For all the bad news about anthropogenic climate change, the world has been lucky so far. Humankind releases some 10 billion tons of carbon into the air each year, but only about half remains in the atmosphere for the long term. Of the other half, some gets dissolved in the oceans, and the rest gets taken up by land ecosystems, with tropical forests such as the Amazon doing a large part of the work.
Forests’ role as a carbon sink is not inevitable. Although trees absorb carbon from the atmosphere during photosynthesis, they also release it again when they die and decay. In the long run, those processes must be in near equilibrium. Apparently, forests have responded to rising atmospheric carbon dioxide levels by taking in more carbon than they release and shielding us from the full impact of our carbon emissions.
But that protective effect may already be waning, concludes a research team led by Luciana Gatti of Brazil’s National Institute for Space Research. Just in the past decade, a large portion of the eastern Amazon appears to have flipped from carbon sink to carbon source—even discounting the carbon emissions from the many fires that are deliberately set to make way for agriculture.
It’s not easy to measure how much carbon is flowing into or out of a patch of forest. Gatti and colleagues took a clever approach: Twice a month from 2010 to 2018, they used aircraft to collect flasks of air from above four sites around the Amazon. The samples’ CO2 content differed from the overall atmospheric CO2 concentration by a few parts per million, depending on whether the air gained or lost carbon as it wafted through the Amazon. After accounting for the winds, which generally blow from east to west in that part of the world, the researchers translated their measurements into carbon fluxes for different regions of the forest.
They also measured the concentrations of carbon monoxide—released in combustion but not in biological processes—to figure out how much of the CO2 was coming from fires. By subtracting the fire-sourced CO2 from the total CO2 flux, they deduced the fluxes for the ecosystems themselves.
The results varied greatly by region. The eastern Amazon, which has a drier dry season and is closer to the populous regions of Brazil, is far more affected by fires than is the wilder, wetter western Amazon. The ecosystem fluxes also varied regionally, with the southeast Amazon standing out by the end of the study as a net carbon source, not a sink.
The researchers attribute that alarming situation to a feedback process: Deforestation makes the dry season hotter and drier, a change that both stresses the ecosystem and makes it more likely that future fires will rage out of control and destroy more forest than intended. All signs point to more bad news to come: Over the past 40 years, average dry-season precipitation has declined in every region of the Amazon. (L. V. Gatti et al., Nature 595, 388, 2021.)
