Does more carbon dioxide mean more forests? And is this all good? Not quite.

  • 10 Jul 2012, 17:00
  • Chris Peters

A paper recently published online in the journal Nature examining how boundaries between grasslands and forest may change as carbon dioxide levels in the atmosphere rise has attracted a bit of attention this week. Particularly, IT blog the Register and subsequently the Daily Telegraph have both published articles by blogger Tim Worstall arguing the paper shows that more carbon dioxide in the atmosphere is going to be a good thing. If only it were that simple.

The Register piece is entitled ' Global warming: It's good for the environment. Don't forget: carbon dioxide is plant food.' Carbon dioxide can indeed boost plant growth, and the argument that CO2 is "plant food" isn't a new argument amongst those who seek to downplay concerns about climate change.

Worstall's approach to summarising the paper's conclusions in the Register is fairly light-hearted, but his interpretation is clear:

"Carbon dioxide is indeed plant food and more plant food means more plants, more forests and thus we're all saved: or perhaps not quite as screwed as some seem to think at least."

The paper's findings

So what does the research show? The scientific paper in question uses global vegetation models to project the consequences of climate change on Africa's savanna complex - the term used by the authors to refer to Africa's "grassland, savanna and forest ecosystems of tropical and subtropical regions."

Trees and grasses compete against each other for dominance in the savanna complex, and they also respond very differently to changing temperatures, carbon dioxide levels and fire frequency.

The new study suggests that in future climate change scenarios, trees on the savanna are likely to prevail. And according to the study, success will breed success in a positive feedback loop. With more trees comes more shade, which inhibits the growth of grasses and further promotes tree dominance.

Fire also plays a large part in stopping trees from establishing in savanna. But the study projects that higher levels of carbon dioxide will mean that trees grow faster and thus reach a size where they are resilient to fire more quickly.

The authors claim that the new paper is an important break from other experimental studies, which have generally shown that plants do not show a large response to carbon dioxide fertilisation. Professor Steven Higgins, the lead author on the Nature study, argues that this may be because most of these studies were either conducted on northern ecosystems or on commercially important plant species.

Is grassland becoming forest the same as forests not being threatened?

According to the paper's results, climate change may lead to grasslands changing into forests.

Worstall's Telegraph piece takes this somewhat further. Headlined ' Climate change will mean new and larger tropical forests', it begins by suggesting that the research means that concerns over how the Amazon will be impacted by climate change are invalid:

"We're told, endlessly, that climate change will mean the end of the Amazon, of the tropical forests, and the Earth will lose its lungs. It appears that this is not wholly and completely true. Actually, an increase in CO2 in the atmosphere is likely to lead to the growth of huge, new, tropical forests."

The paper only analyses potential impacts in Africa. Worstall extends this to discuss Latin America and also presents the paper as a discussion of the health of tropical forests in general.

Worstall writes:

"[T]he major point of this paper is that far from climate change being a threat to the tropical forests, it looks as if it will be the cause of more of them growing."

Professor Higgins pointed out to us that the research suggests savanna may change in the future, but doesn't draw wider conclusions:

"Our paper is actually about savannas. Our paper argues savannas (ecosystems with a mix of grasses and trees) will in future have more trees in them. Some areas currently covered by savannas will have so many more trees in them in the future that a vegetations scientist might classify them as woodlands or forests".

Worstall suggests that  "... the Amazon might get bigger"

Professor Higgins responded that here Worstall is "making his own inferences." He points out that while the authors mention the Amazon in the paper, their analysis is "based on Africa".

Finally, Worstall also writes that "Burning more fossil fuels therefore seems likely to grow several new Amazon style forests across Africa and Latin America".

According to Professor Higgins, this is another case of Worstall "making his own inferences".

Professor Higgins pointed us in the direction of what he described as a more balanced review of his work in the Guardian, which summarised the paper in less dramatic terms as

"A new study shows that increasing carbon dioxide levels favour trees over grass, suggesting that large regions of Africa's savannas may be forests by the end of this century."

Professor Higgins also notes that losing grasslands and savannas is not necessarily good news - they "are ecosystems that have existed for around 6 million years: that is they have their own distinctive fauna and flora - so when grassland becomes savanna or savanna becomes forest we may well loose species typical of these ecosystems".

Online, climate skeptics have lauded the study as proof of the benefits of CO2 fertilisation. Grasslands may shift towards forests as carbon dioxide levels rise, but on the basis of this paper that's the limit of the conclusions that can be drawn. As is often the case, there's a more complicated reality behind the headlines.

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