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IN A world of increasing uncertainty, good news has a particular allure. So it is unsurprising that studies finding that rising carbon dioxide emissions are boosting plant life have attracted a lot of attention in recent years. Some people even go as far as to use this so-called CO₂ fertilisation effect as evidence that an uptick in plants could mitigate the effects of climate change. Is there any truth to it?

Firstly, the studies these claims are based on are far from 鈥渇ake news鈥�. They are well-designed and often reflect a long-established scientific consensus. As CO₂ is one of the essential inputs of photosynthesis, the process by which plants harness solar energy to grow, increasing its availability can indeed boost the process.

We also know from satellite images that Earth鈥檚 surface is measurably greener than in decades gone by, and this is extremely likely to be due in significant part to increases in CO₂ levels. In fact, some studies suggest that this increased growth can remove as much as 25 per cent of the emissions we generate, equivalent to the , the planet鈥檚 largest emitter. But now here come the caveats, and there are many of them.

Collating images of the extent of plants and trees on Earth鈥檚 surface is a beautifully simple way to estimate the level of carbon they suck out the atmosphere, but it isn鈥檛 necessarily a very accurate one. Much of this carbon is stored by plants underground, which is invisible to orbiting satellites. What such studies also can鈥檛 do is distinguish between different types of plant communities, which is a major issue. We know, for example, that old growth forests can store far more carbon than quick-growing commercial plantations. Not all green is created equal.

The same studies that show an increased rate of photosynthesis with higher CO₂ levels also tend to show that these benefits 鈥� although significant 鈥� are by and large short-lived. This is because once CO₂ is no longer a limiting factor, plants eventually reach the next biggest limiting factor, such as the availability of nitrogen 鈥� a mineral essential for plant growth that they usually get from the soil. Ever-increasing CO₂ simply doesn鈥檛 mean ever increasing vegetation, as there is an array of other factors that constrain plant growth.

鈥淚ncreased levels of atmospheric carbon won鈥檛 just impact the amount of food we can grow, but also the quality of it鈥�

One of the outcomes we can predict with some reliability is the effect of extra CO₂ in the atmosphere on global temperatures. Depending on how much we curb emissions, the average global temperature is projected to increase to between 1.5掳C and 4.5掳C above pre-industrial levels this century. Models predict that this could result in as much as 67 per cent of plants losing the range in which they can grow well, which would have such a catastrophic effect on global agriculture as to make any net gain from CO₂ fertilisation seem insignificant.

In these scenarios, the harvest of the three major crops that provide us with the bulk of our calories could decline by 7.4 per cent in maize, 6 per cent in wheat and In some locations, the effect of temperature in tandem with lower rainfall, such as south-eastern Australia, is expected to show far worse effects on crops, with .

However, more atmospheric carbon doesn鈥檛 just affect the yield, but also the quality of crops. It has also been shown that the nutrient density falls.

So, yes, an increase in CO₂ levels may well have direct benefits on plant growth for a while, but it is crucial that these are seen in the context of the downsides of its indirect effects 鈥� not just for plants, but for food crops we all rely on. These downsides are so large, it is tricky to describe rising levels of CO₂ as a 鈥渂enefit鈥� to plants at all. So much for a good news story.