Half of the carbon dioxide emitted by human activities into the atmosphere continues to be absorbed by Earth's oceans, forests and other ecosystems, according to a new study conducted by American researchers.
University of Colorado and NOAA scientists analyzed 50 years of global carbon dioxide (CO2) measurements and found that the processes by which the planet's oceans and ecosystems absorb the greenhouse gas are not yet at capacity.
"Globally, these carbon dioxide 'sinks' have roughly kept pace with emissions from human activities, continuing to draw about half of the emitted CO2 back out of the atmosphere. However, we do not expect this to continue indefinitely," said NOAA's Pieter Tans, a climate researcher with NOAA's Earth System Research Laboratory in Boulder, Colo., and co-author of the study.
Carbon dioxide is emitted into the atmosphere mainly by fossil fuel combustion but also by forest fires and some natural processes. The gas can also be pulled out of the atmosphere into the tissues of growing plants or absorbed by the waters of Earth's oceans.
A series of recent studies suggested that natural sinks of carbon dioxide might no longer be keeping up with the increasing rate of emissions. If that were to happen, it would cause a faster-than-expected rise in atmospheric carbon dioxide and projected climate change impacts.
However, University of Colorado's Ashley Ballantyne, Tans and their colleagues saw no faster-than-expected rise.
Their estimate showed that overall, oceans and natural ecosystems continue to pull about half of people's carbon dioxide emissions out of the atmosphere.
Since emissions of CO2 have increased substantially since 1960, Ballantyne said, "Earth is taking up twice as much CO2 today as it was 50 years ago."
The rest continues to accumulate in the atmosphere, where it is likely to accelerate global warming.
This new global analysis makes it clear that scientists do not yet understand well enough the processes by which ecosystems of the world are removing CO2 from the atmosphere, or the relative importance of possible sinks: regrowing forests on different continents, for example, or changing absorption of carbon dioxide by various ocean regions.
"Since we don't know why or where this process is happening, we cannot count on it. We need to identify what's going on here, so that we can improve our projections of future CO2 levels and how climate change will progress in the future," Tans said.
The study will soon be published in the journal Nature.