Scientists investigate present and future of land carbon sinks

As global decision makers prioritize combating climate change in the coming decades, there is a growing need to understand how much carbon dioxide forests and other land-based ecosystems can absorb and store. While many forest carbon monitoring systems have been developed for various regions, differing techniques, methodology, and assumptions have made it difficult to accurately and consistently measure mitigation performance on a global scale. Additionally, climate change itself may alter the efficacy of land carbon sinks, as the basic biological processes of plants that absorb and release carbon dioxide respond to changing environmental conditions. To meet these challenges and facilitate better forest management decisions, Woodwell Climate scientists have created maps of global forest carbon flows and investigated the sustained potential for land carbon sink mitigation.

Land carbon flux is the flow of carbon back and forth between terrestrial ecosystems and the atmosphere, influenced by land use, soil type, the species present, and environmental conditions. Plants convert carbon dioxide (pulled from the atmosphere) and water into energy via photosynthesis. In respiration, that energy is used and carbon dioxide is released back into the environment. Bacteria and other microbes can also absorb and release carbon dioxide and methane as a result of their metabolic processes. All of these biological processes are influenced by environmental conditions, including temperature, moisture, and the amount and type of carbon available. And, of course, cutting trees or otherwise disturbing an ecosystem can result in the release of previously stored carbon.

The new maps show that, globally, forests currently absorb twice as much carbon as is emitted by deforestation and disturbance. Drs. Richard Birdsey, Richard (Skee) Houghton, and Alessandro Baccini collaborated with other scientists to create a monitoring framework that integrates satellite imagery and ground data. By creating a consistent international standard, the team hopes to increase transparency and improve communication among different regions.

“There has been a steady evolution of forest carbon flux monitoring methods over time and our work builds off methodology that has been proven effective,” explains Birdsey. “Scientists have begun integrating remote sensing and satellite data into their calculations but it is our map-based approach that makes all the difference. Not only do maps provide lots of important data—including information about forest regrowth and changes in carbon density—but they are also incredibly accessible. Our findings are public because we believe everyone should have access to this data, and it should be presented in a way that doesn’t require a supercomputer or PhD to understand.”

Already, the method used here has become useful for decision makers in the U.S. Using scaled down maps developed by Birdsey and Dr. Nancy Harris of the World Resources Institute, 25 counties and communities across the United States are developing strategies for land use modifications to reduce greenhouse gas emissions and maximize efficiency, and access to spatial data about forests and carbon flux has been key to that effort.

However promising these strategies are, scientists are uncertain about the ability of land carbon sinks to continue to mitigate anthropogenic carbon emissions at the rate (~30%) they do today. Climate change poses a distinct threat because plants’ metabolic processes are dependent on temperature. Risk Program Director Dr. Christopher Schwalm was part of a team, led by a former graduate student, that used data from a continuous carbon flux monitoring network to determine the temperatures at which photosynthesis and respiration peak. The work revealed that without drastic changes in emission rates, respiration could soon outpace photosynthesis in many ecosystems, cutting the total global land sink strength by nearly 50% by 2040.

“Seeing such a strong temperature signal globally did not surprise me,” Schwalm told Inside Climate News. “What I was surprised by is that it would happen so soon, maybe in 15 to 25 years, and not at the end of the century.”

In order to limit climate change, it is critical to understand how much we can rely on forests and other natural systems to absorb and store carbon. For decades, Woodwell Climate researchers have played key roles in integrating understanding of basic biological and ecological processes into global accounting of carbon sources and sinks. We continue to collaborate with key stakeholders to develop effective land use practices on both the local and global scales.