The world’s tropical forests are absorbing significantly less carbon dioxide than the models scientists use to understand the global carbon cycle have been predicting.
A new study used airborne measurements collected across the globe to test how well existing models capture the movement of carbon through the Earth system.
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The results cut uncertainty in those models by as much as half and revealed a discrepancy between what the models predict and what the atmosphere actually shows.
The work drew on data from NASA’s Atmospheric Tomography Mission (ATom), a series of airborne campaigns conducted between 2016 and 2018 using a NASA DC-8 aircraft.
The aircraft flew continuous measurement transects from the western Arctic to Antarctica over the Pacific and Atlantic oceans.
Along the way, they sampled the atmosphere from near the surface to more than 40,000 feet.
Where all that carbon goes
About half of industrial carbon emissions stay in the atmosphere and drive the steady rise in atmospheric CO2, from 280 parts per million before industrialization to more than 430 today.
The other half is absorbed by natural carbon sinks: forests, soils, and oceans.
Which sinks are doing how much of that work, and how efficiently, is one of the most consequential open questions in climate science.
“There are big uncertainties in our understanding of the natural carbon cycle at the largest scales,” said study lead author Britton Stephens, a scientist at the National Center for Atmospheric Research.
“By refining our understanding of how much carbon dioxide is taken up and released by the oceans and the land, we can more accurately track where emissions are going and the impacts on the Earth system.”
Tropical forests tell another story
The new measurements revealed something specific and important: there is more carbon dioxide above the tropics than Earth system models predict.
That pattern implies tropical forests are taking in less carbon than the models suggest.
Farther north and south, the picture is different and more ambiguous. Carbon dioxide concentrations there were lower than the models predict.
That could mean those forests are absorbing more carbon than previously thought. It could also mean fossil fuel emission estimates for those regions are too high, or it could reflect some combination of both.
Seeing carbon from above
Scientists already measure atmospheric CO2 using ground-based instruments and satellites. Both have limitations that matter at a global scale.
Ground stations are sparse. Extrapolating regional or global trends from a network of fixed points requires assumptions about how carbon dioxide mixes vertically through the atmosphere.
These assumptions are difficult to verify.
Satellites provide broad coverage, but the technique for measuring CO2 from orbit requires extraordinary precision, and coverage breaks down in cloudy regions and at high latitudes.
Aircraft occupy a different niche, flying at varying altitudes along carefully designed transects. They sample a well-mixed cross-section of the atmosphere.
Those measurements create a detailed, three-dimensional picture of carbon dioxide across regions and seasons.
Flights across all seasons
For the ATom mission, flights were conducted during all four seasons. Five separate instruments measured carbon dioxide simultaneously on each flight, reducing measurement errors considerably.
The same instruments flew every transect, providing a level of consistency that is hard to achieve across a patchwork of different monitoring stations.
“The aircraft and satellites work together synergistically,” said Stephens. “Satellite carbon dioxide measurements have transformed how we observe small-scale sources and large-scale interannual variability.
“But they have struggled when it comes to estimating how carbon dioxide moves through Earth’s sinks and sources on average at the global scale.”
The aircraft fills a critical observation gap, allowing scientists to get much more value from satellite measurements.
Improving climate predictions
The practical stakes of getting the carbon cycle right are high. Climate projections depend on more than knowing how much carbon dioxide humans emit.
Scientists also need to know how much natural systems absorb and how quickly they do it.
If tropical forests are absorbing less carbon than models assume, they may be offsetting less of humanity’s emissions than expected. That leaves more carbon dioxide in the atmosphere, where it drives additional warming.
It also affects how we interpret observed atmospheric CO2 concentrations. If a measurement station records rising CO2, is that because emissions have increased, because a nearby sink has weakened, or both?
Without accurate models of what the natural carbon cycle is doing, that question is difficult to answer confidently.
The need for constant monitoring
The ATom campaign wasn’t designed as a permanent monitoring system; it was a research mission. But the findings make a clear case for regular, sustained airborne measurement programs.
Four years of data, collected systematically across the globe, cut model uncertainty by half.
More data, collected consistently over time, would narrow that uncertainty.
It could also resolve whether the patterns outside the tropics are driven by stronger carbon absorption, overestimated fossil fuel emissions, or a combination of both.
The atmosphere has been tracking where carbon goes for centuries. Getting instruments into it at scale, and keeping them there, seems like the most direct way to read what it has been recording.
The study is published in the Proceedings of the National Academy of Sciences.
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NOTE – This article was originally published in Earth and can be viewed here
