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.”

Read: Forest animals are living on the edge.

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 São 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 backwards. “Could [wet climates] be caused by the forests?” he asked.

Swann arrived at UC 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.