In the week of October 12 to October 18, 49.7 percent of the country and 59.4 percent of the lower 48 states were in drought conditions, affecting nearly 135 million people according to the U.S. National Integrated Drought Information System as of October 27. A drought map can be seen below.
Droughts are not uncommon in some regions of the U.S., but for many the current situation is record-breaking. The southwest U.S. has been experiencing a “megadrought” for the past 22 years, and it is thought to be the most severe one in 1,200 years.
This winter, dry conditions may be set to continue in the south due to the La Niña weather pattern, in which Pacific Ocean surface temperatures periodically cool down. Typically, this pattern occurs once every three to five years or so, but in 2022 La Niña will occur for the third consecutive year, according to the National Oceanic and Atmospheric Administration (NOAA).
Jon Gottschalck, chief of the Operational Prediction Branch at the NOAA’s Climate Prediction Center, said in an NOAA report on October 20 that “parts of the Western U.S and southern Great Plains will continue to be the hardest hit [by drought] this winter”.
The ongoing dry conditions have caused record water-level lows in places along the Mississippi River in October. It dropped to minus-10.75 feet in Memphis, Tennessee—the lowest ever recorded in the city, according to National Weather Service data.
Water levels in Lake Mead—the country’s largest manmade reservoir that supplies water to around 25 million people across several states—have fallen so low that human remains have been found there.
Mathew Barlow, professor of climate science at the University of Massachusetts Lowell, told Newsweek: “Over the winter, the naturally occurring La Niña appears likely to prolong the drought in the Southwest through the southern plains. And the warmer temperatures associated with global warming will continue to stack the odds toward drought over much of that region as well.
“The impact of La Nina is unlikely to last beyond the winter but the global warming influence will continue and will continue to increase as long as we continue to burn fossil fuels. That is, the western U.S. is moving toward drier conditions overall with climate change, directly tied to the level of emissions.
“Estimates of the climate change contribution to the drought in the west over the 2000-2021 period range as high as 42 percent, so we’re talking about a very substantial influence for climate change.”
One way of presenting drought data is to use the Palmer Drought Severity Index (PDSI), which uses precipitation and temperature data to study moisture supply and demand. An example can be seen below.
“We often think of drought in terms of a period of reduced precipitation, but temperature can also play a role in terms of increasing evaporation, and PDSI accounts for both of those factors, although in a somewhat simplified way,” said Barlow.
“There are more direct measures of drought, like soil moisture content, river flows, et cetera, but that data tends to be much more limited in availability, spatial coverage, and period of record than precipitation and temperature. There are many different drought indices available but PDSI, while not perfect, has been shown to work fairly well in a wide range of contexts and can be estimated directly back to the 1800s, and so is one of the most commonly used.”
