James Dinneen is a writer.
Some areas of California could become the new normal due to the extreme heat.
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Natural climate variability can include extremely bad droughts. In places most vulnerable to these extreme events, such as southwestern North America, human-caused global warming could change the weather in ways that make it worse.
This story is part of a series about the worst mega-dry spell in more than 1200 years.
A dry spell is something that ends. Benjamin Cook is an associate professor at Columbia University in New York. The return of wet years ended the megadroughts. What happens when normal ceases to be normal?
Cook and his colleagues looked at research on the global record of dry spells to identify common causes. They looked to studies that looked at natural records to identify the timing and severity of droughts before modern records.
The researchers found that since the beginning of the 20th century, there have been extremely long- lasting, severe or widespread droughts on all of the continents.
The researchers looked at how both natural climate processes and human-caused climate change might affect these extreme events in the future.
Warming is likely to increase the risk of megadrOUGHT in regions already vulnerable. Higher temperatures are the reason for this, not changes in precipitation.
Park Williams is a co-author of the research. Less water is left in waterways or the soil when the air is warmer.
Climate change accounted for about half of the severity of the current megadrought in southwestern North America according to previous work by Williams. Warming reduced the amount of precipitation falling as snow and dried out the soil, making it harder for the water in the mountains to be used.
La Nia is a pattern of sea surface temperature in the Pacific Ocean that can affect the climate in distant regions. Without the added effect of human-caused warming, the drought probably wouldn't have become a mega-drought.
Depending on greenhouse gas emissions, future warming might push things even further in some places, changing average temperature and precipitation to create a new climate normal called a "new climate normal".
Cook says that if you move into a new baseline with climate change, the wet years will be the same as the normal years. This is making us rethink how we think about the problem.
Under a moderate emissions scenario, southwestern North America, Australia, central and South-East Asia, the Mediterranean and western South America are at greatest risk of shifting to dry conditions by the end of this century. West and East Africa, South Asia and northern China have a small increased risk of megadrOUGHT.
The researchers pulled together data from 22 different climate models to project which parts of the country face the greatest risk.
Cook and her team wrote a review of the earth and environment.
Under a very high greenhouse gas emissions scenario, southern Africa, southwestern North America, western Europe and Australia would have a lot of water in the 21st century.
Cook says that we won't be able to go back to the climate of 2000 years. The system is being fundamentally changed and shifted.
Nature Reviews Earth and Environment was published in the journal.
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