Click here to read the report. Here’s the abstract:
Across the Upper Missouri River Basin, the recent drought of 2000 to 2010, known as the “turn-of-the-century drought,” was likely more severe than any in the instrumental record including the Dust Bowl drought. However, until now, adequate proxy records needed to better understand this event with regard to long-term variability have been lacking. Here we examine 1,200 y of streamflow from a network of 17 new tree-ring–based reconstructions for gages across the upper Missouri basin and an independent reconstruction of warm-season regional temperature in order to place the recent drought in a long-term climate context. We find that temperature has increasingly influenced the severity of drought events by decreasing runoff efficiency in the basin since the late 20th century (1980s) onward. The occurrence of extreme heat, higher evapotranspiration, and associated low-flow conditions across the basin has increased substantially over the 20th and 21st centuries, and recent warming aligns with increasing drought severities that rival or exceed any estimated over the last 12 centuries. Future warming is anticipated to cause increasingly severe droughts by enhancing water deficits that could prove challenging for water management.
In much of the western United States (hereafter “the West”), water demand (i.e., the combination of atmospheric demands, ecological requirements, and consumptive use) is approaching or has exceeded supply, making the threat of future drought an increasing concern for water managers. Prolonged drought can disrupt agricultural systems and economies, challenge river system control and navigation, and complicate management of sensitive ecological resources. Recently, ample evidence has emerged to suggest that the severity of several regional 21st-century droughts has exceeded the severity of historical drought events; these recent extreme droughts include the 2011 to 2016 California drought and the 2000 to 2015 drought in the Colorado River basin.
Conspicuously absent thus far from investigations of recent droughts has been the Missouri River, the longest river in North America draining the largest independent river basin in the United States. Similar to California and the Upper Colorado River Basin, parts of the early 21st century have been remarkably dry across the Upper Missouri River Basin (UMRB). In fact, our assessment of streamflow for the UMRB suggests that the widespread drought period of 2000 to 2010, termed the “turn-of-the-century drought” by Cook et al., was a period of observationally unprecedented and sustained hydrologic drought likely surpassing even the drought of the Dust Bowl period.
Northern Hemisphere summer temperatures are now likely higher than they have been in the last 1,200 y, and the unique combination of recent anomalously high temperatures and severe droughts across much of the West has led numerous researchers to revisit the role of temperature in changing the timing and efficiency of runoff in the new millennium. Evidence suggests that across much of the West atmospheric moisture demands due to warming are reducing the effectiveness of precipitation in generating streamflow and ultimately surface-water supplies.
The waters of the Upper Missouri River originate predominantly in the Rocky Mountains of Montana, Wyoming, and Colorado, where high-elevation catchments capture and store large volumes of water as winter snowpack that are later released as spring and early summer snowmelt. This mountain water is an important component of the total annual flow of the Missouri, accounting for roughly 30% of the annual discharge delivered to the Mississippi River on average, but ranging between 14% to more than 50% from year to year, most of which is delivered during the critical warm-season months (May through September). Across much of the UMRB, cool-season (October through May) precipitation stored as winter snowpack has historically been the primary driver of streamflow, with observed April 1 snow-water equivalent (SWE) usually accounting for at least half of the variability in observed streamflow from the primary headwaters regions. However, since the 1950s, warming spring temperatures have increasingly driven regional snowpack declines that have intensified since the 1980s. By 2006, these declines amounted to a low snowpack anomaly of unusual severity relative to the last 800 y and spanned the snow-dominated watersheds of the interior West. A recent reassessment of snowpack declines across the West by Mote et al. suggests continued temperature-driven snowpack declines through 2016 totaling a volumetric storage loss of between 25 and 50 km3, which is comparable to the storage capacity of Lake Mead, the United States’ largest reservoir.
Here we examine the extended record (ca. 800 to 2010 CE) of streamflow and the influence of temperature on drought through the Medieval Climate Anomaly, with a focus on the recent turn-of-the-century drought in the UMRB. The role of increasing temperature on streamflow and basin-wide drought is examined in the UMRB over the last 1,200 y by analyzing a basin-wide composite streamflow record developed from a network of 17 tree-ring–based reconstructions of streamflow for major gages in the UMRB (Fig. 1) and an independent runoff-season (March through August) regional temperature reconstruction. We also explore the hydrologic implications (e.g., drought severity and spatial extent) and climatic drivers (temperature and precipitation) of the observed changes in streamflow across the UMRB and characterize shifts in the likelihood of extreme flow levels and reductions in runoff efficiency across the basin.
From The Washington Post (Darryl Fears):
For the first decade of the century, the Upper Missouri River Basin was the driest it’s been in 1,200 years, even more parched than during the disastrous Dust Bowl of the 1930s, a new study says.
The drop in water level at the mouth of the Missouri — the country’s longest river — was due to rising temperatures linked to climate change that reduced the amount of snowfall in the Rocky Mountains in Montana and North Dakota, scientists found.
The basin has continued to experience droughts this decade — in 2012, 2013 and 2017 — but their severity in comparison with historic drought is unknown. The “Turn-Of-The-Century Drought” study, published Monday in the Proceedings of the National Academy of Sciences, focused only on the 10 years after 2000.
“In terms of the most severe flow deficits, the driest years of the Turn-Of-The-Century-Drought in the [Upper Missouri River Basin] appear unmatched over the last 1,200 years,” the study said. “Only a single event in the late 13th century rivaled the greatest deficits of this most recent event.”
Researchers familiar with drought of this magnitude in the dry Southwest were surprised to find it in the Midwest…
“These findings show that the upper Missouri Basin is reflecting some of the same changes that we see elsewhere across North America, including the increased occurrence of hot drought” that’s more severe than usual, [Erika] Wise said.
The study is the latest to show how human-influenced climate change threatens to reshape the landscape by making naturally occurring drought far more severe.
