Mauna Kea data contributes to CO2, plant adaptation finding

While humans may yet be at a loss for how to respond to rising carbon dioxide levels, a new study suggests that plants have already adapted.

Using data collected from Mauna Loa Observatory and the South Pole, researchers from the Scripps Institution of Oceanography and the University of California at San Diego examined changes in CO2 levels since 1978 and corresponding changes in plant behavior, concluding that land plants have become more efficient at using water for photosynthesis.

The results of the study were published in the Sept. 11 edition of the journal Proceedings of the National Academy of Sciences.

As the researchers note, CO2 levels have risen since the late 19th century due at least in part to human activity. Accordingly, the ratio of the two main atomic forms of carbon — carbon-12 and carbon-13 — has decreased. The scientists cited the low 13C-to-12C ratio associated with fossil fuel combustion as one of the causes.

Updating the record of CO2 isotopic ratios made at Scripps nearly 40 years ago, researchers confirmed that a discrepancy existed between projected changes based on human activity and natural factors and the actual measured changes. They further determined that the only plausible explanation for the discrepancy was that there was a change in how much water plants needed to grow.

This conclusion was based on the understanding that higher levels of CO2 affect the behavior of stomata, the microscopic holes in leaves that allow plants to take up CO2 and lose water in evaporation. With higher CO2 levels, plants can have smaller or fewer stomata, thereby allowing for more photosynthesis with the same amount of water.

The findings support a long-held notion by plant biologists that plants can achieve an optimum response to increases in atmospheric CO2. Further, the greater efficiency in photosynthesis points to the ability of plants to remove more CO2 from the atmosphere, thereby partially offsetting some effects of climate change caused by human activity.

However, lead author Ralph Keeling cautioned that such benefit has to be weighed against other negative changes like extreme weather, biodiversity loss and sea-level rise.

Keeling is the son of Charles D. “Dave” Keeling, a Scripps researcher who began taking C02 readings atop Mauna Loa in 1958. His findings took the form of the Keeling Curve, which shows an inexorable rise in atmospheric carbon, with seasonal variations.