Trees and other plants have been critical in helping to remove carbon dioxide (CO2) from the atmosphere. But newly published scientific findings suggest the clock may be running on vegetation’s forever continuing at the same carbon sink efficiency rate currently taken for granted.
An international team of researchers published their findings in Science. “The enhanced vegetation productivity driven by increased concentrations of carbon dioxide (CO2) [i.e. the CO2 fertilization effect (CFE)] sustains an important negative feedback on climate warming,” they noted in the paper, but “the temporal dynamics of CFE remain unclear.”
This “CO2 fertilization effect” (CFE) describes plants using CO2 to increase their photosynthesis rate to help them grow and thrive. The authors found that “global CFE has declined across most terrestrial regions of the globe from 1982 to 2015, correlating well with changing nutrient concentrations and availability of soil water.”
Plants sequester CO2 in their roots, trunks, and branches and other parts. For decades, the extra CO2 in the atmosphere was a “bonus” for the plants, allowing plant growth to increase as the plants sequestered CO2, leading to more photosynthesis and growth. This process is helpful also to humans in that it reduces the volume of CO2 in the atmosphere. However, as atmospheric CO2 levels keep rising (reaching more than 415 parts per million as of January 2021), water and nutrient levels in the environment – two other essential elements plants need to grow – are not rising in sync with the soaring CO2 levels.
Rate of increasing greenness ‘steadily increasing,’ but at a lower rate
“What we are seeing here is that the pace, the rate of this increasing greening, is decreasing. This is what makes this study special,” says study coauthor and Professor Josep Peñuelas, an ecologist with the Centre de Recerca Ecològica i Aplicacions Forestals (CREAF) in Spain. “It says to us that the rate of increasing of greenness seems to [be] steadily increasing, but with a lower rate.”
“All this different data from satellites, that is something very useful to find out how the land cover has evolved throughout the last decades,” Peñuelas continued.
To examine this issue, the scientists pored over decades of data from hundreds of forests, examining satellite, atmospheric, and ecosystem data, and carbon cycle and climate models. Additionally, the team measured vegetation growth to record how plants were faring.
The researchers found that the CO2 fertilization effect rate of increase declined by nearly 50% during their study period of 1982 to 2015.They believe that decline is occurring because water and nutrient availability are not increasing in the environment in the same way CO2 is. An analysis of plant leaves found that nutrients, such as nitrogen and phosphorus, have also decreased in the leaves. As a result, the effect CO2 has had in boosting vegetation in the past may be declining.
That prospect is concerning since, as the article notes, “terrestrial ecosystems have accounted for more than half of the global carbon sink during the last six decades and have thus substantially mitigated climate warming.”
A decline in the CO2 fertilization effect will impact the carbon cycle, demonstrating a need to adjust our reliance on using forests and plants to sequester carbon. In addition, other climate-related factors will also likely impede plant productivity: Shifting rainfall patterns, drought, and devastating wildfires are just a few of the ways climate change could make it even more difficult for plants to serve as a carbon sink in the way they have all along.
Calling current mitigation strategies into question
The researchers’ study also highlights uncertainties about whether current climate mitigation strategies – such as planting trees and relying on plants to sequester CO2 – are reliable long-term climate solutions, further emphasizing the importance of expeditiously reducing greenhouse gas emissions.
The Science paper builds on previous research on the topic, including an article Peñuelas and colleagues published in Nature Ecology and Evolution in 2017. A CREAF blog about that paper noted that “The greening that has been observed in recent years is slowing and this will cause CO2 levels in the atmosphere to rise, thus increasing temperatures and leading to increasingly severe changes in climate.” It noted also that “signs of saturation are appearing in terrestrial ecosystems, meaning their ability to keep capturing carbon in the future appears to be diminishing.” Those terrestrial ecosystems “typically help immensely in terms of removing CO2 from the atmosphere,” each year removing “about one-third of the carbon dioxide emissions generated by fossil fuel use, industry, and land-use change.”
Looking ahead: Best solution remains reducing, eliminating CO2 pollution
Peñuelas points to how these findings will complicate future planning, carbon budgeting, and carbon cycle models that incorporate the CO2 fertilization effect. The authors noted that current climate models do include a CFE decline, and they believe current modeling may not fully reflect the rate of decline they found. Their paper notes, “Current carbon cycle models also demonstrate a declining CFE trend, albeit one substantially weaker than that from the global observations.” Scientists may have to recalculate models, though that will be a difficult task since no one knows exactly how the rate will change over time – and the stakes are high.
“This means that maybe we’re overestimating, or we are too optimistic with the future capacity for land ecosystems to take up CO2,” Peñuelas says.
In addition to recalculating previous analyses, it may be necessary to readjust strategies for carbon sequestration. For example, tree planting efforts are often hailed as a way to combat climate change, but Peñuelas says people may need to reconsider the efficacy of such strategies. While he says that such efforts can be positive for biodiversity and plant productivity under certain parameters, they alone are not a singular solution to the climate challenge.
“Planting trees is good, but it’s not going to solve the CO2 problem,” Peñuelas says, noting trees require water, nutrients, and other resources not always readily available. “We cannot [rely] completely on these kinds of solutions,” he says. “We need to use these ones, maybe in part, but also try many other ones, and the main one is to avoid the source of the CO2.”
While relying on the world’s forests to sequester endless amounts of carbon is not proving to be a viable solution to climate change, technological innovations could help. Scientists around the globe are working on ways to remove CO2 from the atmosphere and create carbon sinks using physical, chemical, biological, and other methods.
Ultimately, Peñuelas points to the most effective climate strategy: not putting carbon into the atmosphere in the first place. “The main [solution] will be to decarbonize completely our society because this will solve the source of CO2 that is increasing continuously and the concentration of this gas in the atmosphere.”
“We should take care of our environment and our planet and our resources,” Peñuelas says,
“much better than we are doing.”
Kristen Pope is an Idaho-based freelance writer who frequently covers science and conservation-related topics.