Flourishing vegetation enhances carbon dioxide variability

All over the world, measuring stations are recording the increase in the greenhouse gas carbon dioxide in the atmosphere. In high northern latitudes, however, a further trend can be seen: The seasonal fluctuations in carbon dioxide have been increasing since the 1960s. This is mainly due to increased plant growth in the north, write researchers from the Max Planck Institute for Biogeochemistry in Jena together with colleagues from the Potsdam Institute for Climate Impact Research and the USA in the current issue of the journal Science. Due to the higher temperatures, more forests are spreading and photosynthesis is increasing. As a result, vegetation absorbs more carbon dioxide in summer.

The team led by the Jena researchers has thus solved a tricky climate puzzle. This is because the observed trend could not be reproduced and therefore not explained using conventional climate models - presumably because the models do not depict certain processes in the interplay between vegetation and climate accurately enough.

Since 1958, measurements taken at the Mauna Loa volcano in Hawaii have documented how the CO2 content of the atmosphere has increased from year to year. Originally at 315 parts per million (ppm), it has now climbed to over 400 ppm. The famous Mauna Loa curve, however, does not rise uniformly; rather, it fluctuates throughout the year. At the end of each spring, the value climbs to a new high. This is because little photosynthesis takes place in the Northern Hemisphere during winter. Vegetation therefore absorbs hardly any CO2 from the air for months, while human emissions accumulate. In addition, plants and soils release some of the previously absorbed carbon dioxide back into the atmosphere through respiration, which breaks down carbohydrates.

Several hypotheses on increased CO₂ fluctuations

„In September, the CO₂ content then falls to a minimum because the vegetation in the northern hemisphere absorbs carbon dioxide from the air throughout the summer," explains Matthias Forkel, who recently moved from the Max Planck Institute in Jena to the Vienna University of Technology. Together with Nuno Carvalhais from the Max Planck Institute for Biogeochemistry, he was the lead author of the study.

In Hawaii, seasonal carbon dioxide fluctuations were six ppm in the early 1960s; they have since risen to about seven ppm. The increase was even greater in northern regions. For example, annual CO₂ levels in Barrow, Alaska, fluctuated by 15 ppm in the early 1960s; today, the difference is 18 ppm—an increase of almost 25 percent. Using aircraft measurements, a team led by British researcher Heather Graven demonstrated in 2013 that these changes occur over a wide area in northern latitudes, even at an altitude of about six kilometers.

However, Graven and her colleagues did not provide an explanation for the trend in their Science publication at the time. This was because the increase did not occur in climate models. "There were therefore several hypotheses as to why the amplitude of the fluctuation is increasing," says Markus Reichstein, director at the Max Planck Institute for Biogeochemistry in Jena and co-author of the new study. Many researchers already suspected at the time that the phenomenon was related to the observed greening of the Arctic. Some, however, also blamed more intensive agriculture with its higher yields. Others believed a fertilization effect of CO₂ was the cause.

Higher temperatures cause plants to sprout more in the north

The Jena researchers now wanted to clarify the question by combining observations with model calculations. "We improved our vegetation model developed in Jena, Potsdam, and Lund using satellite data from the last 30 years and were thus able to correctly simulate the trend in CO₂ amplitude," says Matthias Forkel.

As they report in Science, climate change is primarily responsible for the greater CO₂ amplitude: The higher temperatures are causing plants north of the 45th parallel to sprout more strongly than before. The boreal forests are expanding ever further. Bushes and shrubs are colonizing the once barren tundra, and the growing season is beginning earlier in the year. All of this means that more CO₂ is disappearing from the atmosphere in summer than 50 years ago. "The respiration of plants and soils, on the other hand, has not increased to the same extent," says Matthias Forkel. Thus, the difference between summer and winter has become greater.

Huge feedback effects between climate and vegetation

The researchers also found that plant growth in the north is barely affected by the increased availability of CO2. "CO2 fertilization plays a negligible role in high latitudes," reports Markus Reichstein. Another finding: Agriculture does not contribute nearly as much to the increase in seasonal CO2 fluctuations as other researchers had previously calculated.

„"It's clear that there are huge feedback effects between climate and vegetation," concludes Markus Reichstein. The vegetation model, which was primarily developed at the Potsdam Institute for Climate Impact Research and tested at the Max Planck Institute in Jena using observations, describes these complex relationships more accurately than conventional Earth system models. For example, it better captures the fact that plants sprout earlier when temperatures rise, or that more water is available to vegetation in permafrost regions when temperatures get warmer.

Whether the increase in seasonal CO₂ fluctuations will continue in the future and what effect this will have on global warming, however, cannot be concluded from the study. The spread of forests in the north probably also has negative consequences for climate change, says Matthias Forkel: "The land surface darkens and absorbs a larger proportion of the incident solar radiation. It is therefore warming up more than before." In turn, this could lead to an increase in forest fires and droughts in the future or an increase in the spread of harmful insects. It is therefore unclear whether the trend towards a greener north will continue.