Text: Trees — beloved sources of shade, shelter, food, raw material and beauty — cover more than 40 million square kilometers of Earth’s surface. The first forests evolved nearly 400 million years ago, before people, or dinosaurs, or flowers. (Inspired by Forests, Climates, and Public Policy, Annual Review of Ecology, Evolution, and Systematics, 2016). Illustration: Forest with sunlight filtering through the tall trees. At foreground is a large tree trunk with big bear peeking out from behind it. Inset illustration shows Benjamin Franklin, a dinosaur and a yellow flower.
Text: These early trees overtook the swamplands of the Carboniferous period and thrived for more than 50 million years. The first primitive conifers grew in these forests. 390 million years ago. Illustration: Silhouettes of conifer trees growing on and around hilly landscape.
Text: Early gingkos appeared around 250 million years ago. The Earth’s landmasses were all attached at the time, and formed a supercontinent known as Pangaea. Illustration: Green gingko leaf superimposed over map of green landmass surrounded by blue water.
Text: Pines showed up around 150 million years ago and thrived alongside dinosaurs. Illustration: Brown pine cone nestled in large dinosaur footprint imprinted in mud.
Text: Angiosperms like oaks and maples, hardwoods whose seeds are hidden in fruits or shed as nuts, were later arrivals, and pushed out many conifers. By this point, Pangaea had long ago split into two, trapping most pines in the northern hemisphere. About 100 million years ago. Illustration: Silhouette of oak branch with two leaves, against backdrop of green hills.
Text: Trees thrived. Scientists think there are more than 60,000 tree species — and around 3 trillion individual trees — on Earth today. They cover about a third of the planet’s land, if you leave out Antarctica. Illustration: World map showing green land and blue water, with various types of trees in different continents.
Text: Benjamin Franklin, Founding Father, Polymath. Illustration: Benjamin Franklin stands in front of a walnut tree, arms out, speaking: “Native trees of North America include everything from the walnut trees in Philadelphia…”
Text: …to the mangroves of the Florida Everglades… Illustration: Trees on mangrove bank, with roots growing into water.
Fir forest in Pacific Northwest. Illustration: Benjamin Franklin says, “…to firs in the Pacific Northwest. They’re gorgeous — and valuable. It’s really no wonder that we sometimes fight over them.”
Text: A scientific consensus has emerged: Earth’s climate is warming at an unprecedented rate. Thanks primarily to human-made carbon emissions. Some researchers and policymakers are exploring what forests have to do with it, too. Illustration: Benjamin Franklin speaks while looking at a graphic. He is saying, “It’s true! Americans have been arguing over how to manage our forests for the better part of 400 years — and today, with battles over climate change, the topics arising anew.” Graphic titled “Variations of the Earth’s surface temperature” with label “Northern hemisphere. Departures in temperature (C) from 1961 to 1990. 50-year average” shows trend line spiking. Source line: Mann, Bradley & Hughes, Nature 1998.
Text: They’re not the first to make a connection between forests, temperatures and precipitation… Illustration: Benjamin Franklin looks out a window on a rainy night, while writing with a feather quill.
Text: … not even close. We haven’t always had the best science at our fingertips, but many Americans have long wanted to understand how America’s trees and forests impacted its climate. Even during the colonial period. Illustration: Four people in American Pilgrim garb stand on shore next to forest, with colonial ship afloat in background, while three people in Native American dress peek out from behind tree trunks.
Text: Europeans back then believed that chopping down their forests to make way for fields and towns had heated up the climate. Getting rid of trees, they thought, had allowed the sun to warm the lands. Illustration: Three farmers till large patch of earth, as shepherd with flock looks from a green hill above.
Text: And that was good. Warmer winters were believed to advance the arts, sciences and culture. There wasn’t much evidence to support the idea, beyond ancient texts, but it was widely accepted. Illustration: Collage showing tree stump, microscope, sheet music and Old World European building.
Text: The first Americans, struggling to survive, wanted the same sort of sunny days for the New World. Illustration: Two men in Colonial American Pilgrim clothing shiver in the snow as two Native American men wearing fur wraps observe them. Inset image of David Hume, Scottish philosopher, saying “Allowing… that Europe is become warmer than formerly; how can we account for it? Plainly, by no other method, than by supposing that the land is at present much better cultivated, and that the woods are cleared, which formerly threw a shade upon the earth, and kept the rays of the sun from penetrating it.”
Text: They started cutting down trees. Illustration: Two Colonial American Pilgrims sweat as they ax down trees in a snowy landscape. One asks, “Do you feel warmer?” The other replies, “I feel warmer!”
Text: They, too, believed they were getting results. They bragged about it. From future president John Adams, in 1756… Prior to European settlement, he wrote: “The whole continent was one continued dismal wilderness.” But after more than a century of colonization… “The forests were removed, the land covered with fields of corn, orchards bending with fruit, and the magnificent habitations of rational and civilized people.” —John Adams. Illustration: John Adams sits at desk, writing with feather quill.
Text: …to me. The early Americans’ explanations for the warming weather weren’t science-based, by modern standards. Illustration: Benjamin Franklin holds up a pointer finger, saying, “When a country is clear’d of woods, the sun acts more strongly on the face of the earth.”
Illustration: President Thomas Jefferson stands outside Monticello, as two plantation workers hold baskets behind him. He is saying, “I thought warming resulted from changes in air circulation when forests were cleared.”
Illustration: Benjamin Franklin stands on a bank of snow melting in the sunlight. He says: “Meanwhile, I thought clearing forests created mild climates because taking away trees let sunlight hit the snow — which then absorbed the heat, melted and allowed the ground to warm.”
Text: Physician/politician Hugh Williamson, on the other hand, held that smooth, clear ground reflected sunlight, warming the air. Illustration: Hugh Williamson points to where sunlight bounces off bare ground. Benjamin Franklin peeks out from corner, saying: “An aside: None of us got it quite right.”
Text: Anyhow, as the 18th century drew to a close, some began to push back on the notion that clearing forests was warming America. Noah Webster, of dictionary fame, wanted to see some convincing date. He mocked Thomas Jefferson for citing older Americans’ remembrances as his evidence for warming. “Men are led into numberless errors by drawing general conclusions from particular facts… Mr. Jefferson seems to have no authority for his opinions but the observations of elderly and middle-aged people.” —Noah Webster Illustration: Noah Webster, writing with a quill, has a thought bubble showing two elderly men with questions marks over their heads.
Text: And as the new, data-based field of meteorology came into its own during the 1800s, others, including the lauded weather scientist Cleveland Abbe, also cried foul. Illustration: Collage showing bearded man wearing suit and earphones, seated at desk with finger on telegraph sounder; utility pole with multiple wires; barometer.
Text: As the 20th century dawned, scientists at the US Department of Agriculture battled fiercely over forests … with meteorologists continuing to insist forests had no impact on climate … while foresters and conservationists, who wanted to save trees, insisted they did. Illustration: Two angry men, nose to nose and holding papers, yell at each other. Text: In 1912, a writer at Nature, attempting to summarize the argument, called the existing literature on the matter “somewhat bewildering.” Illustration: Writer wearing white shirt, tie and eyeglasses scratches his head, while behind him, two men argue heatedly.
Illustration: Benjamin Franklin flies a red kite on a stormy night with lightning in the sky. He is saying: “The thing is, none of these guys had the whole story right, either. The meteorologists had a better understanding of the atmosphere and the water cycle…”
Illustration: Stormy scene with wind blowing through trees and gray skies, as narration from previous panel continues: “… but they didn’t think about the ways that these physical systems might interact with biological systems, including forests.”
Text: Today we know that forests and climate are intimately connected. There are many, many ways they interact. Illustration: Landscape with arrows indicating movement between forest, beach, ocean, mountain, sky and sun.
Illustration: Benjamin Franklin wearing blue ballcap, blue shirt and brown pants, reading ticker-tape output by computer. He is saying, “The challenge now is figuring out how all these processes interact, and whether the net effect of forests is to promote warming or counteract it.”
Text: The answer isn’t straightforward. Illustration: Pair of hands holding strip of paper with zigzag indicator line.
Text: One simulation found that forests in temperate and boreal regions warm surface air temperature by 3 to 4 degrees Celsius because they absorb more sunlight than other types of vegetation… Illustration: Landscape of green treetops, with yellow arrow pointing down toward trees and red arrow pointing up toward blue sky. Text: … while forests in tropical latitudes, including the Amazon, lower the temperature by 1 degree C because evaporation and surface roughness cool the air more than sunlight warms it. Illustration: Landscape of treetops, with white arrow pointing up toward blue sky and half-red-half-clear arrow pointing down toward trees. Text: And there’s more to add to the mix. Forests function as carbon sinks, absorbing the greenhouse gas carbon dioxide. And trees release volatile chemicals, which can also affect climates. Illustration: Landscape showing trees, with gray arrow labeled “CO2” pointing toward them and two yellow arrows labeled “VOC” pointing away from trees.
Text: All of these moving parts create the sort of complexity that fuels the arguments of climate-change skeptics. Yet despite the uncertainties, most agree that rapid deforestation in the tropics spells trouble. Scientists and policymakers also worry that cutting down forests anywhere might increase carbon emissions quickly in the short term, as carbon stored in dead plant material is release into the atmosphere. Trees don’t grow overnight. Illustration: Two men stand in landscape barren except for tree stumps. One says, “We had a record cold winter this year! So much for global warming!” The other replies, “The last three years have been the warmest since reliable record-keeping began in 1880!”
Text: As the developed world continues to spew out human-made emissions, the totals are increasingly as countries like Indian and China modernize. CO2 levels are higher than they’ve been since the Pilocene epoch, 3 million to 5 million years ago. Illustration: Smoke stacks from several buildings, set among green land, spewing dark brown smoke.
Text: And forests are still being mowed down — an estimated 15 billion trees every year. Scientists believe the number of trees on Earth is just over half what it was when the first human civilizations emerged. Illustration: Three bulldozers, emitting dark smoke, mow down green forests.
Text: The 2015 Paris Agreement was the first worldwide climate accord to enshrine protection for forests. In the last ten years, China has spent more than $100 billion on reforestation. In the U.S., forests are expanding too. Illustration: Several people work together to plant trees, some digging with a shovel and others positioning the young trees into the holes. In the background is a barren landscape with tree stumps.
Illustration: Benjamin Franklin stands in a forest, looking thoughtful and stroking his chin with his fingers, saying, “This much is sure: With a preponderance of modern scientists and policymakers agreeing that warming is a real problem, and that now is the time to act… using modern scientific methods to really nail down the role forests play offers a huge opportunity.” Two birds fly in front him, and he continues: “If we better understood the interactions, maybe we could manage Earth’s trees to help regulate its temperature… perhaps 21st century thinkers can get to the bottom of it.”