Yet the computer models that scientists rely on to predict the future climate don’t even come close to acknowledging the power of plants to move water on that scale, Swann said. “They’re tiny, but together they are mighty.”

Scientists have known since the late 1970s that the Amazon rainforest — the world’s largest, at 5.5 million square kilometers — makes its own storms. More recent research reveals that half or more of the rainfall over continental interiors comes from plants cycling water from soil into the atmosphere, where powerful wind currents can transport it to distant places. Agricultural regions as diverse as the U.S. Midwest, the Nile Valley and India, as well as major cities such as Sao Paulo, get much of their rain from these forest-driven “flying rivers.” It’s not an exaggeration to say that a large fraction of humanity’s diet is owing, at least in part, to forest-driven rainfall.

Such results also imply a profound reversal of what we would usually consider cause and effect. Normally we might assume that “the forests are there because it’s wet, rather than that it’s wet because there are forests,” said Douglas Sheil, an environmental scientist at the Norwegian University of Life Sciences campus outside Oslo. But maybe that’s all backward. “Could [wet climates] be caused by the forests?” he asked.

Forests in the Arctic

Swann arrived at the University of California, Berkeley, in 2005 to do her doctoral work with Inez Fung, an atmospheric scientist. In the 1980s, Fung had helped pave the way for climate models that included realistic vegetation and associated carbon dioxide fluxes. (Among her other accomplishments, she was a co-author on the 1988 paper with the NASA scientist James Hansen that helped bring climate change to the public’s attention.) The model she worked with was state-of-the-art at the time, but, like its counterparts at other research institutions, it could only represent the biosphere simplistically.

By the mid-2000s, models had improved enough that scientists could more precisely study the role plants might play in the climate system. Fung suggested that Swann try foresting the Arctic in a climate model. Trees are colonizing higher latitudes as the globe warms, so it seemed reasonable to ask what impact they would have on the region’s climate. Other researchers had previously looked into the potential effects of an expansion of northern spruce forests; unsurprisingly, they found that the Arctic would likely get warmer because those trees’ leaves are dark and would absorb more sunlight than virtually any of the tundra, ice and shrubs they might replace. Swann decided to look into what would happen if the encroaching forests were deciduous trees with lighter colored leaves, such as birch or aspen.

In her model, the Arctic did still warm — by about 1 degree Celsius, which was more than she expected. Swann determined that her simulated forests emitted a lot of water vapor, which, like carbon dioxide, is a greenhouse gas that absorbs infrared radiation from Earth and redirects some of it downward. The vapor then caused ice to melt on land and at sea, exposing darker surfaces that absorbed yet more sunlight and grew even warmer. The new forests had set off a feedback loop, amplifying the impact of climate change. The finding hinted at the power that plants could exert over a region’s climate.

In a separate study, Swann turned all vegetated areas of temperate North America, Europe and Asia into forest. Again, this exercise exaggerated something already happening in the real world: Satellite data have shown that these continents are greening as former farmland returns to forest, perhaps aided by enhanced atmospheric carbon dioxide and longer growing seasons.

As in the Arctic study, the new trees absorbed sunlight and warmed, adding energy to the climate system. Atmospheric currents then redistributed this energy around the planet. Droughts descended on the southern Amazon and rain fell in the Sahara. These effects were caused by a repositioning of the Hadley cell — the massive conveyor belt of air that rises from the equator, dumps its rain over the tropics, and descends again as dry air at around 30 degrees north and south latitudes, where most of the world’s deserts are. Through the influence of plants alone, the Hadley cell had shifted to the north.

Swann had seemingly uncovered a hidden “teleconnection” — a region holding sway over a far distant one through subtle atmospheric mechanisms. Fung wasn’t that surprised: Atmospheric scientists have gotten comfortable with such remote influences. In periodic El Niño events, which have been understood since the 1920s, unusually warm surface water in the eastern Pacific Ocean triggers heavy rain in western South America and Africa and droughts in Southeast Asia and Australia. The novelty in Swann’s simulated events was that forests, not oceans, did the influencing.

“To me that was a really interesting perspective,” said Gordon Bonan, a geoscientist at the National Center for Atmospheric Research in Boulder, Colorado, who also studies the influence of plants on the atmosphere. “If you do enough of this tree planting, you can actually change circulation patterns.”

Distant Effects From Forest Changes

Scenarios such as a green Arctic or a reforested temperate zone are not as far-fetched as they may seem. A recent study in Nature reported that in the last three and a half decades, tree cover has increased by more than 2 million square kilometers in these regions.

Massive tree losses are also part of our modern world. During roughly the same period when temperate and boreal trees gained ground, some 20 percent of the Amazon rainforest was cut down. Since 2010, nearly 130 million trees have died in California alone, mostly because of drought and wildfire.

Much effort has gone into understanding how future climate change will affect forests. Based on severe droughts that occurred in 2005, 2010 and 2015, some scientists believe the Amazon may be nearing a tipping point that would cause much of its rainforest to turn to savanna, with potentially devastating consequences for carbon storage, biodiversity and local climate. A paper from late 2017 provided evidence that future warming would make droughts even more lethal to the forests of the American Southwest. Some scientists predict that many of the Southwest’s forests could become savanna or grasslands, and at least one — Nate McDowell at Los Alamos National Laboratory — has been quoted as saying that a large fraction of the region’s trees could die.

Yet the question of how changes in forests could alter the global climate has barely been considered. “For decades we’ve been looking to see: How well can we do in climate modeling without needing to evoke the influences of vegetation?” Williams said. “Vegetation has kind of been left on the back burner.”

Swann’s and Fung’s research suggested that plants need to be brought to the fore. And other researchers have taken note. Earlier this year, two groups of scientists, both of which included Swann, authored studies of how forest-driven water transport will change as carbon dioxide levels rise. Studies of individual leaves have shown that when plants are bathed in carbon dioxide, they don’t need to make as many stomata per leaf, and they close the ones they do make more of the time. These changes help forest plants conserve water to survive, but they reduce the water vapor available to fall as rain on the surrounding continent. Moreover, when plants transpire, they cool Earth’s surface and warm the air, just as the evaporation of sweat cools your body on a hot day. Leaf-level changes, scaled up across continents, could rob the atmosphere of moisture and warm the planet’s surface.

To Michael Pritchard, a climatologist at the University of California, Irvine, Swann’s results were “very provocative … and a big wake-up call,” he said. “This effect seemed to be rewriting the maps of the drought severity outlook in the future.”

Pritchard said he hadn’t previously been aware of the stomatal closure effect. The knowledge inspired him to join a group led by Gabriel Kooperman, a climate scientist then at UC Irvine, investigating the future effects of enhanced carbon dioxide over the three major tropical forest regions — the Amazon, Central Africa and Southeast Asia.

In a study published in Nature Climate Change in April, the researchers found that the closing of stomata would cause half the rainfall changes the regions would see by 2100. Moreover, the Amazon — home to the world’s most carbon-rich and biodiverse rainforest — would get hit with the most severe declines.

Swann is now probing the effects of forest changes at different scales. In a 2016 paper, she reported that wiping out forests in western North America made forests in eastern South America grow more vigorously, while reducing growth in Europe.

And in a study published in May, she investigated how U.S. forest die-offs would affect forests elsewhere in the country. In her models, she killed off forests in 13 heavily forested regions that the National Science Foundation has identified as being ecologically distinct. The results were dramatic. When she wiped out trees in the Pacific Southwest, forests in the Midwest and eastern U.S. suffered. In recent years, the Pacific Southwest has, in fact, lost an estimated 100 million trees, mostly to droughts and voracious insects.