Dry grasslands have been serving as an unexpected carbon sink in recent decades due to fire reductions: study

Firefighter Cecilia Fernandez works near Altos, Paraguay, Friday, Oct. 2, 2020. (AP Photo/Jorge Saenz)

Dry grasslands may have been providing an unseen helping hand to slow the rate of climate warning, according to a new study which found that these grasslands have been storing more heat-trapping carbon than previously known.

It’s an effect that goes hand in hand with the frequency of fires, researchers found – savannas and grasslands where fires had become less frequent had more carbon-storing potential. While it’s far from a climate-crisis-ending impact, researchers say this means we may have another tool in our arsenal to utilize when planning nature-based interventions to slow the warming of the planet.

"In the grand scheme of things, no, this is not really a massive amount of carbon that will put a dent in heat-trapping anthropogenic emissions," study lead author Adam Pellegrini of the University of Michigan said in a press release. "But no one region—neither the Amazon rainforest nor the U.S. Great Plains grasslands nor Canada's boreal forest nor dozens of other biomes around the world—can alone store sufficient carbon to make a large contribution to slowing climate change. However, in aggregate, they can.”

• Climate Barometer newsletter: Sign up to keep your finger on the climate pulse

• The information you need to know, sent directly to you: Download the CTV News App

The findings are the result of the reanalysis of 53 long-term fire-manipulation experiments worldwide. Researchers also took soil samples from six of the fields in question for the study, which was published Monday in the journal Nature Climate Change.

Researchers wanted to figure out how fire changed how much carbon was stored in topsoil in more arid regions.

As populations expanded and landscapes became increasingly fragmented into croplands, pastures and smaller regions of grassland separated by roads, the size of wildfires in these regions have become smaller. As fire frequency and size changed, researchers observed that this was reflected in the amount of carbon stored in the topsoil, and that this was most dramatic among dry grasslands or savannas.

"The potential to lose soil carbon with very high fire frequencies was the greatest in dry areas, and the potential to store carbon when fires were less frequent was also the greatest in dry areas,” Pellegrini said.

Researchers found that over the past 20 years, there has been an estimated 23 per cent increase in stored carbon in the soil due to the reduction in the size and frequency of wildfires across 2.3 million square kilometres of drier savanna-grasslands.

At the same time, increases in wildfire frequency in more humid savanna-grasslands across 1.38 million square kilometres led to an estimated 25 per cent loss in soil carbon.

The study, which covered sites in South Africa, Brazil and North America, estimates that soils in savannas and grasslands may have gained and stored 640 million metric tons of carbon over the past 20 years.

Most climate models haven’t accounted for arid grasslands storing more carbon as fire frequency decreased, researchers say, meaning these regions have served as an unknown carbon sink.

"Ongoing declines in fire frequencies have probably created an extensive carbon sink in the soils of global drylands that may have been underestimated by ecosystem models,” Peter Reich, professor and director of the Institute for Global Change Biology at the University of Michigan’s School for Environment and Sustainability and second author of the study, said in the release.

"In other words, in the past couple of decades, global savannas and grasslands have slowed climate warming more than they have accelerated it—despite fires. But there is absolutely no guarantee that will continue in the future."

Researchers proposed that fire management be integrated into discussions for climate solutions in savanna-grasslands areas in order to encourage this carbon sink effect and also try and prevent the loss of topsoil carbon due to fires.

Even though the effect is small, it can still have an impact, Pellegrini said.

"Plus, there are several savanna and grassland regions that have soil carbon-credit projects being developed, so understanding their capacity to sequester carbon is relevant to the region—even if it's not a massive flux globally."