Plants are losing their ability to help mitigate climate change

Terrestrial ecosystems are gradually losing their ability to help mitigate the impacts of climate change, according to a study from NASA Goddard. The experts found that 86 percent of land ecosystems are becoming less efficient in absorbing carbon dioxide (CO2) as it increases in the atmosphere.

Trees and plants absorb CO2 as they photosynthesize, which helps to remove the heat-trapping greenhouse gas from the atmosphere, where it accelerates global warming. 

In a phenomenon that is referred to as the carbon fertilization effect (CFE), elevated levels of CO2 cause an increase in photosynthesis and plant growth. CFE has been considered an important mechanism to help reduce levels of atmospheric CO2. 

As the fertilizing capacity of CO2 decreases, there will be major impacts on the carbon cycle and the climate. To investigate, the NASA team analyzed field studies, model-based datasets, and satellite observations. 

“In this study, by analyzing the best available long-term data from remote sensing and state-of-the-art land-surface models, we have found that since 1982, the global average CFE has decreased steadily from 21 percent to 12 percent per 100 ppm of CO2 in the atmosphere,” said study co-author Ben Poulter. “In other words, terrestrial ecosystems are becoming less reliable as a temporary climate change mitigator.”

The findings have important implications for the role that plants will play in offsetting climate change in the future. Poulter said that without the feedback between photosynthesis and elevated atmospheric CO2, we would have already seen climate change occurring at a much more rapid rate. 

It was known that the fertilizing effects of CO2 could not last forever due to other limitations. To pinpoint why CFE has been decreasing, the study authors took into account the availability of water, sunlight, and nutrients, which all have an influence on how much plants can grow. 

“According to our data, what appears to be happening is that there’s both a moisture limitation as well as a nutrient limitation coming into play,” said Poulter. “In the tropics, there’s often just not enough nitrogen or phosphorus, to sustain photosynthesis, and in the high-latitude temperate and boreal regions, soil moisture is now more limiting than air temperature because of recent warming.”

This means that climate change is weakening the ability of plants to mitigate further warming over large areas of the planet.

Remote-sensing observations revealed that the decline in CFE is much more substantial than current land-surface models have shown. Poulter said this is because modelers have struggled to account for nutrient feedbacks and soil moisture limitations.

“By combining decades of remote sensing data like we have done here, we’re able to see these limitations on plant growth. As such, the study shows a clear way forward for model development, especially with new remote sensing observations of vegetation traits expected in coming years.” 

“These observations will help advance models to incorporate ecosystem processes, climate and CO2 feedbacks more realistically.”

According to Poulter, the decreasing carbon-uptake efficiency of land ecosystems means we may see the amount of CO2 in the atmosphere start to increase, shrinking the remaining carbon budget.

“What this means is that to avoid 1.5 or 2°C warming and the associated climate impacts, we need to adjust the remaining carbon budget to account for the weakening of the plant CO2 fertilization effect. And because of this weakening, land ecosystems will not be as reliable for climate mitigation in the coming decades.”

The study is published in the journal Science.

–—

By Chrissy Sexton, Earth.com Staff Writer