How does the photosynthesis rate of plants change when more CO2 is available in the ambient air? Scientists have been dealing with this question for a long time and, depending on the method, has led to different assessments of the extent to which the plants absorb additional CO2. A new study has now come to the conclusion that plants now photosynthesize around twelve percent more than 30 years ago – with a simultaneous increase in the CO2 content in the atmosphere of 17 percent. As a result, the plants, as natural stores of the greenhouse gas, slow down its accumulation in the atmosphere.
During photosynthesis, plants use sunlight to convert water and CO2 into sugar and oxygen. They quickly release part of the bound carbon dioxide through their own cell respiration, while another part is bound in the biomass and only released again later, for example when the plant rots. Plants thus represent an important terrestrial carbon sink. If the CO2 content of the atmosphere rises, the plants can potentially carry out more photosynthesis and thus store more CO2. To what extent they actually do this, however, has so far been controversial. The estimates ranged, depending on the method, from an almost non-existent to a considerable effect.
Combined methods
A team led by Trevor Keenan from the University of California at Berkeley has created a new assessment that shows less uncertainty than previous surveys. To do this, the researchers combined various methods: On the one hand, they relied on data from the Global Carbon Project, which estimates the global carbon sinks year after year. They compared this with satellite data, which provides information about which part of the earth is green, i.e. how many plants in total can photosynthesize.
While such satellite data have shown that the amount of green on earth has increased in recent decades, they do not take into account a possibly increased photosynthetic output of the individual plants. In addition, Keenan and his team used data from so-called terrestrial models of the biosphere. Among other things, plants are specifically exposed to an increased CO2 concentration in field experiments and the extent to which they carry out more photosynthesis as a result is measured. The researchers also combined all of these methods with machine learning algorithms in order to arrive at the most reliable estimate possible.
More photosynthesis
“Our results suggest that the rise in atmospheric CO2 since 1982 has led to a sharp increase in global photosynthesis,” the researchers report. “This response to carbon concentration is underestimated by standard satellite-based methods, but overestimated by terrestrial biosphere models and similar methods.” “This puts our estimate of the increase in the middle of the other estimates,” said Keenan. “In making our calculation, we were able to double-check the other estimates and understand why they were too big or too small. That gave us confidence in our results. “
According to this, the plants absorb 14 billion tons of additional carbon due to the increased CO2 content in the atmosphere. For comparison: the total human-made CO2 emissions in 2020 was around 35 billion tons. “This is a very strong increase in photosynthesis, but it is nowhere near enough to remove the amount of carbon dioxide that we are releasing into the atmosphere,” says Keenan. “We can’t stop climate change, but we can slow it down.”
Unclear future
However, according to the authors, it is unclear how long we can expect a further increase in photosynthesis with increasing CO2 concentration. “We don’t know what the future will look like and how the plants will continue to react to the rising carbon dioxide,” says Keenan. “We assume that there will be saturation at some point, but we don’t know when and to what extent. At that point, the sinks will have much less capacity to offset our emissions. And sinks are currently the only nature-based solution that we have in our toolkit to combat climate change. “
In an accompanying commentary on the study, which was also published in the journal Nature, climate scientists Chris Huntingford and Rebecca Oliver from the UK Center for Ecology and Hydrology in Oxfordshire point out that when predicting the CO2 uptake capacity of plants, in addition to CO2 content of the atmosphere also other factors are relevant. “When CO2 concentrations rise, other geochemical cycles become limiting factors in plant growth,” they write. For example, if there is too little nitrogen and phosphorus in the soil, the plant can no longer produce biomass regardless of the CO2. “If you disregard these relationships, this can lead to an overly optimistic assessment of how much CO2 the plants can bind.”
Source: Trevor Keenan (University of California, Berkeley, USA) et al., Nature, doi: 10.1038 / s41586-021-04096-9