The notorious greenhouse gas also has a beneficial effect on the climate: rising levels of carbon dioxide in the air increase plant growth. But this effect has so far been overestimated, according to a study. Accordingly, the fertilizer effect has decreased significantly in the past few decades. According to the results, growth promotion is now increasingly limited by water and nutrient deficiencies. The scientists say that models for the future development of the global climate should take this into account.
We have given the earth a fever: Mankind is increasingly enriching the earth’s atmosphere with the greenhouse gas carbon dioxide, thereby causing temperatures to rise. In order to assess the further development of the problem and derive the necessary measures, a lot of calculations and modeling are carried out. Information about nature’s absorption potential for carbon dioxide from the atmosphere is an important basis. One thing is clear: the earth’s vegetation swallows large amounts of CO2 and thus acts as a sink in the climate system.
The reason for this is that the gas forms a basic element of photosynthesis: Plants absorb CO2 from the air through their stomata and use the light as an energy source to produce carbon compounds, which form the basis of the food chains. It is known that increased CO2 levels can stimulate plant growth and thus strengthen the buffer effect of the vegetation. This fertilizing effect has already been proven on a global level. The increasing carbon dioxide concentrations have therefore made the world green and the greenhouse gas has in turn been increasingly bound.
More doesn’t help that much anymore
But has this fertilizing effect developed in the context of climate change? To answer this question, an international team of researchers has evaluated and modeled extensive data from satellites and measurements on the ground from the period from 1982 to 2015. As they explain, by looking from space it is possible to draw large-scale conclusions about the development of vegetation and also about the photosynthetic activity of plants. Certain reflective features and fluorescence effects associated with the plant pigment chlorophyll are used for this purpose. In this way, the researchers were able to investigate how the vegetation has changed in the course of the increasing carbon dioxide concentrations in the last few decades and thus conclusions about the development of the fertilizing effect of the gas were possible.
As they now report, it has apparently decreased significantly: “In this study, by analyzing the best available long-term data, we found that the global average CO2 fertilization effect has steadily increased since 1982 from 21 percent to 12 percent per 100 parts per million CO2 in the Atmosphere has sunk, ”sums up co-author Ben Poulter from NASA’s Goddard Space Flight Center in Greenbelt. “In other words: the potential of terrestrial ecosystems as a beneficial factor is dwindling,” says the scientist. In this context, the results show that 86 percent of terrestrial ecosystems worldwide are becoming less and less efficient at absorbing CO2. A downward trend was already assumed, but the new study results show that the decline is more pronounced than previously thought, the scientists explain.
Abundance increasingly meets lack
The scientists see a principle as the cause of the dwindling effect of carbon dioxide in increasing growth, which everyone can also determine in their domestic houseplants: If they are not supplied with sufficient water, more fertilizer will not increase growth. Correspondingly, an excess of CO2 can stimulate the development of vegetation on earth less and less if there is a lack of water and minerals.
This is exactly what is also reflected in the researchers’ results: “According to our data, it appears that both moisture and nutrient limitation come into play,” says Poulter. “In the tropics there are often simply not enough nitrogen or phosphorus compounds and in the temperate and boreal regions, the soil moisture is now more limiting than the air temperature due to the recent warming.” Thus the consequences of global warming weaken the ability of plants in large parts of the Planets to mitigate further climate change, the researchers sum up.
“These observations can now help us to further develop models to take ecosystem processes, climate and CO2 feedback into account more realistically,” says Poulter. Specifically, the decreasing efficiency of carbon uptake by terrestrial ecosystems should now flow into the assessment of carbon budgets. “That means that in order to avoid warming of 1.5 or 2 degrees Celsius and the associated climate impacts, we are allowed to release even less carbon dioxide,” says Poulter.
Source: NASA, technical article: Science, doi: 10.1126 / science.abb7772