Does more carbon dioxide mean more forests? And is this all good? Not quite.
- 10 Jul 2012, 17:00
- Chris Peters
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
"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
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
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
grassland becoming forest the same as forests not being
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
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.
"[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
"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
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".
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
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
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