By John P. Roche
An intriguing part of the carbon cycle is that the concentration of carbon dioxide fluctuates up and down in the atmosphere seasonally each year. In the Northern Hemisphere, as carbon is taken up by plants via photosynthesis during the growing season from April to October, carbon dioxide concentration decreases. Then, when there is a net release of carbon from plants during the winter months from November to March, carbon dioxide increases. This seasonal fluctuation can be seen in graphs of atmospheric carbon dioxide concentration, such as the plot below. (In this graph, the red line shows CO2 measurements and the black line shows a multi-year average. Figure courtesy of the American Museum of Natural History. ©NOAA / ESRL.)
The amplitude in this seasonal fluctuation pattern in carbon dioxide has increased since the 1960s, and the amplitude of the fluctuation is more pronounced at high latitudes above 40 degrees N. These observations create two fascinating questions: (1) What is causing the increase in the amplitude in seasonal CO2 fluctuation? and (2) Why is this amplitude even more pronounced at high latitudes? In a recent study, Matthias Forkel and Nuno Carvalhais of the Max Planck Institute for Biogeochemistry, and colleagues that include Markus Reichstein of Max Planck and Ralph Keeling, the Director of the Scripps CO2 Program, sought answers to these questions by investigating factors that might influence high-latitude fluctuations in CO2.
In their study, Forkel and colleagues looked at the concentration of CO2 in the atmosphere, the Gross Primary Productivity of high latitude ecosystems, and the Net Biome Productivity of high latitude ecosystems. They fed these data into a sophisticated atmospheric transport model that they call LPJmL+TM3. Their atmospheric model improves on other models by looking at a wider range of processes that affect the carbon cycle, including irrigation, agriculture, change in land use, and dynamics of vegetation patterns, including the effect of fire and the amount of permafrost. They optimized the predictive power of their model against observations made from satellites, and they estimated CO2 amplitude at 19 sites, all of which had 20 or more years of data.
Their modeling results estimated that in the 1970s, Net Biome Productivity contributed to 51% of CO2 amplitude trends in sub-arctic high-latitude regions called boreal regions, but in the 2000s, Net Biome Productivity contributed 54%. That is, Net Biome Productivity had a large and increasing effect on CO2 amplitude in boreal regions. They estimated that the current overall contribution of Net Biome Productivity to high latitude CO2 amplitude trends is 57% in boreal regions, and 25% in arctic regions. Conversely, emissions of carbon from fossil fuels, and exchange of carbon with oceans both had only small effects on CO2 amplitude trends.
The investigators concluded that the increase in seasonal fluctuation in CO2 at high latitudes was caused by an increase in net carbon uptake by plants at high latitudes, which was in turn caused by an increase in photosynthesis. The increase in photosynthesis was caused by increased temperatures. Thus, high latitudes have witnessed what the authors call a climate-vegetation-carbon feedback system: as the concentration of CO2 in the atmosphere increased, the CO2 trapped heat and created a warmer climate at high latitudes, which led to an increase in vegetation, which led to more carbon being taken up in the growing season and thus a greater annual fluctuation in CO2.
Does this increased carbon uptake by photosynthesis at high latitudes serve as a sink for carbon released by fossil fuels? When asked this question, Forkel said that the increase in carbon uptake at high latitudes takes up about 2% of the carbon released from the burning of fossil fuels, from cement production, and from changes in land use. "The strength of the northern ecosystems for carbon uptake increased by around 4% per year," Forkel said. But he added that if climate warming continues, carbon stored in permafrost will be increasingly released to the atmosphere.
When asked what are the important next steps in this research on the carbon cycle, Forkel indicated that the climate modeling community needs to further improve their models. "Observations of atmospheric CO2 concentrations," Forkel said, "ecosystem measurements of photosynthesis and respiration, and available satellite observations should be increasingly used to parameterize and test vegetation models." He also spoke to the need to increasingly consider the role that vegetation dynamics plays in the global carbon cycle. Forkel said, "It is necessary to understand how trees grow, how different plant groups compete, and how extreme events such as droughts or insects contribute to plant mortality. All of these process contribute to the strength of ecosystems to take up carbon, and can be traced to changes in atmospheric CO2 concentrations."
To read the full study in Science magazine, click here.
To read a related Carbon in the News article, "Increased Carbon Uptake by Land Plants Leads to Pause in Growth Rate of Atmospheric CO2," click here.