By John P. Roche
Carbon dioxide enters the atmosphere from numerous processes, including plant and animal respiration, changes in land use, and combustion of fossil fuels. Carbon is removed from the atmosphere by additional processes, including absorption into oceans and uptake via photosynthesis by plants on land. A recent study by Trevor Keenan of the Lawrence Berkeley National Lab and colleagues found that since 2002, there has been a pause in the rate of increase in CO2 in the atmosphere, and that this pause may be caused by an increase in carbon sequestration by land plants.
The amount of carbon taken up by land plants and oceans is significant. The 2015 Global Carbon Budget study concluded that each year, about 45% of CO2 released by human activities is taken up by terrestrial ecosystems and oceans. The amount of carbon absorbed by land ecosystems and oceans has doubled in the past half century.
Keenan and his colleagues wanted to explore what is causing this increase in carbon uptake by terrestrial ecosystems, and what effect this increase in uptake has on atmospheric CO2 levels. Surface measurements show a plateau in temperature over land since the start of the 21st century, while the concentration of atmospheric CO2 over the same period continued to rise. This provides scientists with an excellent opportunity to explore the relative effects of CO2 and temperature on carbon uptake by plants.
In their study, Keenan and colleagues collected data on CO2 levels in the atmosphere, and used satellite measurements of land vegetation to estimate components of the terrestrial carbon cycle. They also conducted modeling simulations with ten predictive models known as dynamic global vegetation models, and they calculated the average predictions from those ten models.
Carbon dioxide uptake and release by plants is affected by photosynthesis and respiration. Photosynthesis uses carbon dioxide to produce carbon compounds called carbohydrates (releasing oxygen in the process). Respiration (in both plants and animals) releases carbon dioxide when those carbohydrates are broken down. Photosynthesis is stimulated by increased atmospheric CO2—with more CO2 available, plants can use more CO2 to produce more carbohydrates, many of which are structural and contribute to increased plant biomass. This is called CO2 fertilization, and it tends to increase the amount of carbon in the terrestrial "carbon sink." Respiration, on the other hand, is stimulated by an increase in temperature.
Keenan and colleagues concluded that the increase in uptake of carbon by terrestrial ecosystems resulted from an increase in photosynthesis caused by rising atmospheric CO2 levels, combined with a reduction in respiration caused by the plateau in temperature. They also calculated that whereas the atmospheric CO2 growth rate increased from 1959 to the 1990s, there was no increase in the atmospheric CO2 growth rate from 2002 to 2014. In other words, while the total concentration of atmospheric CO2 was still increasing, there was no increase in the rate of increase of atmospheric CO2 during that period. They also found that although the proportion of CO2 emissions from human activities that remain in the atmosphere increased from the 1960s to the 1990s, this proportion has decreased since 2002. This is noteworthy because this proportion decreased even though the magnitude of human-made emissions has been increasing since 2002. They attributed these patterns to the increased rate of photosynthesis-driven carbon uptake in plants.
What are the implications of this study? It is dramatic that land plants can take up such a huge amount of carbon. But they don't take up all the carbon—the oceans and terrestrial ecosystems combined only take up about half of human-released CO2. As a result, CO2 levels are still rising—just more slowly than they would be without the high rate of uptake by plants. Keenan and colleagues emphasize that going forward, it will be important to maintain the carbon sequestration provided by land plants and preserve the ecosystems in which those plants exist.
Another intriguing aspect of this research is the insights it provides about environmental uncertainty. Temperature over land plateaued between 2002 and 2014, and atmospheric CO2 growth rate paused between 2002 and 2014. In addition, observations show no decrease in soil moisture, and no increase in drought, on a global level. All of these observations are contrary to many earlier predictions. These findings highlight that climate is complex and that variables related to climate are difficult to predict in the short term. They also highlight that care needs to be used in analyzing climate measurements, and in interpreting model predictions.
When asked about the next phase for this research, Keenan said, "The next step is identifying where the carbon sink is changing the most rapidly. This is important information as it can help policy makers decide where to focus efforts to protect ecosystems in order to ensure that the carbon sink continues." Another, longer-term goal, Kennan said, "is to develop models that can predict how these ecosystems will affect the carbon cycle in the future. It is not clear whether the sink will continue, or whether ecosystems will actually become a source of carbon, instead of a sink, in this century."
You can read the complete study by Keenan and colleagues in Nature Communications at:
Recent pause in the growth rate of atmospheric CO2 due to enhanced terrestrial carbon uptake.
To read a related Carbon in the News article, "Increased Vegetation Cover Affects Seasonal Fluctuations in Carbon at High Latitudes," click here.
Keenan, T.F., Prentice, I.C., Canadell, J.G., Williams, C.A., Wang, H., Raupach, M. and Collatz, G.J., 2016. Recent pause in the growth rate of atmospheric CO2 due to enhanced terrestrial carbon uptake. Nature Communications, 7, p.13428.
Le Quéré, C., Moriarty, R., Andrew, R.M., Canadell, J.G., Sitch, S., Korsbakken, J.I., Friedlingstein, P., Peters, G.P., Andres, R.J., Boden, T.A. and Houghton, R.A., 2015. Global carbon budget 2015. Earth System Science Data, 7(2), pp.349-396.