An OpEd in today’s New York Times by Yale professor of atmospheric chemistry Nadine Unger starts with the headline “To Save the Planet, Don’t Plant Trees.” The article claims that – contrary to both conventional wisdom and the scientific consensus – planting trees and conserving forests is not an effective solution to climate change. While the headline is eye-catching, and attacking conventional wisdom can attract an editor’s attention, the article’s conclusions simply do not have the backing of science.
There are at least four major scientific fouls that need to be called.
Foul #1: Extrapolation (part A). First, the opinion is given the air of scientific seriousness by supposedly being based on new findings by a whole group of researchers from a respected institution. Dr. Unger did in fact just publish in a top-notch journal what could prove to be an important paper. Her conclusion in that article, “that atmospheric chemistry must be considered in climate impact assessments of anthropogenic land cover change and in forestry for climate protection strategies” is probably correct, and I look forward to the results of studies that do exactly that. But to say that a certain process should be included in models is not the same as having the results of such modeling in hand – even if you have a sense that the process you want added will push the overall model in a particular direction.
Foul #2: Overstatement. The Times article talks about new research suggesting that chemicals released by trees have a warming effect on the climate, and therefore that removing trees can have a cooling effect. Call this the “atmospheric chemistry” effect. It also suggests that this cooling impact of forest destruction (or, conversely, the warming impact of planting new forests) is of a similar scale to the warming impact of forest destruction from adding carbon to the atmosphere (the “carbon effect”), and to the cooling impact of forest destruction by replacing dark forests that absorb sunlight with lighter-colored landscapes that reflect more (the “albedo effect”).
But in fact, the underlying study doesn’t even say this for sure. The paper finds the atmospheric chemistry effect of removing forests is likely to be between negative 0.28 (cooling) and positive 0.06 (warming), with a midpoint of negative 0.11. This is much smaller than the known warming impact from the carbon effect (cited in the paper as positive 0.17 to positive 0.51, with a midpoint of 0.34). So not only is the carbon effect three times bigger than the atmospheric chemistry effect, the underlying article can’t even say with statistical certainty the direction of the atmospheric chemistry effect – whether it is in the direction of causing warming or cooling.
Foul #3: Hidden variables. The conclusion in the Times article is based on adding the potential cooling effect from removing trees’ chemical releases when forests are cleared, to two other climate-relevant impacts of forests. But these are not the only three ways that forests impact climate. Dr. Unger ignores at least two other important effects: the cooling of the evaporation effect from healthy forests, whereby trees cool the air and supply rain clouds with vapor, thus further cooling the planet by changing the albedo of the atmosphere (rather than the albedo of the surface); and the warming of the black carbon effect when forest clearing fires release particulate matter into the atmosphere. These also need to be added to the models, and will push the conclusion in the same direction as “conventional wisdom” – more forests mean a healthier, cooler climate.
Foul #4: Extrapolation (part B). The overwhelming majority of the “precious dollars for climate change” directed to forests is going towards reducing the rapid loss of tropical forests. And in fact, as the article admits, the albedo effect in the tropics is flipped from that in the colder regions – cloud production by tropical forests reflects light back into the atmosphere, cooling the Earth. So the article has extrapolated from the appropriate conclusion of “It is possible that large scale forest expansion in the north may not protect the climate as much as we thought” … to the wholly wrong conclusion of the article that investments in reversing forest loss (including in the tropics) are a waste of climate money. In fact, they are one of the cheapest climate control tools we have.
Normally, this type of scientific debate would take place in specialist journals with lengthy peer review processes to ensure accuracy. And for good reason – it is a process that keeps scientists from jumping to conclusions that aren’t implied by their work, and that should not be cited as fact by others.
When you work on one piece of a huge puzzle with thousands of people and it starts to look like your corner could impact the whole picture, calling attention to it is perfectly reasonable. But to jump from working on that corner to the conclusion that the whole puzzle shows “blue” instead of “red” is just plain wrong. Especially when bigger sections of the puzzle are more put together and more red; when not all the pieces of the puzzle are even on the table yet; and – most damning of all – when you can’t even say for sure whether your corner area is more blue than red.