Exceptionally bad droughts – or megadroughts – are part of natural climate variability. Yet in places most vulnerable to these extreme events, such as southwestern North America, human-caused global warming could shift background temperatures and precipitation such that drought becomes the new normal.
Parched Earth: Megadrought in North America
This story is part of our series about the ongoing megadrought in southwestern North America, the worst such drought in more than 1200 years.
A drought is, by definition, something that comes to an end. “A drought is a temporary period of below-normal water availability,” says Benjamin Cook at Columbia University in New York. Even megadroughts that lasted for centuries eventually ended with the return of wetter years. But what happens when normal itself changes?
Cook and his colleagues looked at research on the global record of drought over the past 2000 years to identify common patterns and causes of megadroughts. They looked to studies that had examined distinctive patterns left in tree rings, lake sediments, stalagmites and other natural records to identify the timing and severity of droughts prior to modern records.
The researchers found exceptionally long-lasting, severe or widespread droughts have played out on every continent except for Antarctica in the past two millennia.
With the context provided by this long-term record, the researchers then considered how both natural climate processes and human-caused climate change might influence these extreme events in the future, synthesising results from different climate modelling studies.
They found that regions already vulnerable to megadrought under natural climate variability are likely to see this risk increase with warming. This is mostly due to higher temperatures, rather than expected changes in overall precipitation.
“As you warm the air, the air’s demand for water increases,” says Park Williams at the University of California, Los Angeles, a co-author of the research. Warmer air can hold more moisture, meaning less water is left in waterways or the soil.
In previous work, Williams and his colleagues found that human-caused climate change accounted for about 46 per cent of the severity of the current megadrought in southwestern North America. They concluded that warming contributed to the drought mainly by drying out soil and reducing the amount of precipitation falling as snow, leaving less snowpack to provide a reservoir of water for the drier, warmer months.
They found the rest of the drought was mainly driven by the recurring pattern of sea surface temperature in the Pacific Ocean known as La Niña, which can interact with the atmosphere to affect the climate in distant regions. Without the added effect of human-caused warming, however, the drought probably wouldn’t have become a megadrought, says Park.
Depending on greenhouse gas emissions, future warming might push things even further in some places, changing average temperature and precipitation to create a sort of perpetual drought – what the researchers call a “new climate normal”.
“If you move into a brand-new baseline with climate change, where even the wet years are just equal to the normal years in the past, then by that definition you’re kind of permanently in drought,” says Cook. “This is challenging the way we think about drought.”
Under a moderate emissions scenario, the researchers found the regions at highest risk of shifting to drier conditions by the end of this century are southwestern North America, Australia, central and South-East Asia, the Mediterranean and western South America – where there is a megadrought currently. West and East Africa, South Asia and northern China would see a small increased risk of megadrought as well.
Under a very high greenhouse gas emissions scenario, modelling by Samantha Stevenson at the University of California, Santa Barbara, and her colleagues found that southern Africa, southwestern North America, western Europe and Australia would have megadrought-like conditions for much of the 21st century.
“We’re not going to be able to go back to the climate of the past 2000 years,” says Cook. “We are fundamentally changing the whole system and shifting it.”
Journal reference: Nature Reviews Earth & Environment, DOI: 10.1038/s43017-022-00329-1
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