Medieval megadroughts that lasted for decades may soon return to American Southwest due to climate change
Decades-long droughts, known as megadroughts—which occurred in the American Southwest between the 9th and 15th centuries—could reappear soon as the climate warms, scientists warn.
According to experts, it is critical to understand the causes of past megadroughts as it can help scientists predict whether, how, and where they might happen in the future. Over a dozen megadroughts struck the American Southwest during the 9th through the 15th centuries, but then they mysteriously ceased around the year 1600. What caused this clustering of megadroughts - that is, severe droughts that last for decades - and why do they happen at all? Scientists now have answers to these questions.
The team - comprising researchers from Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, and NASA Goddard Institute for Space Studies, New York - found that the medieval megadroughts were due to a combination of multiple features - atmospheric warming, which resulted from Earth absorbing more sunlight than it radiated back into space, as well as unusual ocean temperature changes caused by El Niño. The study suggests that these phenomena from a bygone era are likely to reappear soon.
“This assessment of past megadroughts provides the first comprehensive theory for the causes of megadroughts and their clustering, particularly during the Medieval era. This work also provides the first paleoclimatic support for the prediction that the risk of American Southwest megadroughts will markedly increase with global warming,” says the study published in the journal Science Advances.
According to the National Weather Service (NWS) in the US, the El Niño-Southern Oscillation (ENSO) is a climate pattern that involves changes in the temperature of waters in the central and eastern tropical Pacific Ocean. “On periods ranging from about three to seven years, the surface waters across a large swath of the tropical Pacific Ocean warm or cool by anywhere from 1°C to 3°C, compared to normal. This oscillating warming and cooling pattern, referred to as the ENSO cycle, directly affects rainfall distribution in the tropics and can have a strong influence on weather across the US and other parts of the world. El Niño and La Niña are the extreme phases of the ENSO cycle; between these two phases is a third phase called ENSO-neutral,” says NWS.
Based on their research, the authors hypothesize that atmospheric warming due to greenhouse gas emissions, combined with future ENSO events, may be responsible for the drying trend in the Southwest dating to the 1980s and will probably lead to megadroughts in the near future. “If the past is a useful guide to the next few decades, then megadroughts could also arise through unforced and unpredictable ENSO variability,” says the study.
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The researchers used a first-of-its-kind global reconstruction of aquatic climate data and sea surface temperatures spanning the last 2,000 years to identify 14 megadroughts in the American Southwest, all of which occurred before the year 1600.
They found that each medieval megadrought can be explained by frequent cold dips in Pacific Ocean surface temperature, warming in the North Atlantic Ocean and atmospheric warming due to increases in net absorbed sunlight - a conclusion that brings together previous, separate theories about megadrought origins during this era. By assessing the relative importance of each climate driver, they determined that La Niña has the greatest influence on megadrought conditions in the region, although climate cycles affecting sea surface temperatures can also cause them to occur.
“Three key factors led to megadroughts in the American Southwest: radiative forcing, severe and frequent La Niña events - cool tropical Pacific sea surface temperatures that cause changes to global weather events - and warm conditions in the Atlantic. High radiative forcing appears to have dried out the American Southwest, likely due to an increase in solar activity (which would send more radiation toward us) and a decrease in volcanic activity (which would admit more of it) at the time. The resulting increase in heat would lead to greater evaporation. At the same time, warmer than usual Atlantic sea-surface temperatures combined with very strong and frequent La Niñas decreased precipitation in the already dried-out area. Of these three factors, La Niña conditions were estimated to be more than twice as important in causing the megadroughts,” say the findings.
During the time of the medieval megadroughts, increased radiative forcing was caused by natural climate variability. But today, say the researchers, many locations around the globe are experiencing increased dryness due to human-made forces. They explain because of an increase in the baseline aridity, if there is a big La Niña in the future, or several of them in a row, it could lead to megadroughts in the American West. "While El Niño and La Niña activity remain challenging to simulate, human-driven climate change is stacking the deck towards more megadroughts in the future through greater aridity,” add the researchers.