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Plants absorb carbon dioxide more slowly than previously thought

21 December 2020

Although rising temperatures boost the productivity of photosynthesizing vegetation, nutrient and water limitations may have diminished the positive trend over the past 30 years.

Sycamore tree
Sycamore tree. Credit: Maatpublishing, CC BY-SA 4.0

In 1958 atmospheric scientist Charles Keeling began measuring atmospheric carbon dioxide at the Mauna Loa Observatory in Hawaii. The data helped identify the CO2 fertilization effect—a negative feedback loop in which rising concentrations of CO2 in air raise temperatures and thereby spur plants to increase their rate of photosynthesis, which reduces the CO2 concentration. Modelers have since incorporated the effect in simulations to improve climate predictions. But Songhan Wang and Yongguang Zhang, both of Nanjing University, and their colleagues have now determined that the rate of the CO2 fertilization effect has declined over the previous three decades.

The researchers found the negative trend by analyzing three satellite-derived data products. The Advanced Very High Resolution Radiometer (AVHRR) aboard the NOAA-19 satellite and the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite measure the near-IR reflectance of vegetation (NIRV), which estimates the amount of plant cover over land. Wang, Zhang, and their colleagues complemented the reflectance data with multiple space-based observations of Sun-induced chlorophyll fluorescence (SIF)—the light that chlorophyll molecules reemit when excited by sunlight.

CO2 fertilization effect estimations over time
Adapted from S. Wang et al., Science 370, 1295, 2020

The researchers then used those data to estimate the CO2 fertilization effect β as the total change in plant productivity in response to increasing CO2 concentrations. The time series displayed here shows various estimates of β (blue, red, and yellow solid lines) and the agreement between them within the 1 standard deviation of uncertainty (gray shading). The average of the three trends from 1982 to 2015 shows a decline of 0.92% per year for every 100 ppm increase in CO2 concentration.

The decrease is consistent with two potential causes. Independent observations show that declining concentrations of nitrogen and phosphorus in leaves could be starving plants of critical nutrients. Plant productivity could also be stymied by a restriction in the amount of water in the soil, a possibility supported by comparing the findings with independent terrestrial water-storage data. The authors conclude that land-based carbon sinks may be less effective than predicted for limiting the effects of climate change, but the community can still improve its understanding of Earth’s future terrestrial carbon budget by focusing research on other parts of the biogeochemical cycles. (S. Wang et al., Science 370, 1295, 2020.)

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