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SF Special: Will a Megadrought Slam the U.S. as Soon as 2050?

When the most recent drought, which devastated California for more than five years, ended this winter with record rainfall, the impact of the event was unprecedented in U.S. history. The drought itself was determined to be the worst dry spell that state had suffered as a result of record hot temperatures. All told, the effects of this drought were estimated at over $5 billion. In 2015 alone, the California drought cost agriculture $1.8 billion.

This weather event is child’s play, however, compared with a potential megadrought predicted to hit the country at the end of this century.

For a comparison, the 10-year Dust Bowl drought, which desiccated the High Plains during the Great Depression, doesn’t even qualify to be classified as a megadrought!

“What these results (climate-model studies) are saying is that we’re going to get a drought similar to those events, but it will probably last at least 30 to 35 years,” warns Benjamin Cook, a climate scientist at NASA’s Goddard Institute for Space Studies. Cook was part of an extensive research team that includes scientists from Cornell University, the Lamont-Doherty Earth Observatory of Columbia University, and the NASA Goddard Institute for Space Studies.

cook-drought-model
Columbia University

The map above shows the findings of the climate models the team generated. The map projects drought conditions in the year 2090. Brown shadings show soil that’s drier than the 20th century average at 30 centimeters (just less than 1 foot) below the surface. Blue shadings show wetter-than-average soil at the same depth. Note that this predicted drought would not only affect the southwest quarter of the U.S., as well as all of Mexico – where it would be the most intense – but would also spread into Southern states as far east as Florida and invade parts of the Midwest.

The map and related research from the research team is the most extensive reconstruction of massive past drought events (see below). In the study, researchers used 17 computer models of droughts and three models of soil moisture for their predictions. After the researchers determined a high degree of agreement among the models, they applied the models to data gathered from tree rings going back to the year 1000.

A drought that would eclipse anything we’ve ever seen

The researchers concluded from their works that such a megadrought is highly likely to happen as soon as 2050 as the result of greenhouse gas emissions. Such a dry period would be devastating beyond description and certainly worse than anything the U.S. has ever experienced, Cook claims.

“Even when selecting for the worst megadrought-dominated period (from the past), the 21st century projections make the droughts from the past seem like quaint walks through the Garden of Eden,” says study coauthor Jason E. Smerdon of Columbia University’s Lamont-Doherty Earth Observatory.

The most poignant drought to afflict recent history, the Dust Bowl, affected 100 million acres around the Texas and Oklahoma panhandles and adjacent lands in Kansas, Colorado, and New Mexico. The dry period created dust storms that spread across the High Plains and deposited dust as far east as New York and Washington. It is estimated by historians that the Dust Bowl drought event caused more than 500,000 people to be homeless and 3.5 million refugees to move west to try to find work. In just one year alone, 1935, it was estimated that 850,000,000 tons of topsoil was blown off the Southern Plains.

What is a megadrought?

By definition, a megadrought is a prolonged drought that lasts two decades or longer. The issue isn’t if a megadrought will reappear, but, rather, when it will happen and how severe such an extended dry period could be.

The NASA/Columbia University/Cornell University research team estimates the current chance of a megadrought hitting the U.S. is more than 60% if greenhouse gas emissions level off by the middle of the 21st century. However, if greenhouse emissions continue to increase in the world along current trends throughout this century, there is more than an 80% likelihood of a megadrought occurring between the years 2050 and 2099.

Megadroughts from the past

Whether you accept global warming or not really doesn’t matter when it comes to the existence of megadroughts. Several such events have already occurred in the U.S., explains Toby Ault of Cornell University. Ault led the team that also researched future potential severe droughts.

Evidence from tree rings shows that the American Southwest suffered abnormally dry periods during the Middle Ages (900 to 1300 AD) as illustrated below.

medieval-droughts
E.R. Cook of Earth-Science Reviews

That 400-year period, sometimes called the Medieval Megadrought, doesn’t represent one long drought, but rather a series of massive regional droughts centering on the years 936, 1034, 1150, and 1253.

Climatologists call this entire period the Medieval Warm Period. During this time, the incidence of wildfires increased and the sand dunes in the Great Plains mobilized, reports University of Nebraska researchers Song Feng and R.J. Oglesby.

Nebraska’s Sand Hills Are Evidence

nebraska-sand-hills
NASA
The sand dunes (shown at right) the researchers refer to are now the grass-covered Sand Hills of Nebraska, an area covering 19,300 square miles, or one third of that state. Some of the dunes in this area are 400 feet high and up to 20 miles long. Paleoclimate data reveals the Nebraska Sand Hills were active during the Medieval Megadrought. 

What is troubling is that the Medieval Megadrought that caused the Sand Hills to be active was nothing compared with a dry period that devastated Mexico from 1540 to 1589. This was the most severe sustained dry period in North American history. It has been estimated that the impact of that drought caused the loss of 15 million people in Mexico.

How it Was Determined the Medieval Megadroughts Ever Happened

Of course, the Medieval Megadrought happened long before there were any weather records. Data has only been recorded for the past 150 years.

Past massive drought events have been determined by the effects they had on trees living at the time, as mentioned before. Those trees can be found as stumps often covered with water and residing in river valley bottoms in lakes in the Sierra Nevada mountains. Ancient tree stumps populate margins of famous lakes such as Mono Lake in California.

In the 1990s, University of California scientist Scott Stine began using carbon dating to determine when those trees were living and found that they pretty much all grouped in the medieval period. As mentioned before, Nebraska’s Sand Hills also provide evidence of the Medieval Megadrought. 

Not everything is gloom and doom when it comes to the predicted megadrought at the end of this century, however. Its likelihood will be greatly affected by increasing greenhouse gas emissions. If emissions are curtailed, so are the chances of the U.S. Southwest suffering from a megadrought.

“I am optimistic that we can cope with the threat of a megadrought in the future because it doesn’t mean no water. It means significantly less water than we are used to,” says Cornell’s Ault.

Predicting the impact of a Megadrought

Noble Prize winner Jonathan Overpeck isn’t so positive. When he was at the University of Arizona, Overpeck and colleague Connie Woodhouse invented the word “megadrought” in the 1990s. Overpeck was the lead author on the 2007 and the 2010-2014 climate assessments that was published by the Intergovernmental Panel on Climate Change authorized by Congress.

A “searing megadrought” is what Overpeck fears could strike an area stretching from California in the west, Iowa to the east, and Louisiana in the south. Such an event would lead to dust storms across the region with the majority of trees perishing. Agriculture would become all but impossible in this drought area.

Furthermore, a megadrought, Overpeck predicts, would lead to major water shortages that would cause vegetation to dry up causing massive wildfires across the Southwest.

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