Drought is an easily identifiable weather phenomenon that refers to a period when an are or region experiences below-normal precipitation. As the climate heats up, droughts are expected to become more frequent and severe in some locations. The lack of adequate precipitation can result in many things such as diminished streamflow, reduced soil moisture or groundwater, damaged crops, and a general water shortage.
Unlike with sudden weather events such as hurricanes, tornadoes, and thunderstorms, it is often difficult to know when a drought has started or when it has ended. Nonetheless, it would still have severe impacts not only on our environment but also on our economy. For instance, nationwide losses from the US drought of 1988 exceeded $40 billion - more than the losses caused by the San Francisco earthquake in 1989, the Mississippi River floods of 1993, and Hurricane Andrew in 1992.
But, droughts are just the tip of the iceberg. Megadroughts, droughts that occur on an extreme scale and for much longer than a drought would usually last, is now happening in the Western US. This isn’t the first time a weather phenomenon like this happened. Previous reports revealed that megadroughts occurred during the medieval times in south-west America, between the 9th and 16th Centuries. And, they lasted for years.
Research published in the science journal Science Advances showed what causes the megadroughts using ice samples, sea beds, and tree rings. The researchers identified three reasons: cooling water temperatures in the Pacific Ocean, warming water in the Atlantic Ocean, and radiative forcing. According to BBC, an operational business division of the British Broadcasting Corporation responsible for the gathering and broadcasting of news and current affairs, radiative forcing occurs when the Earth's atmosphere traps more energy from the Sun.
Just like droughts, megadroughts are very difficult to predict. But an important indicator is La Niña, a weather phenomenon that cools the temperature of the water in the Pacific Ocean. It can cause droughts by pushing very wet weather storms to certain areas. While this can cause floods, it would leave others without rain for very long periods of time. "It certainly looks like with warmer temperatures that we're going to see more megadroughts, but we can't predict precisely how many or how big they'll be,” Peter Fawcett, a paleoclimatologist at the University of New Mexico, said.
38% More Severe
Climate scientists have long debated that the Western US has been in the midst of a megadrought for 19 years and counting. This is mainly due to the increased wildfires but a huge decline in groundwater supply. As a result, there’s been an invasion of destructive bark beetles and lower precipitation, which dries out lakes and rivers.
Experts have seen a difference between the previous and current megadroughts. Before, only a few parts of the West were dry with the surrounding area had fairly normal conditions. Today, there are also only a few dry parts but it is also kind of dry across a huge area. Park Williams from Columbia's Lamont-Doherty Earth Observatory said that this may be a signal of global warming.
According to Phys.org, an internet news portal that provides the latest news on science, the researchers used climate models to measure global warming's fingerprints on the West's megadrought. They estimated what temperatures and precipitation would have been like in the absence of global warming, then subtracted the average of those projections from what was actually measured. The results revealed that climate change likely made the drought 38% more severe.
Climate change, as Williams said, "caused what would have been a fairly severe drought to become a drought as severe as the most severe droughts of the last millennium." The team also reconstructed drought conditions in the American West over the past 1,200 years using tree ring records. They found out that "the drought severity of the last 19 years is almost as bad as the worst 19-year period of the worst megadrought.”
The Worst Megadrought in the Western US
A recent study published in Science argues that the drought conditions experienced in the western US since 2,000 are the start of a megadrought equal to the worst the US has experienced in more than 1,200 years. Williams said that this megadrought is “a drought bigger than what modern society has seen.”
According to Time, an American weekly news magazine and news website, University of Michigan environment dean Jonathan Overpeck called this study “the first observed multidecadal megadrought in recorded US history.” The researchers examined 1,200 years of tree ring data, dozens of climate models, and modern weather observations. Tree ring data allowed them to infer yearly soil moisture for centuries before humans began influencing climate. The study covered an area stretching across nine US states, which include Oregon, Montana, California, New Mexico, and others.
The researchers compared four megadroughts that have extreme aridity lasting decades: the late 800s, the mid-1100s, the 1200s, and the late 1500s with observed weather in the 19 years from 2000 to 2018. They found out that the current drought is already outdoing the three earliest ones, which have led to massive drops in lake levels throughout the western US, most notably in Lake Powell and Lake Mead. These conditions have also caused increasing wildfire activities and the need to rely on deep underground aquifers for water.
According to Forbes, a global media company focusing on business, investing, technology, entrepreneurship, leadership, and lifestyle, the worst megadrought occurred between 1575 to 1603, which lasted 28 years and was followed by an abnormally wet period once again. The researchers revealed that the current drought matches the severity of this one. "Because the background is getting warmer, the dice are increasingly loaded toward longer and more severe droughts. We may get lucky, and natural variability will bring more precipitation for a while. But going forward, we'll need more and more good luck to break out of drought, and less and less bad luck to go back into drought,” Williams said.
Daniel Swain, a UCLA climate scientist who wasn’t part of the study, called the research important because it provides evidence “that human-caused climate change transformed what might have otherwise been a moderate long-term drought into a severe event comparable to the ‘megadroughts’ of centuries past.”