THE planet is getting lusher, and we are responsible. Carbon dioxide generated by human activity is stimulating photosynthesis and causing a beneficial greening of the Earth's surface.
For the first time, researchers claim to have shown that the increase in plant cover is due to this "CO2 fertilisation effect" rather than other causes. However, it remains unclear whether the effect can counter any negative consequences of global warming, such as the spread of deserts.
Recent satellite studies have shown that the planet is harbouring more vegetation overall, but pinning down the cause has been difficult. Factors such as higher temperatures, extra rainfall, and an increase in atmospheric CO2 – which helps plants use water more efficiently – could all be boosting vegetation.
To home in on the effect of CO2, Randall Donohue of Australia's national research institute, the CSIRO in Canberra, monitored vegetation at the edges of deserts in Australia, southern Africa, the US Southwest, North Africa, the Middle East and central Asia. These are regions where there is ample warmth and sunlight, but only just enough rainfall for vegetation to grow, so any change in plant cover must be the result of a change in rainfall patterns or CO2 levels, or both.
If CO2 levels were constant, then the amount of vegetation per unit of rainfall ought to be constant, too. However, the team found that this figure rose by 11 per cent in these areas between 1982 and 2010, mirroring the rise in CO2(Geophysical Research Letters, doi.org/mqx). Donohue says this lends "strong support" to the idea that CO2 fertilisation drove the greening.
Climate change studies have predicted that many dry areas will get drier and that some deserts will expand. Donohue's findings make this less certain.
However, the greening effect may not apply to the world's driest regions. Beth Newingham of the University of Idaho, Moscow, recently published the result of a 10-year experiment involving a greenhouse set up in the Mojave desert of Nevada. She found "no sustained increase in biomass" when extra CO2 was pumped into the greenhouse. "You cannot assume that all these deserts respond the same," she says. "Enough water needs to be present for the plants to respond at all."
The extra plant growth could have knock-on effects on climate, Donohue says, by increasing rainfall, affecting river flows and changing the likelihood of wildfires. It will also absorb more CO2 from the air, potentially damping down global warming but also limiting the CO2 fertilisation effect itself.
Donohue cannot yet say to what extent CO2 fertilisation will affect vegetation in the coming decades. But if it proves to be significant, the future may be much greener and more benevolent than many climate modellers predict.