Fires Are Keeping Arctic Forest Growth From Offsetting CO2 Release

Scientists hoped new forest growth would trap CO2 released by melting ice and permafrost due to global warming, but a new study shows it’s not enough because wildfires are reducing forest growth.

In this July 18, 2018 photo, smoke billows from a fire outside Ljusdal, Sweden. Sweden enlisted its military and volunteers to help fight wildfires above the Arctic Circle. (Maja Suslin/TT via AP)

(CN) — Climate change is causing the Arctic to get greener, but those thicker forests will not help battle climate change as well as was hoped, a new study says.

Scientists believed that although the warming atmosphere is melting Arctic ice and permafrost, and therefore releasing sequestered carbon dioxide, the warmer temperatures could spur more plant growth in the same area that might be able to offset the CO2 releases.

A group of researchers representing the University of California at Irvine, Boston University and Montana State University published a study Thursday in Nature Climate Change, a peer-reviewed scientific journal, finding otherwise.

The Arctic’s boreal forests are important in the fight against climate change because they are widespread and can hold a lot of carbon, but they are also susceptible to disturbances like fire, harvest, insect infestations and drought, according to the study.

“Understanding how disturbance affects large-scale AGB [above-ground biomass, i.e. vegetation] dynamics is therefore crucial for understanding how changing disturbance regimes in boreal ecosystems will influence the high-latitude terrestrial carbon budget,” the scientists wrote.

Although there have been gains in Arctic plant life, “much of the new green biomass isn’t storing carbon — it’s combusting during wildfires,” reads the the group’s press release.

“What we estimate is that 430 million metric tons of biomass has accumulated over the last 31 years,” said UCI Earth system scientist Jon Wang, the study’s lead author, in the release. “But across this domain it would’ve been nearly double if it weren’t for these fires and harvests that are keeping it down.”

The researchers used satellite data from 1984 to 2014 to look at 2.8 million square kilometers, about 1 million square miles, of land in Canada and Alaska, tracking changes in above-ground biomass over time.

They found a very small increase in overall AGB, but it was far lower than increases shown by many other studies, which didn’t accurately account for disturbances, the scientists said.

Overall, the study area experienced positive growth early in the 20-year record in areas that saw no fires or harvests, and then fluctuated between positive and negative growth from 2003-2014.

“These gains may result from a combination of recovery from undetected disturbances, climate-driven expansion of forest or shrub cover, increases in forest density, and growth enhancement from multiple global change drivers,” the study said.

However, this small amount of accumulation was lessened by the destruction wildfires caused.

Since 1984, fires burned 14.8% of the study area; harvests affected just 1%. Looking back to 1940, the earliest data, another 11.5% was lost to fire.

“Annual rates of both fire-driven losses and recovery of AGB have increased considerably since 1940 with the majority of both cumulative losses and gains occurring in the last 23 years of the 75-year fire record,” the group said. “This result is driven by increases in annual burned area that have occurred since 1940 and especially in the past few decades.”

The study’s models showed that only 59% of AGB loss from fires since 1940 has been regained in forest recovery.

They also found that fire disturbances were more of a predictor of AGB variation between years than climate change was, making them not only a major driver of changes in the forest but important for its future.

Climate change, however, is a major driver of increased wildfires, and is predicted to make the Arctic both warmer and drier. Years with drier weather tend to have larger and more frequent fires.

“These results suggest that as the climate continues to warm, areas of fire and post-burn recovery will expand, and fires will play an increasingly important role in boreal AGB dynamics,” the study continues. “In these forests, disturbance regulates the spatial and temporal dynamics of AGB change with projected increases in the frequency, extent and severity of high-latitude disturbance likely to further limit the potential of these ecosystems to serve as a terrestrial carbon sink.”

Co-author of the study James Randerson of UCI said in the release that “the rates of carbon accumulation in this region are lower than what previous studies have indicated and will push the science community to look elsewhere for the main drivers of the terrestrial carbon sink.”

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