All major recent climate reports say nature plays a crucial role in the effort to stop global warming, and many countries are relying on forests and other ecosystems to help fulfill their commitments under the Paris Agreement. Research published Thursday in the journal Nature Communications quantifies the climate benefits of protecting natural areas—especially carbon-storing forests—at a global scale.
The researchers used more than 400 million satellite measurements to draw a detailed 3D snapshot of global forests in 2020, which they compared to another set of satellite images from 2000 to 2020. They matched each protected area to ecologically similar unprotected areas based on climate, human pressure, land type, country and other factors, and were able to show how much more carbon the protected areas stored.
Since 2000, the researchers reported, protected forests worldwide have stored 9.65 billion metric tons more carbon in their trunks, branches and stems than ecologically similar unprotected areas. That is equal to about a year’s worth of annual carbon dioxide emissions from human activities. But that doesn’t mean that nature is a silver bullet that will stop climate change, said lead author Laura Duncanson, an assistant professor and remote sensing scientist at the University of Maryland who studies global carbon stocks.
“We don’t want this to be interpreted as another ‘forests could save us’ paper, because while absolutely critical as part of the solution, they don’t come even close to offsetting fossil fuel emissions,” she said. “Our results showed that in approximately 20 years, protected areas effectively avoided the equivalent of one year of annual fossil fuel emissions.”
This is not drawing down emissions, but only avoiding additional ones, she emphasized, adding that the study is a global-scale affirmation of other research showing that the climate and carbon benefits of forest protection are mainly from avoiding deforestation in protected areas.
“It meant that substantial forest carbon was being lost from unprotected forests during that period,” she said. “Forests take a long time to amass carbon, so I think the message is that protected areas are preventing things from getting worse, but no matter what, forest management will never cancel out fossil fuel emissions.”
Land ecosystems like forests and grasslands absorb about 25 to 30 percent of annual fossil fuel emissions, mostly through photosynthesis by plants. Of course, those ecosystems are also a source of carbon dioxide, as decomposing plants and soil release it back to the atmosphere through respiration. For now, land ecosystems take up more carbon than they release and numerous local studies have suggested that protection increases the climate benefit.
But a good way for measuring those effects accurately at a global scale just recently became available, said co-author Patrick Roehrdanz, a planning expert with Conservation International, a nonprofit science-based conservation group. The new data is from a lidar instrument aboard the International Space Station, part of the NASA and University of Maryland Global Ecosystem Dynamics Investigation, which measures forest structure in great detail.
The data streams were new and exciting and a good chance to “pose the question of how good a job our protected areas are actually doing in terms of storing carbon, versus other areas that are essentially the same in terms of their ecological structure,” he said. The findings “help confirm for us that particular areas are doing something positive in relation to climate mitigation … and provide some real quantification as to what that has been over the last 20 years or so.”
Lidar shoots laser beams at the surface and uses the reflected energy to see the surface in 3D, Duncanson said. For forests, this results in height and canopy density measurements that show the mass of the forest.
“About half of the dry mass of a tree is carbon,” she said. “So the bigger and taller the tree, the more carbon it stores. The basic methods for using lidar to estimate and map forest carbon have been around for decades. But doing it globally from space, with a lidar system powerful enough to map even the tallest, most carbon-rich forests? That’s the novelty here, and why we are finally able to do this kind of global scale analysis.”
The study focused on what happened to carbon stocks in the first 20 years of this century. But, Duncanson said, “the use of forests as a tool to combat climate change has really only become popular in the past few years. We have yet to see the carbon outcomes from activities such as the expansion of protected areas, rewilding forests and mass tree planting. As the satellite lidar record continues, we will start getting those answers, and hopefully guide forest management to improved carbon benefits.”
Roehrdanz said forests are key to global climate goals because trees are the “primary place where carbon is stored in the living environment. In terms of the above ground biomass that we can look [at] and see with our eyes, forests are where it’s at.”
The new study didn’t focus exclusively on trees, but that’s where the climate benefits of ecosystem protection were most clear. And that signal varied regionally. In Asia and South America, tropical moist forests showed the most carbon storage gain with protection. In Africa, the signal was mixed, with the climate benefits of forest protection not as clear, but with protected savanna ecosystems showing a gain in carbon storage with protection.
“We looked at all vegetated ecosystems,” Duncanson said, adding that it was “interesting and surprising” to see that, in Africa, dryland ecosystems had more “carbon effectiveness” than African forests. She said the research team speculated that may be due to significant woodland degradation from charcoal production in African forests, which releases CO2, but that they plan to use the data to try and figure out exactly why. Overall, she said, “tropical moist forests are where forest protection is most important for protecting carbon stores.”
Roehrdanz said the biggest single signal of the climate benefits of forest protection came from the Amazon, and Brazil, specifically, which contributed 36 percent to the global total.
“If you look at satellite images of Brazil, the Brazilian Amazon, you can see the outlines of protected areas,” he said, “and there’s clearly been either deforestation or degradation outside of protected areas.” The detailed results of the new study show how much emissions are reduced by stopping deforestation and degradation, he added.
“Thankfully, we got some confirmation,” he said. “It helps show what we’re getting out of our investments in forest protection, and what we can expect over the next 30 years. It shows what we will gain if we add additional protected areas.”
