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The decline of CO2 fertilization effects on vegetation photosynthesis

Plants play a critical role in removing part of the excess CO2 released to the atmosphere from human emissions. This is mainly because photosynthesis by plants has been enhanced by the increasing availability of CO2 in the atmosphere (CO2 fertilization effect).

However, whether this crucial feedback in the climate system, which dampens man-made climate change by removing CO2 from the atmosphere, will persist for the future decades remains unclear. In particular, because the temporal dynamics of the CO2 fertilization effect in the recent past has not yet been comprehensively explored.

In a new study published in the journal Science, an international team with researchers from Augsburg University found that the fertilizing effect of CO2 deduced from satellite observations has been declining on global scale since the 1980s.

The international research team found that the global CO2 fertilization effect has unexpectedly declined during the past four decades. They used novel proxies of vegetation photosynthesis from long-term (1982-2015) satellite data of near-infrared reflectance and satellite retrievals of vegetation sun-induced fluorescence. Further analyses based on other independent atmospheric and ecosystem datasets also showed the similar declining rate. Interestingly, although current carbon cycle models also could reproduce such global decreases in CO2 fertilization effect, they fail to adequately detect the sharp declining trend identified in the satellite data.

The reason(s) for a decline in CO2 fertilization effect are difficult to pin down. It may be linked to declining availability of nutrients or water which reduce the CO2 fertilization effect. In support of this view, the authors found that plant nutrient concentration in leaves has been declining since 1990s in parallel to the CO2 fertilization effect. The team also found that water availability and temporary changes in water supply could play a role. They found increasing sensitivities of photosynthesis to water supply in parallel to the declining trends in the CO2 fertilization effect.

"Despite the increasing carbon uptake from vegetation in the last decades, we provide robust and consistent evidence that the positive CO2 fertilization effect on vegetation photosynthesis has declined in recent decades. This decline may imply an emerging saturation of the vegetation carbon uptake, with important implications on the potentials of land-based mitigation strategies and policies," explains Prof. Yongguang Zhang, a researcher in Nanjing University, China.

"Plants need an adequate balance of CO2 , water and nutrients to grow. If the availability of water and nutrients does not increase in parallel to the increase of atmospheric CO2 concentrations, plants will not be able to take advantage of the increase availability of this gas. The resulting decrease of the capacity of nature to sequester carbon increases the dependence of society on future strategies to curb greenhouse gas emissions", said Dr. Daniel Goll from Augsburg University, a co-author of this study.

Professor Wolfgang Buermann from Augsburg University's Institute of Geography, who was not involved in this study, further explains: "The study provides first evidence that the land-based carbon sink, which is largely driven by the CO2 fertilization effect, is already weakening. These important results must therefore also be taken into account in revised estimates of the remaining budget of carbon emissions that would limit global warming to 1.5 or 2.0 C, in accordance with the Paris climate goals. "

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Source: University of Augsburg