A new study indicates that the natural uptake of atmospheric carbon dioxide by land and oceans has in the recent past been more efficient than previously thought. Researchers came to the conclusion by drawing up a new time series for global carbon emissions from deforestation—up to now a missing link in understanding the global carbon cycle.
The study shows that carbon emissions from deforestation between the 1960s and 1980s were lower than previous studies had assumed. By combining the time series with other datasets, it indicates that the uptake of carbon dioxide by nature has so far been influenced less by climate change than has been thought. The study was published today in the scientific journal Nature.
To estimate carbon emissions in areas of South America and Indonesia undergoing deforestation, the scientists used ground measurements of air visibility, which over decades have been recorded alongside data on temperature and precipitation. Smoke reduces visibility, and thus can be used to estimate deforestation in these regions. Consistent records of visibility at airports and other sites go back much further in time than satellite data. “By linking them to deforestation and forest fires on the basis of satellite measurements for more recent periods, we were able to establish a new global time series of carbon emissions from deforestation, and in that way to clarify the role of deforestation in the global climate system,” said joint lead author Margreet van Marle, a climate researcher at the Dutch research institute Deltares.
“These records are shared in real time by weather services around the world and have fortunately been archived. We were able to use them as a proxy for fire before the modern satellite record,” said study coauthor Robert Field of Columbia University.
Since the 1970s, scientists have known that about half of our carbon emissions from the burning of fossil fuels and deforestation stay in the atmosphere. The other half is absorbed by oceans and land. Extensive research has been conducted into whether this ratio has changed in the long term, possibly because climate change impairs the capacity of vegetation and oceans to absorb CO2. This could lead to an acceleration of climate change, because more CO2 is left in the atmosphere. Previous studies have provided evidence for this, but it has been difficult for researchers to draw firm conclusions, in part because emissions from deforestation have been a source of uncertainty.
The new time series shows that emissions from deforestation were lower from the 1960s to the 1980s than indicated by earlier studies. The scientists therefore believe that a larger fraction of the total emissions stayed in the atmosphere during that time. Over the entire 60-year time series, this means there has been a negative trend in the proportion of carbon that ends up in the atmosphere—a sign that CO2 uptake by nature has kept pace with growing emissions from fossil fuels, and may even have become more efficient over time.
According to joint lead author Dave van Wees of Vrije University Amsterdam, the cause for this trend remains unclear. “It may well be that some of the climate feedback loops that we are concerned about, such as the thawing of permafrost or more forest fires, are already making their mark but are being offset by other mechanisms,” he said.
According to Guido van der Werf, a professor at the Vrijje University Amsterdam who set up the study, it is difficult for now to draw firm conclusions about future climate change. “What we can mainly prove is that the worst nightmare scenarios have not yet materialized,” he said. That is primarily because the new insights relate to the 1960s, 1970s and 1980s, he said. “Looking at the last few decades, it might be that the improvement in efficiency has stalled,” he said.
The study was also conducted by scientists from Woodwell Climate Research Center and Wageningen University.
Adapted from a press release by Vrije University Amsterdam.