The greening of the earth is approaching its limit

A new study published in Science reveals that the fertilizing effect of excess CO2 on vegetation is decreasing worldwide. The lack of water and nutrients limit the greening observed in recent years and can cause CO2 levels in the atmosphere to rise rapidly, temperatures to increase and there to be increasingly severe changes in the climate.

Vegetation has a key role in mitigating climate change because it reduces the excess CO₂ that we humans emit into the atmosphere. Just as when sportsmen and women are doped with oxygen, plants also benefit from the large amounts of CO₂ that accumulate in the atmosphere. If more CO₂ is available, they photosynthesize and grow more, which is called the fertilizing effect of CO₂. When plants absorb this gas to grow, they remove it from the atmosphere and it is sequestered in their branches, trunk or roots.

An article published in Science on December 2020 shows that this fertilizing effect of CO₂ is decreasing worldwide, according to the text co-directed by Professor Josep Peñuelas of the CSIC at CREAF, PhD student Songhan Wang (first author of the article) and Professors  Yongguang Zhang of the University of Nanjin, with the participation of CREAF researchers Jordi Sardans and Marcos Fernández. The study, carried out by an international team, concludes that the reduction has reached 50% progressively since 1982 due basically to two key factors: the availability of water and nutrients. "There is no mystery about the formula, plants need CO₂, water and nutrients in order to grow. However much the CO₂ increases, if the nutrients and water do not increase in parallel, the plants will not be able to take advantage of the increase in this gas", explains Professor Josep Peñuelas. In fact, three years ago Prof. Peñuelas already warned in an article in Nature Ecology and Evolution that the fertilising effect of CO₂ would not last forever, that plants cannot grow indefinitely, because there are other factors that limit them.

If the fertilizing capacity of CO₂ decreases, there will be strong consequences on the carbon cycle and therefore on the climate. Forests have received a veritable CO₂ bonus for decades, which has allowed them to sequester tons of carbon dioxide that enabled them to do more photosynthesis and grow more. In fact, this increased sequestration has managed to reduce the CO₂ accumulated in the air, but now it is over. "These unprecedented results indicate that the absorption of carbon by vegetation is beginning to become saturated. This has very important climate implications that must be taken into account in possible climate change mitigation strategies and policies at the global level. Nature's capacity to sequester carbon is decreasing and with it society's dependence on future strategies to curb greenhouse gas emissions is increasing", warns Josep Peñuelas.

The study published in Science has been carried out using satellite, atmospheric, ecosystem and modelling information. It highlights the use of sensors that use near-infrared and fluorescence and are thus capable of measuring vegetation growth activity.    

Less water and nutrients

According to the results, the lack of water and nutrients are the two factors that reduce the capacity of CO₂ to improve plant growth. To reach this conclusion, the team based itself on data obtained from hundreds of forests studied over the last 40 years. "These data show that concentrations of essential nutrients in the leaves, such as nitrogen and phosphorus, have also progressively decreased since 1990," explains researcher Songhan Wang.

The team has also found that water availability and temporal changes in water supply play a significant role in this phenomenon. "We have found that plants slow down their growth, not only in times of drought, but also when there are changes in the seasonality of rainfall, which is increasingly happening with climate change," explains researcher Yongguang Zhang.

Reference article:

Wang S, Zhang YG, Ju W, Chen, J, Ciais P, Cescatti A, Sardans J, Janssens IA, Fernández-Martínez, M, … Penuelas J (2020). Recent global decline of CO₂ fertilization effects on vegetation photosynthesis. Science, DOI: 10.1126/science.abb7772