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The fire that started on Tubbs Lane in Calistoga, California, on the night of October 8, 2017, was like nothing the region had ever seen. It crackled and fumed its way swiftly through forests of oak, fir, bay laurel, and buckeye, over the hills in the night. It raced downhill, which is hard for fires to do, hurling fireballs to the south and west, and eventually laid waste to block after block of Santa Rosa, stopping about five miles from the house where one of us—John—lives. Most of Sonoma County’s 500,000 residents came away physically unscathed, but with a permanently altered awareness of the world.
We are living climate change, fully immersed in the future that, until recently, we were only talking about. In 2020, the routine continued for the fourth straight year—fires handily set a new California record for charred acreage, alternately coloring the world orange, sepia, and, as one writer for The New York Times put it, “yellow-gray, like a smoker’s teeth.” One Wednesday in September, San Francisco went red, like a city under a darkroom light. Equivocation is gone from the media accounts and scientific discussions; the drought-baked landscape and fires that rip through it are results of a changing planetary reality.
Societies around the world need to adapt, transforming energy systems, transportation, manufacturing, and what we eat. The human population needs to stabilize and start to decrease in order to cut the amount of energy, food, transportation, and other things we ask our finite planet to provide. There are glimmers of progress on all of these fronts.
But to meet the climate challenge, we have to accomplish one other essential task: Save the world’s biggest forests. The planet is a linked physical-biological system in which large wooded expanses keep both local and global conditions stable and livable. They metabolize the carbon our economies so relentlessly put in the air in a process that circulates life-giving water around our landscapes. This physical work is accomplished with a biological mechanism involving trillions of organisms belonging to millions of distinct species in a constant whir of transacted matter and energy, moving from one being to another, from earth to sky and back.
Our world keeps its carbon in four places. One is the lithosphere, a term that comes from Greek words meaning “rock ball.” Carbon made solid by ancient photosynthesis is stored in Earth’s rock layer in combustible forms such as oil, gas, and coal, as well as other substances, such as graphite and diamonds. The second place is the atmosphere (from the Greek, meaning “vapor ball”), where the element mostly takes the form of carbon-dioxide gas. The third place is the hydrosphere, the planet’s surface water, 97 percent of which is ocean. When seas absorb carbon dioxide from air, their water becomes more carbonated, like an oh-so-slightly fizzier soda.
Finally, there is the biosphere, the layer of living stuff between rock and air. Plants slurp carbon-dioxide molecules through tiny pores, cleave off the carbon, and build themselves out of it. Carbon makes up about half of plants’ mass. Growing things drop leaves, cones, seeds, flowers, branches, and eventually trunks and stems onto the ground. Some decomposing biomass goes back into the air and some into the soil, the proportion depending on the speed of decay. Of the carbon that is buried, some is compressed over the eons into the fossil fuels we’re now quickly burning. As for the carbon that stays topside, vegetarian animals eat the plants and incorporate the carbon into their bodies and are, in turn, eaten by carnivores, the apex carbon collectors.
The distribution of carbon in these four realms has varied over time. During periods of rapid cooling, Earth made a lot of plants into coal. When the planet had extensive shallow seas, the ocean floors became vast graveyards for tiny plants and animals that were eventually transformed into oil and gas. Over the 200,000 years of our species’s existence, the atmosphere’s CO₂ has oscillated between 170 and 280 parts per million. The past 10,000 years are a period called the Holocene Optimum, a time when temperatures have been very stable, a hair below their current levels. This has been the climatic stage on which the human dramas of agriculture, industry, and explosive population growth have played out. These plot twists that were supported by the Holocene Optimum are now ending it, as we withdraw carbon from the biosphere and lithosphere and deposit it into the atmosphere and waters.
All forests can help, but large forests are of supreme importance for the climate. The five largest ones left—the megaforests—include boreal forests in Russia and North America, and the tropical forests in the Amazon, Congo, and New Guinea. Intact forests are 20 percent of the tropical total and store 40 percent of the aboveground forest carbon in the low latitudes. New research led by Sean Maxwell, of the University of Queensland, and 11 collaborators suggests that the carbon benefit of intact tropical forests is six times greater than the Intergovernmental Panel on Climate Change and others have estimated to date. That’s because in the years after a big forest is broken up by roads or farms, its edges dry out and winds whistle through, blowing over big trees. Fires invade it more readily, and overhunting eliminates animals that disperse seeds. And on top of all the carbon vaporized from the space actually deforested, over the next several decades the climate will be stuck with 14 metric tons of extra carbon per acre that the lost tropical forests would have absorbed had they remained standing.
The consequences of fragmentation are similar in the boreal forest. Even small amounts of deforestation create hot, dry forest edges and warm the forest interior, far from the bits actually cleared. That makes the understory highly flammable. Michael Coe, a climate scientist at the Woodwell Climate Research Center, is an Amazon expert who collaborated with temperate- and boreal-forest specialists on a 2020 study of forest-climate dynamics across all latitudes. He says that fragmenting the boreal can lead even more directly to the incineration of the remaining trees than is the case in the tropics. “Any kind of an edge, it doesn’t have to be a big edge, causes a problem,” Coe says.
When forests are kept intact, they deliver a double climate benefit. They cool the planet, thanks to CO₂ removed from the atmosphere, and cool the local environment through the processes of evaporation and transpiration. Evaporation is the familiar process of liquid water, on all the forest surfaces in our case, warming and turning into vapor. Transpiration is the exhaling of vapor that originates inside the leaves and escapes through pores. The combined process is called “evapotranspiration.” Just like sweating cools people, as water turns to vapor it absorbs energy and cools the surrounding environment. You can feel this air-conditioning in the forest interior, which is cooler than a treeless shady spot, say, under an awning.
Tropical and boreal forests have different rhythms for harvesting and storing carbon. The tropical forest grows riotously all year, minting solid biomass from CO₂ and shaping it into trees, shrubs, ferns, ground covers, orchids, and other plants. Its pollinators, seed dispersers, and bacterial and fungal partners are of unfathomed number and diversity. Fallen leaves and wood decay into a thin layer of soil whose nutrients rainforest roots tap immediately to grow more plant matter. Liquid water is available year-round to support plant growth, evaporating and transpiring continually from plants into clouds that coalesce, move, grow heavy, and spill onto another patch of woods that does the same thing all over again.
The boreal forest, by contrast, is a patient, seasonally photosynthesizing interface between the sky and underground carbon caches. In the northern parts of the boreal, trees can take many years to get as tall as a person. Throughout the ecosystem, they grow during a short summer and shower the forest floor with needles, leaves, cones, and twigs. Some material falls into oxygen-deprived waters and changes extremely gradually, like specimens preserved in laboratory jars. In the winter it’s too cold for microbes to process the vegetation into soil. Vegetative “sediments” are thus packed into ever thicker deposits of soil and the proto-coal called “peat,” a semi-decomposed layer that comprises 47 to 83 percent of carbon in boreal ecosystems.
Intact forests are only now being fully recognized as central to the climate crisis and its solutions. In 1992, the United Nations Earth Summit in Rio de Janeiro produced a climate treaty, which largely excluded forests, and one for biodiversity, which embraced them. Among those fighting to keep forests out of the climate accord were some environmental advocates, who argued that forest carbon was hard to measure and that giving countries credit for dodgy forest-emission reductions might permit very real increases in industrial CO₂ pollution. The measurement problem was largely solved by 2010 thanks to advances in aerial and satellite technology, wide availability of data, and improved computing power. At the same time, tropical-forest countries started playing more prominent roles in treaty negotiations. In the past five years, as the urgency of the climate crisis has heightened, researchers have begun to confirm the surpassing climate advantages of very large forests.
In 1973, one of us—Tom—took up a post as head of programs for the U.S. office of the World Wildlife Fund. He realized that WWF needed to know more about habitat fragmentation. How else could they determine whether their conservation projects were big enough to save species? Then he remembered that Brazil’s forest law required landowners to leave 50 percent of Amazon rainforest standing as they mowed down the rest for cattle ranching or crops. He proposed to the United States National Science Foundation (NSF) in 1976 that a Brazilian landowner might be persuaded to leave that 50 percent in a configuration that would provide a giant forest-fragmentation experiment. With the NSF’s backing and that of the Brazilian National Institute for Amazonian Research (INPA), in Manaus, he approached the Brazilian bureaucracy in charge of fomenting cattle ranching with a request: Ask ranchers to leave their required reserves in squares of various sizes surrounded by pasture. The agency agreed.
This experiment began in 1979. It ended up with five plots measuring two and a half acres, four at 25 acres, and two covering 250 acres. Matching control plots in continuous forest were also established. By 2002, the project had produced a simple answer about fragmentation: Large intact areas are very important, the larger the better. Even the 250-acre reserves were too small for forest-interior bird species, half of which vacated these patches in less than 15 years. The edges were hotter and drier, with great mats of desiccated leaves from trees either dying or losing foliage to wind. There were more vines, thicker undergrowth, and fewer mushrooms.
Species that need continuous tree cover decamped. Black spider monkeys, for example, who move fast through large areas of forest eating fruit from widely spaced trees, abandoned all the forest fragments immediately. They stayed in nearby continuous forest. Howler monkeys, by contrast, are leaf eaters and not particularly choosy. They remained in all the fragments. The white-plumed antbird, so named for the spiky crest between its eyes, could not persist in the fragments. Antbirds follow raiding ant armies and eat the bugs flushed out by the lethal column. Though 250 acres is sufficient territory for one ant colony, each colony marches only about a week per month. So, to avoid going hungry for weeks at a time, the white-plumed antbirds need to follow several colonies on a rotating basis. The 250-acre fragments were at least three times too small for the birds. No antbirds means no antbird droppings, which deprives shimmering blue-and-black skipper butterflies of their sustenance. They left too.
Birds such as the black-tailed leaftosser, which finds insects by turning over leaves on the forest floor, also ran into problems. The forest fragments were pummeled by wind, which felled trees up to a quarter mile from the edge. Resulting gaps were filled by trees in the Cecropia genus, which you can see along almost any Amazonian roadside, riverbank, or regrowing pasture. The Cecropia leaves are like lobed umbrellas that can easily measure a foot across, too large for the leaftosser to upend. Most insectivorous birds and bats, along with arboreal mammals, dung beetles, wild pigs called peccaries, and orchid bees found even a narrow clearing insuperable. A couple hundred feet of treeless ground, typical of a highway, was enough to prevent their using the forest fragment habitats.
Orchid bees, vital pollinators for towering Brazil-nut trees, left the fragments, depriving nut-devouring mammals called agoutis of their preferred meal. At least four frog species that live in the wallows created by white-lipped peccaries vanished from the fragments; the pigs that dig their pools wouldn’t use the forest islands. These amphibians were replaced by “generalist” frogs common in cattle pastures.
The forest-fragments project, with its emblematic squares, spawned a field of study focused on what happens when big forests are made smaller. Its findings firmly established forest fragmentation as an urgent environmental problem. Hundreds of graduate students from Brazil and elsewhere have earned advanced degrees studying plants, animals, soil, and carbon in the original plots. Many more have investigated the unscripted fragmentation of forests across the world. This body of science corroborates an observation made by Charles Darwin during the voyage of the Beagle: Intact nature has more diversity than nature in pieces.
The urgency of climate change has compelled many scientists, economists, and environmentalists to think about how much carbon there is in everything. Carbon, the element, becomes a currency, the unit of measurement in a chemical accounting system we use to chart survival paths for civilization. The peril, of course, is that this carbon myopia conceptually distills the intricacy of a forest ecosystem into a colorless idea small enough to fit in a beaker.
Some studies show that when animals are gone, the forest sheds plant carbon. But what do we make of the forest animals whose removal has a negligible impact on carbon? Do we write off the gibbons of certain forests of Southeast Asia, the pollen and seeds of which are wind-borne? What will become of boreal creatures, even famous ones like caribou, if they are found to be contributing too little to the production of peat?
An engineering mindset may also lead us to muse whether the forest might be force-fed a bit more biomass. Some scientists say it might indeed, with performance-enhancing genes that augment photosynthesis and carbon transfer from plant to soil; it’s possible to juice the jungle. Another idea, about which scientific papers are written, is to cut down the boreal forest—all of it—so the snow reflects sunlight in winter. This wouldn’t work because it’s impossible to cut down the whole boreal and keep it from growing back, and razing it incrementally would emit more carbon than the reflective cooling could make up for. In any case, says Michael Coe, of the Woodwell Climate Research Center, “an engineering solution that destroys biodiversity is a bad idea. There are always unintended consequences.”
Big forests are a linchpin in a planetary system. They are vivid stages for stories about energy and matter that we describe severally with our physical, biological, and chemical sciences, but are really a single story whose intricacies and meaning we don’t fully understand. Orchid bees make Brazil nuts, feed agoutis, take carbon from the air, breathe water back into it, make clouds that make rain a hundred miles away that feeds a stream, where a catfish, having migrated from the mouth of the Amazon, is caught by an otter or by a person, surrendering its protein to enliven the woods. The bee makes all these things, and these things make the bee.
Losing the forest would change more than the reading on the thermometer. Wind, rain, fire, and ocean currents would be rewritten. If we lose too many trees, everything changes.
This article is adapted from Thomas E. Lovejoy and John W. Reid’s forthcoming book, Ever Green: Saving Big Forests to Save the Planet.