The world’s tropical forests are exhaling — and it’s not a sigh of relief.
Forests are sometimes called the “lungs of the planet.” That’s because trees and other plants take in carbon dioxide gas and release oxygen. Past analyses had estimated that forests soak up more carbon dioxide than they release. Because carbon dioxide is a climate-warming greenhouse gas, that trend was encouraging. But new data suggest the trend no longer holds.
Trees and other plants use the carbon in that carbon dioxide as an ingredient in all of their cells. A study now suggests that tropical forests today return more carbon back into the atmosphere than they remove from it as carbon dioxide (CO2). As plant matter (including leaves, tree trunks and roots) break down — or rot — their carbon will be recycled back into the environment. Much of it will enter the atmosphere as CO2.
Deforestation refers to the cutting down of forests to open up room for such things as farms, roads and cities. Fewer trees mean there are fewer leaves available to take up CO2.
But far more of the forests’ release of CO2 — more than two-thirds of it — comes from a less visible source: a drop in the number and types of trees that remain in tropical forests. Even in seemingly intact forests, the health of trees — and their uptake of CO2 — can be diminished or disturbed. Selectively removing certain trees, environmental change, wildfires, disease — all can all take a toll.
For the new study, scientists analyzed satellite images of tropical Asia, Africa and the Americas. Deforestation is easy to see in these images. Areas may look brown, for example, instead of green. Other types of damage can be harder to spot, notes Alessandro Baccini. He is a forest ecologist at the Woods Hole Research Center in Falmouth, Mass. He specializes in remote sensing. That’s the use of satellites to collect information about Earth. To a satellite, Baccini explains, a degraded forest still looks like forest. But it’s less dense. There will be less plant matter and, therefore, less carbon.
“Carbon density is a weight,” Baccini says. “The problem is that there is no satellite in space that can give an estimate of [a forest’s] weight.”
Seeing the forest and the trees
To get around that problem, Baccini and his colleagues came up with a new approach. To estimate the carbon content of the tropics from satellite images, they compared such images to what they could observe for the same sites, but from the ground. They also used a mapping technique called lidar (LY-dahr). They divided each lidar image into square sections. Then, a computer program compared each section of each image to the same section in images taken each year from 2003 to 2014. In this way, they taught the computer program to calculate year-to-year gains — or losses — in carbon density for each section.
Using this approach, the researchers calculated the weight of carbon entering and leaving the forests from year to year.
It now appears that tropical forests have been emitting 862 teragrams of carbon to the atmosphere annually. (A teragram is one quadrillion grams, or 2.2 billion pounds.) That’s more than the carbon released (in the form of CO2) from all the cars in the United States in 2015! At the same time, those forests absorbed 437 teragrams (961 billion pounds) of carbon each year. So the release outweighed the absorption by 425 teragrams (939 billion pounds) of carbon each year. Of that total, almost 7 in every 10 teragrams came from degraded forests. The rest was from deforestation.
Some six in every 10 teragrams of those carbon emissions came from tropical America, including the Amazon Basin. Africa’s tropical forests were responsible for about one-fourth of the global release. The rest came from Asia’s forests.
The researchers shared their findings October 13 in Science.
These findings highlight what changes could give climate and forest experts the biggest benefits, says Wayne Walker. He's one of the authors. A forest ecologist, he also is a remote sensing specialist at the Woods Hole Research Center. “Forests are low-hanging fruit,” he says. By that he means that keeping forests intact — or rebuilding them where they might have been lost — “is relatively straightforward and inexpensive” as a way to prevent the release of too much climate-warming CO2.
Nancy Harris manages research for the forest program of the World Resources Institute in Washington, D.C. “We’ve known for a long time that forest degradation is happening,” she notes. Until now, however, scientists “haven’t had a good way to measure it.” She says that “this paper goes a long way to capturing it.”
Joshua Fisher points out there may be more to the story, however. Fisher works at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. There, he is a terrestrial ecosystem scientist. That is someone who studies how living organisms and Earth’s physical environment interact. Fisher says that measurements of atmospheric releases of CO2 from tropical forests don’t agree with the new calculations.
Forests are still taking up more carbon than they emit, the atmospheric data show. He says one reason might be dirt. Like plants, soil itself can absorb a large amount of carbon. The new study focuses only on the trees and other things above ground. It doesn’t account for what the soils have absorbed and now hold in storage.
Still, Fisher says, the study shows how important it is to include forest degradation as well as deforestation in studies of climate change. “It’s a good first step ,” he concludes.
atmosphere The envelope of gases surrounding Earth or another planet.
basin (in geology) A low-lying area, often below sea level. It collects water, which then deposits fine silt and other sediment on its bottom. Because it collects these materials, it’s sometimes referred to as a catchment or a drainage basin.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
carbon dioxide (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall.
climate The weather conditions that typically exist in one area, in general, or over a long period.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
colleague Someone who works with another; a co-worker or team member.
computer program A set of instructions that a computer uses to perform some analysis or computation. The writing of these instructions is known as computer programming.
density The measure of how condensed some object is, found by dividing its mass by its volume.
ecology A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
ecosystem A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.
environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).
forest An area of land covered mostly with trees and other woody plants.
greenhouse gas A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.
lidar (short for light detection and ranging) A tool to measure the shape and contour of the ground from the air. It bounces a laser pulse off a target and then measures the time (and distance) each pulse traveled. Those measurements reveal the relative heights of features on the ground struck by the laser pulses.
matter Something that occupies space and has mass. Anything on Earth with matter will have a property described as "weight."
NASA Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.
physical (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, the object must be big enough to have pulled neighboring objects into the planet itself or to have slung them around the planet and off into outer space.
quadrillion A very big unit of measure equal to 1,000 trillion. It would be written with a 1 followed by 15 zeros.
remote sensing Collecting data about an object or area from a distance, such as by using satellite cameras to take images of Earth.
satellite A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.
terrestrial Having to do with planet Earth, especially its land. Terra is Latin for Earth.
tropics The region near Earth’s equator. Temperatures here are generally warm to hot, year-round.