AMERICA’S WESTERN states are burning—again. After an extremely dry 2020 and a spring heatwave, fires are blazing across the south-west. Lake Mead, the country’s largest reservoir, hit its lowest-ever water level on June 9th. Utah’s governor recently invited his constituents to “pray for rain”. Drought and wildfire are perennial characteristics of the American West, but in the past year conditions have worsened dramatically. Is the region experiencing a megadrought?

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The definition of megadrought varies, but it is usually understood to be a multi-decade period of intense aridity. Jason Smerdon, a paleoclimatologist at Columbia University’s Lamont-Doherty Earth Observatory, says the defining feature of a megadrought is its length. Because the current drought can be traced back to 2000, Mr Smerdon suggests it is an “emerging megadrought.” But that is not to understate its severity. A paper published in Science last year found that 2000-2018 was the second-driest 19-year period for the south-west in the past 1,200 years, exceeded only by a megadrought in the late 1500s.

There are two main reasons things look so dire. The first concerns natural weather patterns. La Niña events, when cooler surface temperatures in the Pacific Ocean push the polar jet stream north, tend to coincide with drought in the American south-west. Successive La Niña events are largely to blame for previous megadroughts, says Mr Smerdon. But it’s not just the south-west that is suffering. Hot and dry conditions have plagued the Pacific Northwest for the past year as well affecting states such as Oregon and Washington. Indeed, the most striking feature of the current drought may be its geographic spread. Whereas the drought in the 1500s was mostly concentrated in the Four Corners region—where Arizona, Colorado, New Mexico and Utah meet—today’s drought encompasses most of the area west of the continental divide, most of which runs along the Rockies. The United States Drought Monitor estimates that as of June 10th 97% of the region was at least “abnormally dry”, and 55% was experiencing “extreme” or “exceptional” drought (see map).

Climate change is also to blame. The water that fills the country’s biggest reservoirs comes from melting snowpack, which is delivered to Lake Mead, for example, via the Colorado River. But average snowpack across the West has been decreasing for decades as rising carbon-dioxide emissions have led to warmer winters. Dan McEvoy, a climatologist at the Western Regional Climate Centre in Reno, Nevada, observed record melt rates in the first two weeks of April throughout the West. Snowpack in California was at 0% of normal as of June 1st—it had all melted four to five weeks ahead of time.

The effects of such a climate cocktail can be wide-ranging. Severe drought can decrease streamflows and increase river temperatures. This can harm local species, deprive forests and soils of nutrients, and heighten the risk and severity of wildfires. Officials across the region fret that a long and dangerous fire season looms. Some states and counties are already imposing water restrictions.

Questions about water scarcity have long been central to the region. When white settlers were lured west in the 19th century it was, in part, by promises of irrigable farmland—though the land they would attempt to tame was mountainous and arid. Writing in 1980, Wallace Stegner, a novelist, warned that “the history of the West until recently has been a history of the importation of humid-land habits…into a dry land that will not tolerate them”. Climate change has only upped the stakes. The amount of water available to westerners may be shrinking, but eight of the ten fastest-growing states in the last decade are west of the Mississippi River.