Land ecosystems becoming less efficient at absorbing CO2: Study

New York

Because CO2 is a main 'ingredient' that plants need to grow, elevated concentrations of it cause an increase in photosynthesis, and consequently, plant growth - a phenomenon aptly referred to as the CO2 fertilization effect, or CFE.

CFE is considered a key factor in the response of vegetation to rising atmospheric CO2 as well as an important mechanism for removing this potent greenhouse gas from our atmosphere - but that may be changing.

For a new study published in the journal Science, researchers analyzed multiple field, satellite-derived and model-based datasets to better understand what effect increasing levels of CO2 may be having on CFE.

Their findings have important implications for the role plants can be expected to play in offsetting climate change in the years to come.

"In this study, by analysing the 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 per cent to 12 per cent per 100 ppm of CO2 in the atmosphere," said study author Ben Poulter from NASA's Goddard Space Flight Center.

"In other words, terrestrial ecosystems are becoming less reliable as a temporary climate change mitigator," Poulter added.

Without this feedback between photosynthesis and elevated atmospheric CO2, the researchers said said we would have seen climate change occurring at a much more rapid rate.

But scientists have been concerned about how long the CO2 Fertilization Effect could be sustained before other limitations on plant growth kick in.

For instance, while an abundance of CO2 won't limit growth, a lack of water, nutrients, or sunlight - the other necessary components of photosynthesis -- will.

To determine why the CFE has been decreasing, the study team took the availability of these other elements into account.

"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," Poulter said.

"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," Poulter added.

In effect, climate change is weakening plants' ability to mitigate further climate change over large areas of the planet.

The results of the study also highlight the importance of the role of ecosystems in the global carbon cycle.

According to Poulter, going forward, the decreasing carbon-uptake efficiency of land ecosystems means we may see the amount of CO2 remaining in the atmosphere after fossil fuel burning and deforestation start to increase, shrinking the remaining carbon  budget.