Climate change is messing with photosynthesis in unexpected ways

Increases in CO₂ in the atmosphere brought about by anthropogenic activity was expected to increase the rate of photosynthesis in plants and perhaps increase plant yield and growth.

New science has showed the rate of photosynthesis around the globe has been increasing, but now there is evidence the rate has slowed and might soon plateau.

During photosynthesis plants take water and CO₂ and convert it into oxygen and carbohydrates – storing carbon inside the plant and soil. A higher availability of CO₂ increases the rate of this process, acting as a sort of brake on global warming by sequestering more CO₂.

However, a new modelling study, published in the journal Science, has found that the increase in photosynthesis has slowed since 2001 due to an adverse effect of climate change.

The study looked at satellite images of various environments covered by foliage – such as savannas, croplands, and forests – and used machine-learning to find changes, such as leaf colour, to reveal rates of photosynthesis. They also studied data on CO₂ and water vapour levels in the air between 1982-2016.

Combining these datasets, they modelled changes in global photosynthesis rates from 1982 to 2016 and found that, as CO₂ levels rose from 1982 to 2000, global rates of photosynthesis also increased significantly. But from the year 2000 onwards, this increase in the rate of photosynthesis began to slow.

The researchers think this is probably due to an increased vapour pressure deficit, or VPD. VPD is the difference between the amount of moisture in the air and how much moisture the air can hold when it is saturated – basically it’s a measure of how dry air is.

Increased VPD (drier air) imposes water stress on photosynthesis because it causes more water to evaporate from plants’ tissues through transpiration.

Transpiration predominantly occurs through a small opening in the leaves of plants, called stomata. But, if too much water is lost too quickly, plants close the stomata to slow transpiration. This effects photosynthesis because CO₂ also enters the plant through these pores when they’re open.

“As a result of temperature rise-induced increases in VPD, global ecosystem photosynthesis has become suppressed and, thus, so has the ability of global ecosystems to assimilate carbon,” the authors write in their study.

The authors suggest that existing climate data and projections indicate that this trend will likely continue into the future. Increasing VPD is “projected to persist at least into 2050, and possibly beyond in response to rising air temperature” and the resulting impact on photosynthesis is expected to be long-lasting.

They conclude that “this study emphasises that human reliance on nature-based climate sinks to achieve [carbon] neutrality may be undermined by the adverse effects of climate change.”