Hurricanes jeopardize carbon-storing New England forests

Sequestering carbon in forests is an example of a nature-based solution that is being used to address climate change, but a new study suggests that hurricanes could pose a risk.

Offset programs involve investments organizations or individuals can make in projects that cut carbon emissions, such as solar energy, or that can store carbon, such as preserving and enhancing forests.

As it happens, New England is one of the most heavily forested regions in the U.S. with Maine at 83%, New Hampshire at 80%, and Vermont at 74%.

In the California carbon market, the largest regulatory U.S. carbon market, 3% of a carbon offset project is reserved for catastrophic risks such as hurricanes and other storm events.

Wildfires, which comprise their own separate risk category, can also deplete forests that store carbon, known as "carbon stocks," and have typically been the focus of prior research on disturbances to such stocks.

The study finds that a single hurricane may wipe out 5% to 10% of total aboveground forest carbon, through tree damage, in New England. The results are published in Global Change Biology.

"Our results reveal that carbon offset programs in the U.S. do not adequately account for the risk of hurricanes, as a single storm could wipe out everything the program has set aside to ensure against risk," says lead author Shersingh Joseph Tumber-Dávila, an assistant professor of environmental studies at Dartmouth and investigator at Harvard Forest where he conducted this work.

While New England hasn't experienced many severe hurricanes in recent decades, they are an important driver of long-term ecosystem change. The Hurricane of 1938, for example, caused widespread tree damage in New England, leading to the salvaging of 500 million board feet of lumber in the Granite State alone, according to the Society for the Protection of New Hampshire Forests.

Hurricanes obtain their energy from the ocean and typically impact the southeastern coastal regions of the U.S.; however, they are a dominant disturbance agent in the Northeast as well.

"As the climate warms and sea surface temperatures continue to rise, hurricanes could get stronger and will have the capacity to stay on land longer, with the potential to move inland and northward into the heavily forested regions of the Northeast," says Tumber-Dávila.

For the study, the research team examined the 10 most powerful hurricanes that had an impact on land in New England over the past century, including Hurricane Bob in August 1991, and analyzed how the region's forests would be impacted if one of those storms were to hit today.

They mapped the trees -- the aboveground forest carbon in New England -- using USDA Forest Service Forest Inventory and Analysis Program data and mapped the hurricanes using tracking and wind speed data to simulate a storm's path and strength in a geographic area. They determined how susceptible a forest was to wind damage based on the height and type of the trees. The team applied meteorological predictions to estimate the potential future strength of hurricanes.

The study finds that a projected 8% and 16% increase in hurricane wind speeds leads to a nearly 11- and 25-fold increase in high-severity impacts that would likely cause widespread tree death.

"In the context of climate change mitigation, the forest sector is unique in that carbon moves both into and out of the system," says senior author Jonathan Thompson, a senior ecologist and research director at Harvard Forest, which is based in Petersham, Massachusetts. "When mitigation programs look to forests, they often focus only on carbon moving into the forest through sequestration but our research shows the potential for carbon to move back into the atmosphere via hurricanes."

To estimate how long it would take for the forest carbon to be emitted from the downed trees due to the given hurricane, the researchers took into account wood decay rates and estimated the timber products that could be made from the salvaged wood based on regional timber product reports.

The results show that it takes nearly 19 years for trees knocked over by a hurricane to become a net emission and 100 years for most of the downed carbon (90%) to be emitted.

One hurricane however, can lead to the release of the 10-year equivalent of carbon sequestered in New England's forests.

Tumber-Dávila says the study suggests that future hurricanes need to be considered as using New England forests to capture and store carbon from the atmosphere becomes more popular.

"If forest carbon stocks are going to continue to be used as a nature-based climate solution, we have to be critical about evaluating its longevity and risks, to make sure that we're doing something that actually has an impact," says Tumber-Dávila.