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Exceptionally severe and persistent droughts 鈥� or megadroughts 鈥� have wreaked havoc on human societies for thousands of years. A megadrought in northern China between AD 1627 and 1643, for instance, caused a famine that killed an estimated 20 million people and may have sparked a revolt that toppled the Ming Dynasty.

Today, there are two megadroughts underway. One is in south-western North America and has been going on for 22 years. The other is in Chile and Argentina, where the years since 2008 have been among the driest of the past millennium.

Megadroughts vary widely in terms of their duration, severity and extent, but like hurricanes or ocean currents, these events aren鈥檛 entirely random. They are influenced by Earth鈥檚 larger climate system, including both natural processes and, more recently, human-caused climate change.

Using climate models and records of past droughts, researchers are starting to make sense of the complex links between the climate and these extreme dry periods.

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How do we know about droughts in the distant past?

Researchers identify droughts that occurred prior to modern records by looking at 鈥渘atural archives鈥� like tree rings, lake sediments and stalagmites, which respond to wet and dry years in predictable ways, says at Columbia University in New York. During dry years, tree rings tend to be narrower, for instance. In stalagmites built by dripping rainwater, isotopes preserved in the layers provide signatures of what the climate was like when that rain fell.

These kinds of natural records can provide a reliable view of when a drought occurred and how severe it was, though there are significant gaps. 鈥淭here are whole parts of the globe where we don鈥檛 have data, so we don鈥檛 know,鈥� says at the University of Arizona. These gaps are especially large in tropical regions. Trees there often lack visible growth rings, says Belmecheri, and palaeoclimate research has been concentrated in the Global North.

The palaeoclimate records that we do have show that megadroughts have occurred on every continent except Antarctica in the past two millennia, running from events lasting a few years to dry spells lasting for centuries, according to a recent by Cook, Belmecheri and their colleagues.

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Are megadroughts a normal part of climate variation?

The prevalence of megadrought in the record demonstrates that such extreme events are part of the natural variability of the climate, says at the University of California, Los Angeles. Megadroughts are effectively built into the climate in many regions.

Researchers have used models to test how variables like ocean temperature or the odd volcanic eruption can impact broader climate trends including megadroughts, and how these correspond with physical records. 鈥淭here are a few hypotheses, some of which are better than others,鈥� says Cook.

In several regions, Cook says the clearest driver of megadrought is changing surface temperature in the oceans, especially in the tropical Pacific. 鈥淲hen you get changes in the tropical ocean, those changes can affect the atmosphere, and those changes can propagate around the world,鈥� he says.

The past and current megadroughts in North and South America, for example, are associated with the recurring pattern of surface temperature in the Pacific known as La Ni帽a, which occurs when strong east to west winds known as trade winds blow warm water off the coast of South America west towards Indonesia. As warm waters accumulate in the west, cold water wells up along the coast of South America. This cold water cools the atmosphere above it, creating a high-pressure area further north. This in turn shifts the jet streams that circle the globe from west to east in both hemispheres, and that pushes water-bearing storms towards the poles and diverts precipitation from some areas.

The cooler temperatures also affect enormous atmospheric waves of high and low pressure called Rossby waves: they trigger particular patterns that can lead to drought or flooding in faraway places , says at the National Center for Atmospheric Research in Colorado.

These climatological relationships between distant regions are known as teleconnections. Other La Ni帽a teleconnections include a drying influence on the South Asian monsoon and a wetting influence on Australia and South-East Asia. A pattern of warmer temperatures in the tropical Pacific 鈥� an event known as El Ni帽o 鈥� has drought-inducing teleconnections with eastern Australia and South-East Asia. El Ni帽o is associated with wetter years in south-western North America.

This cycling between La Ni帽a and El Ni帽o, which usually takes place about every two to seven years (although a rare 鈥淭riple Dip鈥� La Ni帽a just developed for the third year in a row), is itself influenced by longer-term trends in sea surface temperature, such as the and the even slower Interdecadal Pacific Oscillation. These protracted changes are good culprits for drivers of megadrought, says Meehl.

颅颅鈥淵ou need something that鈥檚 going to sustain [a megadrought] for some long period of time,鈥� he says. 鈥淟ooking at the ocean gives you these long-term, slow changes.鈥�

Zooming out even further, these long-term patterns can themselves be influenced by changes in climate driven by the composition of the atmosphere, the brightness of the sun and wobbles in Earth鈥檚 orbit, though whether these processes can provide a 鈥渟trong enough bump鈥� to push things towards megadrought conditions is uncertain, says Cook.

Megadrought can also be influenced by more local interactions between the land and the atmosphere. For instance, drought can kill plants, reducing the amount of water they transpire into the atmosphere, thus increasing heat at the surface. Resulting erosion can also cause more dust to blow around, inhibiting the formation of clouds and suppressing rain.

鈥淥nce you get rid of the vegetation, you get this feedback with surface heating,鈥� says at the University of Arizona. 鈥淢ore heating, more drying, more heating, more drying.鈥� This vicious cycle to the 1930s Dust Bowl drought in the US Midwest, which devastated the drought-sensitive wheat crops that had replaced native prairie grass. Natural records point to other examples as well: in Central America, deforestation by the Maya and other Mesoamerican peoples might have by as much as 15 per cent during a megadrought there between AD 800 and 1000.

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How does human-caused climate change add to this picture?

Simply put, human-caused climate change is likely to increase the risk and severity of megadroughts in places that are naturally vulnerable to it. The risk grows with more warming, which is mainly driven by the amount of greenhouse gases people emit into the atmosphere. 鈥淎ny warming that we can avoid is going to translate to a reduction in hazard risk,鈥� says Cook.

Much of this increased risk comes from the direct effect of warmer temperatures, he says. Hotter air is thirstier; it can hold more moisture, so takes up more from the soil and from transpiring plants. If not offset by increased precipitation, this leads to drier conditions, says Williams. Hotter temperatures also cause more snow to evaporate without melting and more snow to fall as rain, which is an issue in places like the Colorado river basin that rely on snowmelt to replenish rivers and reservoirs.

Human-caused climate change can also influence the amount of precipitation, increasing it in some places and decreasing it in others. A rule of thumb is that wet places will get wetter and dry places will get drier, says Meehl, though models disagree on precisely how precipitation will change everywhere.

Research by Williams and his colleagues, for example, that human-caused climate change and natural climate variability contributed roughly equally to the current megadrought in south-western North America. Most of the human-caused effect was due to higher temperatures rather than changes in precipitation, which their climate models found increased slightly.

There would still have been a bad drought without global warming, says Cook, but it wouldn鈥檛 have become a megadrought. He expects other places vulnerable to drought face a similar future.

鈥淚 think what we鈥檙e going to see over the next several decades and centuries is that these types of events are going to become more frequent,鈥� says Cook. 鈥淭hey鈥檙e going to become more intense, and the wetter periods between are going to become probably shorter.鈥�

Less clear is whether human-caused climate change, in addition to the local effects of warming and changing precipitation, will alter the large-scale forces that drive megadroughts. 鈥淒id global warming cause the ocean to be the way it is?鈥� says Williams. 鈥淔or that, we have no idea.鈥�

Take the La Ni帽a example. Williams says there are some good physical reasons to think that global warming could drive a La Ni帽a-like conditions, such as increasing the difference in temperature between the ocean鈥檚 surface and deeper waters, but models of the Pacific Ocean and atmosphere disagree on how warming might affect La Ni帽a. He says this is a key area for researchers to make headway.

But it doesn鈥檛 take such seismic shifts in weather patterns to make the link between human-caused climate change and megadrought. From the local effects of higher temperatures and changing precipitation alone, researchers expect the risk of megadrought in the places most vulnerable to it will increase.

鈥淓very place on the planet will still have bad-luck dry years,鈥� says Williams. 鈥淎 warmer atmosphere may mean that that bad luck may be more impactful than it would have been if the atmosphere was cooler.鈥�

In a scenario with very high emissions, some places could even see a shift to a hotter and drier average, with drought becoming a 鈥�new climate normal鈥�. This is what happened to the Sahara desert around 6000 years ago, forcing people to find ways to adapt to the drier conditions or leave, says Williams, and it could happen again.

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