Droughts usually evoke visions of cracked earth, withered crops, dried-up rivers and dust storms. But droughts can also form over oceans, and when they then move ashore they are often more intense and longer-lasting than purely land-born dry spells.
A Sept. 21 study published in the journal Water Resources Research found that, of all the droughts that affected land areas globally from 1981 to 2018, about 1 in 6 started over water and moved onto land, with a particularly high frequency along the West Coast of North America, said senior study author Noah Diffenbaugh, a Stanford climate researcher.
"Because they usually take a number of months to migrate onto land, there is a potential that tracking moisture deficits over the ocean could provide advance warning to help protect against at least some of the most severe droughts," he said, adding that the landfalling droughts, as those that move from the ocean to terra firma are known, grow three times as quickly as land-only droughts.
The research zoomed in on West Coast landfalling droughts and linked them with Pacific Ocean weather patterns that are changing in a warming world. Those dry spells happen when large areas of stable air persist much longer than normal off the West Coast.
That pattern lingered so long in the period from 2012 to 2017 that scientists named it the "ridiculously resilient ridge," and a 2016 study showed that global warming made that pattern more likely. The long drought dried forests and brush, killing millions of trees, and partly setting the stage for the 2020 siege of wildfires.
Currently, more than half the West—nearly 1 million square miles—is experiencing acute drought conditions, according to the National Integrated Drought Information System. The epicenter is the Southwest, where drought conditions have mostly prevailed since about 2000, the start of a megadrought that could last for decades, according to recent research.
The current Western drought could soon rise to a crisis level, with federal water managers warning that Lake Powell and Lake Mead, the two key Colorado River reservoirs, may drop to levels that could result in economically damaging cuts to water allocations in the Southwest and California. In April, the U.S. Bureau of Reclamation projected inflows would be 75 percent of average; by late September, the projections had been lowered to 55 percent of average.
In the last four decades, severe droughts caused $252.7 billion in economic losses and damage in the United States, about 14 percent of the total cost of climate disasters in that span. Only hurricanes, at $954.4 billion, and severe storms, at $268.4 billion, have caused more damage.
Droughts often damage or destroy crops and natural ecosystems like forests and wetlands. They also affect water supplies and can reduce water available for hydropower, and for cooling coal or nuclear plants. Globally, about 55 million people are affected each year, and worsening droughts could displace up to 700 million people by 2030, according to the United Nations.
But the climate processes leading to such deep dry spells are not fully understood, and that makes the landfalling drought study important, said Dim Coumou, a climate extremes researcher with the Potsdam Institute for Climate Impact Research.
The new study tracks moisture-deficit areas that can meander for months over oceans before making landfall, he said.
"That might provide opportunities for early warning," he said. "And not surprisingly, these landfalling droughts are associated with persistent high-pressure systems, so to me, understanding the dynamics of those is key."
High pressure systems are areas of relatively stable air over land or sea that are generally associated with dry conditions. They alternate with stormy low pressure systems that bring rain or snow. In a 2018 study, Coumou and other scientists showed how global warming is changing that rhythm, leading to more persistent extremes.
Rewinding the Drought Movie
Co-author Julio Herrera-Estrada, a drought and sustainability researcher with Descartes Labs, said it may seem counterintuitive to think about droughts starting over the oceans. But there have always been vast ocean deserts where very little rain falls. The problem is that long-stable patterns are changing, he said, which is shifting more of those deserts over land.
He's been studying the genesis and movement of droughts for quite some time, and a few years ago, showed that droughts in the United States often follow geographic patterns, starting, for example, in the Southwest, then heading north and east.
"During drought in North America, what is the transport of moisture from ocean to over land?" he asked. Tracking the newly defined category of landfalling droughts was a matter of "rewinding the drought movie" to try and learn where they started.
"We usually think of droughts as stationary hazards that start and stop in one area and just [affect] that area," he said. "But maybe we can do analyses similar to what we do for hurricanes, to see where there are more tracks over decades. Droughts don't stop at borders, or at the land-ocean border. We're thinking of droughts as a dynamic hazard. That is somewhat new."
Droughts are "creeping hazards," making it hard to define where they begin and end, and lags in the water cycle make predicting where and when their impacts will manifest difficult. For example, a lack of rain one year can affect downstream parts of the hydrologic system, like groundwater and wetlands, much later.
"Once you have a better understanding of their nature, and how they can be caused, you have a better understanding of drought in the context of climate change and adaptation," Herrera-Estrada said.
In the context of global warming, he said, the findings are a step toward more accurate drought forecasts that will make communities more resilient to climate extremes.
"In climate change, our focus is often on the mean, like the increase of average global temperature," he said. "But I think in some ways we underestimate the impacts of the extremes. There's a disconnect—people are going to feel climate change mostly through extreme events. Those are going to affect more people."
Other areas affected by droughts that start over the ocean include Chile, Argentina, New Zealand and Eastern Australia. Regional studies can show what is driving the droughts from the oceans onto every continent, but "getting from these studies to prediction is still a big step," said drought researcher Peter Greve, with the International Institute for Applied Systems Analysis, a climate research center in Vienna.
Oceans, Atmosphere and Land are Linked
Along with devastating impacts to people, important ecosystems are also damaged by the impacts of droughts and heat waves that move between oceans and land, said Dave Breshears, a climate researcher at the University of Arizona School of Natural Resources and Environment.
Breshears was part of a research team that studied those linked effects after an extreme heat wave and drought straddled Western Australia and the adjacent Indian Ocean in 2011. The team looked at anomalies in temperatures of the sea surface and over the land.
"(We) pasted those maps together, and what we saw matched up," he said. "We see that these landfalling droughts have enormous ecological impacts."
In Western Australia, the impacts spread across an area as large as California, killing forests on land and coral reefs in the ocean at the same time, he added. There was a loss of seagrass and kelp, a hampering of penguin breeding and outbreaks of tree-killing bugs.
Drought studies are also important for efforts to reduce Earth-warming greenhouse gas emissions, he said. "We're moving toward an urgent need to manage carbon at a huge scale, and we have to understand these connections and their ecological impacts." Plans to reduce atmospheric carbon dioxide could be crippled if more frequent and severe droughts kill even more carbon sequestering trees than they have already, he added.
"These landfalling droughts are bigger, badder and more frequent in terms of the drought impacts, and yeah, we're seeing ecosystems getting hammered," he said. "This new study is a really important step in helping us understand the larger-scale connections."
Adapting to Deepening and Lengthening Dry Spells
One thing is sure: global warming is intensifying droughts and will continue to do so for the foreseeable future, said Niko Wanders, a drought expert at the University of Utrecht who has calculated that the number of people worldwide facing water stress could double, to nearly 800 million, by 2050.
"The landfalling droughts are interesting when you consider the droughts in California and the teleconnections with things like El Niño," he said, referring to how large-scale changes in winds and patterns of sea surface temperatures can affect the emergence and shape of a drought over adjacent land areas.
The new study shows a global pattern, with a focus on the West Coasts of large continents. The findings suggest there is a lead time of many months between detecting the pattern over the ocean and the arrival of drought impacts on land.
"That gives you time to prepare, and even if it's only a probability of drought impacts, you can plan low-cost, low-regret drought mitigation measures," he said. That can include up-front water conservation, before there are shortages, adjusting water storage to anticipate drought conditions and actively recharging underground aquifers and related systems like wetlands and riparian forests, he added.
Where Wanders lives in the Netherlands, he said, that means storing water underground and recharging aquifers. Urban areas will also need to send as much water as possible into natural and man-made storage systems, he said. Instead of letting water pass swiftly through storm drains back to the sea, it should be slowed and distributed to fill up underground reservoirs.
But that's currently not a high priority for urban planners in his country. It will probably take a catastrophic drought before people start to think in those terms, he said.
The danger of landfalling droughts could be even greater in areas that historically don't have a lot of experience dealing with dryness, added Greve, the IIASA researcher. Areas that periodically have droughts have developed adaptation tools, but where water has always been plentiful, people are unprepared. The threat to many of them, however, is growing.
"Compounded risks of heat waves and drought are a big concern," he said. "The frequency and duration are increasing, and this is clearly attributable to climate change."