Land ecosystems currently play a key role in mitigating climate change. The
more carbon dioxide (CO2) plants and trees absorb during photosynthesis, the
process they use to make food, the less CO2 remains trapped in the atmosphere
where it can cause temperatures to rise. But scientists have identified an
unsettling trend - as levels of CO2 in the atmosphere increase, 86 percent of
land ecosystems globally are becoming progressively less efficient at
absorbing it.
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 Dec. 10 in 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 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 Ben Poulter, study
co-author and scientist at NASA's Goddard Space Flight Center. "In other
words, terrestrial ecosystems are becoming less reliable as a temporary
climate change mitigator."
What's Causing It?
Without this feedback between photosynthesis and elevated atmospheric CO2,
Poulter 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."
In effect, climate change is weakening plants' ability to mitigate further
climate change over large areas of the planet.
Next Steps
The international science team found that when remote-sensing observations
were taken into account - including vegetation index data from NASA's
Advanced Very High Resolution Radiometer (AVHRR) and the Moderate Resolution
Imaging Spectroradiometer (MODIS) instruments - the decline in CFE is more
substantial than current land-surface models have shown. Poulter says this
is because modelers have struggled to account for nutrient feedbacks and
soil moisture limitations - due, in part, to a lack of global observations
of them.
"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," he said. "These
observations will help advance models to incorporate ecosystem processes,
climate and CO2 feedbacks more realistically."
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.
"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," he said. "And
because of this weakening, land ecosystems will not be as reliable for
climate mitigation in the coming decades."
Reference:
Songhan Wang, Yongguang Zhang, Weimin Ju, Jing M. Chen, Philippe Ciais,
Alessandro Cescatti, Jordi Sardans, Ivan A. Janssens, Mousong Wu, Joseph A.
Berry, Elliott Campbell, Marcos Fernández-MartÃnez, Ramdane Alkama, Stephen
Sitch, Pierre Friedlingstein, William K. Smith, Wenping Yuan, Wei He, Danica
Lombardozzi, Markus Kautz, Dan Zhu, Sebastian Lienert, Etsushi Kato,
Benjamin Poulter, Tanja G. M. Sanders, Inken Krüger, Rong Wang, Ning Zeng,
Hanqin Tian, Nicolas Vuichard, Atul K. Jain, Andy Wiltshire, Vanessa Haverd,
Daniel S. Goll, Josep Peñuelas. Recent global decline of CO2 fertilization
effects on vegetation photosynthesis. Science, 2020; 370 (6522): 1295 DOI:
10.1126/science.abb7772