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Forests Emerge as a Major Overlooked Climate Factor

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Gabriel Popkin writes in Quanta:

When Abigail Swann started her career in the mid-2000s, she was one of just a handful of scientists exploring a potentially radical notion: that the green plants living on Earth’s surface could have a major influence on the planet’s climate. For decades, most atmospheric scientists had focused their weather and climate models on wind, rain and other physical phenomena.

But with powerful computer models that can simulate how plants move water, carbon dioxide and other chemicals between ground and air, Swann has found that vegetation can control weather patterns across huge distances. The destruction or expansion of forests on one continent might boost rainfall or cause a drought halfway around the world.

Swann is now a professor at the University of Washington, where she runs the Ecoclimate lab. She is in the vanguard of a small but growing group of scientists studying how plants shape Earth’s weather and climate. Their results could shake up climate science. “None of the atmospheric scientists are thinking about” how plants could influence rainfall, Swann said, though hints had appeared in the scientific literature for decades. And, she added, “it blows the ecology community’s mind … that the plants over here could actually influence the plants over there.”

“Many of us are surprised at what a powerful role plants actually play,” said Park Williams, a bioclimatologist at Columbia University. “The influence of Earth’s surface on large-scale climate is currently a really booming topic, and Abby Swann is one of the emerging leaders in that field.”

The Ignored Influence of Plants

The schism between the atmospheric and life sciences that Swann encountered was a holdover from the late 1800s, when the U.S. government proclaimed that planting crops and trees would turn the arid Great Plains wet. The government had embraced a dubious theory pushed by land speculators and rejected the counsel of one of the nation’s top scientists, John Wesley Powell. Spurred on by such optimistic but dubious claims, thousands of would-be farmers headed west, only to find that greening the land did not, in fact, make it rain. Many struggled to scrape a living from the dry ground, and the ill-conceived agricultural experiment eventually contributed to the devastating Dust Bowl.

Scientists reacted strongly. Early meteorologists, hoping to save their young field’s credibility, rejected the notion that forests influence weather. “Much of the discussion of it, unfortunately, has not been of a purely scientific character,” one wrote in 1888 in Science. Meteorology, and later climate science, became the study of air and water. Plants were relegated to passive participant status.

Atmospheric scientists — and everyone else — could be excused for thinking of a stoically standing tree or a gently undulating wheat field as doing little more than passively accepting sunlight, wind and rain. But plants are actually powerful change agents on the planet’s surface. They pump water from the ground through their tissues to the air, and they move carbon in the opposite direction, from air to tissue to ground. All the while, leaves split water, harvest and manipulate solar energy, and stitch together hydrogen, oxygen and carbon to produce sugars and starches — the sources of virtually all food for Earth’s life.

The key features of this molecular wizardry are pores, called stomata, in plant leaves. A single leaf can contain more than 1 million of these specialized structures. Stomata are essentially microscopic mouths that simultaneously take in carbon dioxide from the air and let out water. As Swann notes, the gas exchange from each stoma — and indeed from each leaf — is, on its own, tiny. But with billions of stomata acting in concert, a single tree can evaporate hundreds of liters of water per day — enough to fill several bathtubs. The world’s major forests, which contain hundreds of billions of trees, can move water on almost inconceivably large scales. Antonio Nobre, a climate scientist at Brazil’s National Institute for Space Research, has estimated, for example, that the Amazon rainforest discharges around 20 trillion liters of water per day — roughly 17 percent more than even the mighty Amazon River. 

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, . . .

Continue reading.

Written by Leisureguy

9 October 2018 at 2:14 pm

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