Climate Change, Photosynthesis, and El Niño

The way carbon is moved between the atmosphere, the biosphere, oceans, and other parts of the Earth system plays an important role in the current scientific consensus on climate change. While the various processes involved are known well enough to predict that the planet will continue warming if CO₂ concentrations keep increasing from fossil fuel emissions, better understanding of any part of the so-called carbon cycle can improve predictions of how exactly how much warming will occur. The image below shows a model of the carbon cycle.

In particular, the approximation of the conversion of CO₂ to sugars by plants during photosynthesis (called primary production) could have its range of possible quantities narrowed. In a recent study in the journal Nature, the authors use measurements the rare, stable oxygen isotope, oxygen-18, in CO₂ to estimate the value of primary production. Since CO₂ can exchange oxygen atoms with water in leaves without undergoing photosynthesis, the ratio of oxygen-18 to oxygen-16 in CO₂ is related to how much CO₂ is converted to sugars in plants. Using these measurements, they actually found that primary production might be greater than previously thought! Their estimate has the advantage of not relying on assumptions about biology and provides further constraints on primary production. This increase in the estimate of primary production certainly may turn out to be good news because it could mean that CO₂ concentrations will rise (slightly) more slowly. However, especially because primary production does not count how much of the sugars produced are consumed by the plant itself (and thus changed back into CO₂), it does unfortunately not mean that the biosphere can completely offset all changes in CO₂ from fossil fuel emissions.

By observing the oxygen-18 to oxygen-16 ratio over 30 years at various locations around the world, the scientists found occasional small increases in the ratio from year to year. Oddly enough, these increases occurred at the same time as El Niño! They explained this increase in the oxygen-18 to oxygen-16 ratio by decreases in rainfall over rain forests (where lots of photosynthesis occurs) in Southeast Asia and northern South America during El Niño. Because the water with oxygen-18 is "heavier" than water with oxygen-16, the water in clouds tends to have more oxygen-16 since "lighter" oxygen-16 containing water evaporates first. During periods with less rainfall, the water still evaporates over rainforests, reducing the amount of oxygen-16 in water in the soil. Without enough rainfall to return oxygen-16 back to the ground, the relative amount of oxygen-18 increases, thus the water in plant leaves also has more oxygen-18. Although this effect is very small (a 0.05% change), it can be measured in CO₂. As the southern ocean returns to a La Niña pattern and rainfall in these regions increases, the oxygen-18 to oxygen-16 ratio in CO₂ eventually returns to its "normal" level. The scientists used the rate of change in oxygen-18 to calculate an estimate of primary productivity. Additional measurements of oxygen-17 to oxygen-16 ratios may provide additional constraints to help improve this estimate further. (For a more detailed explanation of isotopes in geology and chemistry, see this previous post)

(h/t Jeremiah J.)