Forests: how well do they really store carbon?
Forests are an important part of the global carbon cycle. Trees retain a portion of the CO2 they remove from the atmosphere, sequestering that carbon in the tree for as long as it lives. Dead trees and fallen leaves and branches decompose on the forest floor, at which point the carbon is either released back into the atmosphere or stored in the soil. The amount of carbon a forest is able to store varies based on the tree species, soil type, underlying geology, regional climate and the forestry management practices used. Typically, younger forests that are still growing rapidly and forests with younger underlying rocks store the most carbon.
Forests’ ability to sequester carbon has made them a prime target for policy initiatives to offset CO2 emissions and curb global warming. However, because of deforestation and forest degradation, tropical rainforests today are actually a source of carbon rather than a carbon sink. Tropical forests are releasing carbon at a greater rate than the slow-growing trees are able to absorb it, contributing to around 15% of global greenhouse gas emissions annually.
To address this problem, the United Nations has proposed REDD+ (Reducing Emissions from Deforestation and Degradation, Plus Related Pro-forest Activities) policies. As part of its effort to preserve existing tropical forests, REDD+ would pay developing countries to set aside forest land that would be otherwise converted for uses such as logging, farming, mining and road construction.
Recent studies complicate the matter
However, recent studies show that previous estimates of carbon storage by tropical, temperate and boreal forests were overly optimistic. Research at the University of Guelph in Canada revealed that temperate and boreal forests are not storing as much carbon as expected. Results indicated that stress on trees, partly due to a warming climate, caused a decline in tree growth. These results contradict the expectation that trees will store more carbon as atmospheric CO2 levels rise.
In another study, scientists from the Carnegie Institution for Science compared their assessment of the amount of carbon stored in a region of lowland tropical forest in Peru to that from the Intergovernmental Panel on Climate Change (IPCC), and found there to be 398 million tonnes of carbon stored, rather than 587 million. This discrepancy is not small; it is approximately one third lower than the IPCC estimate.
These findings have implications for REDD+ or any policies that rely on quantifying the amount of carbon stored in forest land to accurately offset emissions. The REDD+ strategies of restoring degraded forests, seeding new forests, and managing them sustainably (in addition to conserving existing forests) may have to become more aggressive in order to reach the goal of zero net greenhouse gas emissions from tropical forests by 2030.
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