America’s forests help mitigate climate change by storing some 15 billion metric tons of carbon in the leaves, trunks and branches of trees. But climate change also makes their future very uncertain. Rising carbon dioxide concentrations in the atmosphere promote tree growth, but also exacerbate environmental disasters associated with rising temperatures, such as wildfires.
According new search released on Thursday, the future of forest carbon sequestration varies widely depending on the analysis used. Some scientific models show that US forests store more carbon over the coming decades, while others show greater regional variability or global losses in carbon storage.
The results highlight forest carbon offsets as a climate bet. These offsets consist of tree planting or forest conservation projects that polluters pay for when they cannot reduce their own greenhouse gas emissions to zero. In theory, carbon-storing forest offsets can offset some residual carbon emissions from polluting corporations and governments, but they don’t always deliver. One of the models analyzed in the new study showed that, by the end of the century, wildfires and disease could contribute to carbon losses in some 1.73 million acres of forests currently designated by l State of California to Offset Ongoing Greenhouse Gas Emissions.
“Forest offsets face very significant climate risks,” said William Anderegg, director of the Wilkes Center for Climate Science and Policy at the University of Utah and co-author of the study. “We need to keep that in mind when considering whether we want to pursue offsets.”
To project what will happen to the forests over time, researchers have several tools at their disposal. Some scientists use the complex dynamics of ecosystems — like the movement of carbon and water — to model whether trees will grow or die more over time. Others use large datasets of tree species, size and health from existing forests, and still others use machine learning to understand what will happen to the “climate niches” in which trees like to live.
Anderegg’s study contrasted these three approaches to envisioning the future of US forests, considering only the carbon they would store in aboveground biomass like leaves and tree trunks (rather than tree roots). and forest floor, which the authors say represent a minority of the carbon storage potential of forests). The results were everywhere.
The first approach, which is based on ecosystem dynamics and poorly models the effect of wildfires, suggested that US forests could store an average of 45% more carbon by 2100. A second model based on the machine learning predicted that US forests would store 4% less carbon in the same time frame, and a third based on data from more than 100,000 long-term forest monitoring plots across the US ended up somewhere goes in the middle. He predicted that forests in the eastern United States would store more carbon, but that growth would be more than offset by the death of trees in the wildfire-ridden West.
Anderegg said the third model is probably the most reliable because it’s based on extensive field data for US trees. But the results of the study show how difficult it is to predict the growth and decline of forests.
“(U)sing a one-size-fits-all approach to projecting carbon storage potential is likely inadequate” with respect to forest carbon storage, according to the paper.
This is particularly relevant for forest-based carbon offsets, which only work if the carbon remains locked up for a very long time – ideally on the order of centuries. Anderegg’s study shows how difficult it is to guarantee this extended time frame: the intermediate model conservatively predicted that 36% of the forest area designated as offsets by the California Air Resources Board – an agency that oversees state climate and air pollution programs — would lose carbon to wildfires, disease, and other climate-related stresses by the end of the century. The machine learning model’s projection was even worse: it predicted that carbon storage would decline for three-quarters of the regulator’s offset projects by 2100.
“Offsets are risky and a heavy proposition,” Anderegg told Grist. Other experts have come to the same conclusion, considering the number of offset plots that have caught fire in recent years, including plots representing more than 80 percent California’s “buffer pool” — an insurance system designed to protect the offset system against climate disasters over the next 100 years.
“These buffer basins are probably far too small for these climate hazards,” Anderegg added. He said mitigating carbon emissions and less reliance on offsets would reduce those risks, help keep forests healthy and advance other pressing climate goals.