More green on Earth means less carbon in the atmosphere. A photosynthesizing plant captures carbon in the air, in the form of carbon dioxide (CO₂), and converts it into plant matter. As CO₂ levels rise, ecosystems generally grow more efficient at capturing carbon. However, as temperatures rise, that efficiency wanes.

This feedback between the efficiency of plant carbon sequestration and air temperature has been demonstrated in the tropics, in models, and in studies at leaf level. McGowan et al. investigated this relationship on Bribie Island, a barrier island off the east coast of Australia near Brisbane, setting up sensors in three subtropical ecosystem types: a wetland, a swamp, and a pine plantation. The researchers recorded an array of environmental data, from weather and radiation levels to soil temperature and water content.

From these data, the scientists calculated that the optimum temperature range for photosynthesis, and thus for CO₂ uptake, in these environments is 24.1°C–27.4°C. And they found that when temperatures exceeded this optimum window, which they often did during the study, an ecosystem’s ability to capture CO₂ drops sharply. This trend was especially visible during dry periods. The monoculture of the pine plantation did not handle hot or dry conditions well. The natural wetland ecosystem, in contrast, was the most productive of the three sites and responded best to changing conditions, likely because of its natural diversity and dense vegetation, the authors note.

Earth’s temperature continues to climb, and experts predict that the subtropics will experience less rain as the climate warms. In the face of these changes, the ability of plants in subtropical ecosystems to scrub carbon from the air will decline, the authors say. They suggest that similar research is needed in other climate zones and ecosystems to develop a more complete picture of plant-climate feedbacks. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2020JG005678, 2020)

—Elizabeth Thompson, Science Writer