Global warming drying the air may have slowed a rising rate of photosynthesis around the world. With this plant process involving the uptake of carbon dioxide, some researchers hoped that a boost to photosynthesis rates would help to remove the greenhouse gas from the atmosphere, but the latest study suggests this effect has faltered since 2000.
Photosynthesis is the chemical reaction that plants use to convert CO2 and water into carbohydrates. Scientists generally think that a rise in CO2 emissions has been leading to more and more photosynthesis, but Jingfeng Xiao at the University of New Hampshire says that few studies have actually looked into this on a global scale.
To learn more, Xiao and his colleagues analysed ground measurements taken between 1982 and 2016 from sensors scattered around the world that measure fluctuations in CO2 and water vapour in various environments, such as forests and savannahs.
They then used satellite images to estimate plant growth in different locations. Using machine learning, the team combined these datasets to broaden the fluctuation measurements to a global scale.
The models suggest that, on average, increases in global photosynthesis levels have slowed since 2000, despite the amount of CO2 in the atmosphere continuing to rise. This is probably due to the rise in the so-called vapour pressure deficit offsetting some of the increase in CO2, says Xiao. This deficit is a measurement of how dry air is.
The higher the deficit, the more water evaporates off plants’ leaves, in a process called transpiration. A higher rate of transpiration generally supports plant growth, as they suck up water to replace what they lost, resulting in their cells receiving more water and the nutrients it contains, says Xiao.
But if plants lose water too quickly, it can cause them to close the pores in their leaves to slow this down. These pores take in CO2, so their closure can hinder photosynthesis. This effect has probably been occurring worldwide since 2000 as a result of the vapour pressure deficit increasing alongside rising temperatures, says Xiao.
The findings mean that using tree planting as a way to soak up CO2 may not have as big an impact as previously thought, says Xiao. Trees can store carbon for centuries, but we may have to rethink the idea that the amount they take up will get greater due to higher levels of photosynthesis from raised CO2 levels, he says.
However, Iain Colin Prentice at Imperial College London is sceptical about the findings, saying that there are uncertainties involved in scaling up the meteorological fluctuation measurements the researchers took to a global level.
Xiao agrees the model has uncertainties, but says the team still found a consistent trend in photosynthesis rates worldwide.
Kevin Griffin at Columbia University in New York says that while the study is based on a fair few assumptions, the researchers acknowledged this in their paper. “It’s important to note that this isn’t a direct measurement of what has happened, but is instead a projection,” he says.