Tree planting to offset carbon could be a gamble

But researchers are warning that forests can be a risky way to offset the production of greenhouse gas emissions.

Trees are capable of storing carbon in their trunks and roots, which helps to lower the concentrations of carbon dioxide found in the atmosphere. Impact of climate change such as severe droughts or wildfires, however, could see much of that stored carbon could go up in smoke.

This was notably experienced during the Australian wildfires in January, which saw an estimated 900 million tons (820 tonnes) of carbon dioxide released into the atmosphere: almost double the country's total annual fossil fuel emissions.

“There have been optimistic assessments of how valuable forests could be in mitigating climate change over coming decades,” said Professor Scott Goetz of Northern Arizona University. “But all of those have somewhat surprisingly overlooked or underestimated the factors that constrain forest carbon sequestration in the face of extreme temperatures, drought, fire and insect disturbance.”

“This paper tempers that enthusiasm while also more realistically recognising the potential of forests to remove massive amounts of heat trapping carbon dioxide from the atmosphere.”

Professor Deborah Huntzinger, who also worked on the project, said: “Terrestrial ecosystems absorb and store about a third of the carbon emissions human activities release each year. Reducing the amount of carbon dioxide that accumulates in the atmosphere year after year. As a result, land ecosystems serve as a thermostat of sorts, regulating climate by helping to control carbon dioxide levels.”

Governments in many countries are looking to “forest-based natural climate solutions” which include preventing deforestation, managing natural forests and reforesting.

Forests could be some of the more cost-effective climate mitigation strategies, with co-benefits for biodiversity, conservation and local communities. But built into this strategy is the assumption that forests are able to store carbon for at least 50 to 100 years.

Such permanence is not guaranteed, with the very real chance that the carbon stored in forest mitigation projects could go up in flames or be lost due to insect infestations, severe drought or hurricanes in the coming decades.

Forests have long been vulnerable to all of these factors, and have been able to recover from them when they are episodic or come one at a time.

However, the risks connected with climate change, including drought and fire, increase over time. Multiple threats at once, or insufficient time for forests to recover from those threats, can kill the trees, release carbon and undermine the entire premise of forest-based natural climate solutions.

“Not fully accounting for the range of climate- and human-driven risks to forests can result in an overestimation of the carbon storage potential of forest-based mitigation projects,” said Huntzinger.

The researchers want increased attention on assessing forest climate risks so that policymakers can make more robust climate strategies in the future.