Jonathan Overpeck/Brad Udall

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Discussions of droughts and their impacts often center on the lack of precipitation, just as assessments of hydrologic impacts under a changing climate most often focus on how average precipitation in a given locale is likely to change in the future. Within climate science, however, focus has begun to include the growing role warming temperatures are playing as a potent driver of greater aridity: hotter climate extremes; drier soil conditions; more severe drought; and the impacts of hydrologic stress on rivers, forests, agriculture, and other systems. This shift in the hydrologic paradigm is most clear in the American Southwest, where declining flows in the region’s two most important rivers, the Colorado and Rio Grande, have been attributed in part to increasing temperatures caused by human activities, most notably the burning of fossil fuels. Warmer summers are also likely to reduce flows in the Columbia River, as well as in rivers along the Sierra Nevada in California. Now, an important study documents how warming is also causing flow declines in the northern Rocky Mountains and in the largest river basin in the United States, the Missouri. This work further highlights the mechanisms behind the temperature-driven river flow declines and 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.

Climate change is causing the Southwest to aridify. (Left) Since the 1930s, increasing temperatures have caused the percentage of precipitation going to evapotranspiration (ET) to increase at the expense of precipitation going to Colorado River flow, resulting in an unprecedented and still ongoing megadrought (shading) starting in 1999. (Right) Higher temperatures have already reduced Colorado River flow by 13%, and projected additional warming, assuming continued high emissions of greenhouse gases, will increase ET while reducing river flow even more through the 21st century. Data on Left are 20-y running means from ref. 5, and data on Right are calculated from Representative Concentration Pathways (RCP) 8.5 multimodel Coupled Model Intercomparison Project–Phase 5 (CMIP5) ensemble temperature increases projected for the Upper Colorado River Basin combined with temperature sensitivity of −9.3%/°C estimated by ref. 5, assuming no change in precipitation. Graphic credit: Jonathan Overpeck/Brad Udall

The work by Martin et al. on the temperature-driven flow reductions in the Upper Missouri River has broader implications. As they note, many aspects of river management could be increasingly impacted by a more arid river basin, including agricultural water deliveries, river management and navigation, and ecosystem services associated with the river; economies of a large region will likely suffer if the aridification continues. This mirrors the change occurring in the Southwest, where rivers provide the only large sustainable water supply to the region and more than 40 million water users, yet flows have already declined significantly since just the late 20th century. Across the US West, warming is also contributing to drier soils, widespread tree death, and more severe wildfires. The recent unprecedented drought conditions in California also have been tied to human-caused warming. 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.

Martin et al. also highlight how increasing temperature-driven aridity is more often framed in the West in terms of episodic drought. Just as in the Southwest, where an unprecedented drought began in 1999 and has continued through 2020 with drier-than-normal soils, reduced river flows, and low levels in major reservoirs, the worst drought of the instrumental era gripped the Upper Missouri River Basin between 2000 and 2010. This drought framing is also common among many water and land managers, as well as the public, and implicitly assumes an end to arid conditions must come with the return of rain and snow. However, anthropogenic climate change calls this assumption into question because we now know with high confidence that continued emissions of greenhouse gases into the atmosphere guarantee continued warming and that this continued warming makes more widespread, prolonged, and severe dry spells and drought almost a sure bet. This translates into an increasingly arid Southwest and West, with progressively lower river flows, drier landscapes, higher forest mortality, and more severe and widespread wildfires—not year on year, but instead a clear longer-term trend toward greater aridification, a trend that only climate action can stop.

In the West, declining river flows and soil moisture, as well as more severe wildfires and drought, are now a matter of record. The search for “why” starts with the increasing moisture-holding capacity of the atmosphere. As the atmosphere has warmed, the amount of water vapor in the atmosphere has gone up, consistent with the Clausius–Clapeyron relationship. This, in turn, has increased the demand for moisture by the atmosphere from the land surface and water bodies. Whereas the latter translates to increased evaporation, the former takes place primarily via greater evapotranspiration from soils and vegetation. Soils dry out in a straightforward manner understood by anyone gardening on a hot day, and they dry out faster the warmer it gets. Plants also lose more water to a hotter atmosphere through the small pores (stomata) in their leaves. Although plant physiologists have postulated that increased concentrations of carbon dioxide in the atmosphere help plants keep these pores less open and thus retain water, evidence now suggests that this water efficiency gain is often more than offset by greater plant growth owing to the same carbon dioxide increases. It makes sense that longer growing seasons enabled by warming temperatures mean more total evapotranspiration, drier soils, and reduced river flows.

Martin et al. showcase additional processes likely implicated in the observed aridification in the West. Warming is leading to more rain, rather than snow; a reduction in snowpack; and thus a reduction in surface reflectivity (albedo), further increasing warming, evaporation, and evapotranspiration. Others have focused on yet more ways warming can reduce river flows and soil moisture. Once there is too little water in soil and vegetation to continue evapotranspiration, solar radiation has a bigger warming impact on the atmosphere—the cooling effect of evaporation can no longer do its job, and excess heat can accumulate in ways that amplify the warming and the drying impact of the warming (15).

The impact of warming on the West’s river flows, soils, and forests is now unequivocal. Will higher precipitation save the day? It is true that precipitation is increasing in many regions outside the Southwest, but as in the past, there will always be dry spells and droughts throughout the West and across the High Plains, as well as in eastern North America (16). Importantly, during these dry periods, some lasting just weeks in the summer and some lasting years like the High Plains “Dust Bowl” drought of the 1930s, the full impact of warming temperatures will be fully felt without the relief of increased rainfall. The net result of these more frequent and severe hot–dry events translates into a climate that can manifest increasing aridity and extreme event impacts, particularly in summer, even if the mean annual climate paradoxically becomes wetter in response to anthropogenic climate change.

Increasing aridity is already a clear trend in the West, but greater aridity is also expanding its reach eastward with continued warming. Recent exceptional “flash droughts” in 2012 and 2017 on the High Plains of the United States and Canada, as well as the Dust Bowl drought of the 1930s, highlight how extreme spring and summer temperatures can speed the onset, and worsen the impact, of dry spells and droughts. Climate change farther east, in the Midwest, also means that summer dry spells will tend not only to become hotter but also to lengthen. It is no surprise that irrigated agriculture is expanding eastward in response to climate warming. 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 of the way to the East Coast of North America, north deep into Canada, and south into Mexico. It is no surprise that the boreal forest of Canada is starting to show a substantial increase in wildfire and a related net increase in carbon emissions to the atmosphere.

“Aridity” means many things to many people, but at its core it means extreme dryness of the kind that can have serious impacts on humans and the natural systems upon which they depend. Climate change, and in particular warming, will continue as long as humans burn fossil fuels or otherwise increase the concentrations of greenhouse gases in the atmosphere. This is known with high scientific confidence and means that the warming that is already driving an increase in aridity in many parts of North America is sure to continue and expand geographically until climate warming ceases. In the southwest United States and adjacent Mexico, the implications are dire for water security and ecosystems. More severe extreme heatwaves and dust storms are also already occurring, and these and other impacts of aridity will only increase until the cause is halted. Across North America, greater aridity is being offset with increased groundwater use, but this strategy has limits in the many places, such as the Southwest and the High Plains, where groundwater use exceeds recharge and is thus unsustainable.

Other parts of North America likely will not see the widespread aridification and decadal to multidecadal 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. Even in many places where mean annual precipitation is increasing, there will be an increase in aridity and the deleterious impacts that come with this increase. The good news is that we know the cause of expanding aridification. Unfortunately, climate change and this aridification are likely irreversible on human timescales, so the sooner emissions of greenhouse gases to the atmosphere are eliminated, the sooner the aridification of North America will stop getting worse.

COVID-19 Brings Extra Hazards To Wildfire Season — Aspen Public Radio

Firefighters work to contain the Ryan Fire in northern Colorado on Wednesday, Oct. 3, 2018. Photo credit: USFS via Firehouse.com

From Aspen Public Radio (Elizabeth Stewart-Severy):

Local governments, the U.S. Forest Service and the Bureau of Land Management each implemented fire restrictions in April in an attempt to prevent further taxing first responders and firefighters.

“The last thing we need right now, with COVID-19, is a wildfire,” [Valerie] MacDonald said, noting that crews have to congregate to effectively fight fires. If even one firefighter has the coronavirus, the disease could spread rapidly…

Wildland firefighters with the Upper Colorado River Fire Management Unit, a combined fire organization for the Forest Service and BLM that covers the White River National Forest, have made changes in light of the pandemic.

Lathan Johnson, a manager with UCR, said it is moving to radio briefings and virtual briefings. But wildland firefighting is a hazardous job, and trying to be mindful of social distancing guidelines adds an extra hurdle…

So far, the UCR unit has not had a positive case of COVID-19. The team is developing contingency plans and protocols to plan for cases. But there isn’t a system for testing in place and many firefighters are still living in bunkhouses…

Johnson said wildland firefighters across the country are working together to share information and best practices, mostly through a website called Wildland Fire Lessons Learned. Firefighters share experiences and ideas, such as a recent post about how a fire unit in Michigan responded to a crew member’s COVID-19 diagnosis by tracing contacts and encouraging exposed team members to get tested.

Wildland firefighting uses a national system that dispatches and moves resources — including firefighters and the specialized equipment they use — across the country to respond to the most dangerous and pressing fires. That presents an additional risk of potential exposure to COVID-19 as fire crews travel across the country and set up camps in new communities…

If a large wildfire caused evacuations in Pitkin County, MacDonald said the county would typically work with the American Red Cross to set up evacuation centers for affected residents. During the Lake Christine Fire, the Red Cross set up a shelter at Basalt High School.

But this year, “we wouldn’t do that,” said Courtney Strother, a disaster program manager for the Red Cross in western Colorado. Traditional evacuation shelters in gyms or large cafeterias make social distancing — and limiting the spread of disease — challenging.

In light of COVID-19, Strother said the best option is to evacuate residents to hotels. Red Cross volunteers across the region are reaching out to hotels to inquire about availability through the fire season…

It might also be possible to use dorms at nearby colleges or cabins at campgrounds that still provide private areas for evacuees. Strother said the Red Cross is also developing plans in case a congregation shelter such as a gym is the only option.

MacDonald said citizens need to do their own planning ahead of fire season, too.

“We definitely want people to take advantage of this time at home,” she said. “With all the extra time everyone’s got, be planning. Be working on their emergency plan: What are their evacuation routes? What would they take? What’s their communication plan with their family?”

People can also work on wildfire mitigation near their homes by clearing brush and other flammable materials.

But it’s a dry heat: #ClimateChange and the #aridification of North America — The University of Michigan

Map of the Missouri River drainage basin in the US and Canada. made using USGS and Natural Earth data. By Shannon1 – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=67852261

From the University of Michigan (Jim Erickson):

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.”

Graphic credit: Western Water Assessment