Planting Trees Won’t Save the Planet
By David Shearman
The Guardian Weekly January 5, 2007 used this heading to summarise a research paper by Ken Caldeira from the Carnegie Institute of Washington. It said “planting trees to combat climate change is a waste of time, according to a study by ecologists who say that most forests do not have any overall effect on global temperature whilst those furthest from the equator could actually be making global warming worse”. The newspaper then quotes Caldeira, “The idea that you can go out and plant a tree to help reverse global warming is an appealing feel-good thing. To plant forests outside the tropics is a waste of time”.
Let us look in more detail at the information reported in the press. The researchers used complex climate modelling to simulate changes in forest cover and then examined the effects on global climate. They found that while tropical forests help cool the Earth by evaporating water, northern forests tend to warm the Earth because they absorb a lot of sunlight without losing much moisture. In one simulation, the researchers covered much of the northern hemisphere (above 20° latitude) with forests and saw a rise in surface air temperature of more than 6° F.
If one goes to the abstract of the study, it was published in December 2005 and was presented as a news story by the Guardian in December 2006. Looking at the claims in the study, climate change researchers recognise the importance of using several models to look at the same problem. One study using one modelling system is insufficient to establish these findings.
The topic was presumably deemed newsworthy because of the big business of forest creation schemes and for guilty travellers to mitigate their flights by paying for trees to be planted and for gross polluters such as Grand Prix to do the same.
What can be learned from these events?
The Guardian, perhaps one of the best of the UK and international press, has drawn conclusions from one scientific paper that are unwarranted and could be used by climate change sceptics and those with an interest in deforestation. Lesson—check all material at source. Furthermore the study was completed over one year ago and should not be presented as news.
The scientist, on the basis of one unconfirmed study, has made some sweeping statements on the implications of the findings. It behoves scientists to be judicious in the implications they draw from their findings. In fact the stated intent of the study was “When changing from grass and croplands to forest, there are two competing effects of land cover change on climate: an albedo effect which leads to warming and an evapotranspiration effect which tends to produce cooling. It is not clear which effect would dominate”. It is surely a huge leap to conclude that planting trees in temperate zones is a waste of time.
In conclusion the scientific consensus on trees remains that old growth forest must be retained for carbon storage, biodiversity and many other reasons (see DEA policy on forests at www.dea.org.au). Re-afforestation must continue for most of the same reasons