Last week, the New York Times published an article that argues that, “The science says that spending precious dollars for climate change mitigation on forestry is high-risk”. It is written by Nadine Unger, an assistant professor of atmospheric chemistry at the School of Forestry and Environmental Studies, Yale University.
Unger’s article, “To Save the Planet, Don’t Plant Trees“, argues that protecting forests and planting trees is not an effective solution to climate change.
Unger’s argument is based on two main points:
- The albedo effect: Forests are dark and absorb more of the sun’s heat than agricultural fields which are lighter and reflect the sun’s heat. (That’s why dark snow in Greenland is such a problem.)
But as Unger points out, climate scientists have calculated that “planting trees in the tropics would lead to cooling, but in colder regions, it would cause warming”.
CIFOR’s Louis Verchot points out that,
Stopping tropical deforestation and increasing tree planting in REDD+ countries, where the albedo effect is smaller than the carbon effect, makes sense from an energy balance point of view.
A commentary by Dan Nepstad of the Earth Innovation Institute and colleagues notes that forests create water vapour and clouds, especially in the tropics. This increases sky albedo (i.e. increases reflectivity), a point that Unger doesn’t consider in her article.
- Volatile organic compounds: Trees emit gases that help protect them from stresses like heat and insect attack. These volatile organic compounds combine with pollution from fossil fuels to form methane and ozone, both of which are powerful greenhouse gases.
But as Doug Boucher from the Union of Concerned Scientists writes, “reducing fossil fuel pollution is the most direct and effective way to reduce this effect, not blaming the forest”.
The second question relating to volatile organic compounds is whether cooling effect of fewer gases being released is as big as the warming caused by the release of carbon from forests when they are converted to cropland.
In Unger’s recent paper in Nature Climate Change, she calculates the cooling effect of converting forest to cropland between 1850 and 2000 as −0.11 ± 0.17 W/m2. That is, somewhere between -0.28 (cooling) and +0.06 (warming). The IPCC Fifth Assessment Report provides global radiative forcing values of land use change for CO2 of between +0.17 and 0.51 W/m2, and for albedo of −0.15 ± 0.10 W/m2.
Michael Wolosin at Climate Advisers sums up:
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.
So far, so good, then. A climate scientist who focusses on atmospheric chemistry wrote an opinion piece in the New York Times arguing against planting trees to address climate change. Lots of people responded on the article.
But there’s an elephant in the room. Unger’s conclusion happens to be right: “spending precious dollars for climate change mitigation on forestry is high-risk”.
Climate change is only one of the many reasons for protecting forests and for planting trees. To address climate change we need to stop digging out and burning fossil fuels.
On the EDF website, Steve Schwartzman argues that,
Emissions from tropical deforestation are, from the perspective of the atmosphere, just the same as emissions from burning fossil fuels – carbon that was wood, coal, oil or gas is turned into CO2 and released to the atmosphere.
While it is true that the carbon emitted from burning forests is chemically the same as the carbon emitted from burning fossil fuels, from the climate perspective they are different.
Carbon stored underground as fossil fuel is stable. It will only enter the atmosphere if it is dug out and burned. Carbon stored in forests, soils, and peatlands is difficult to measure, is not stable, and can far too easily enter the atmosphere.
Climate change makes the problem worse, by making the carbon stored in forests even less stable. Carlos Nobre, Brazil’s National Secretary for Research and Development Policies, in a recent interview with CIFOR notes that,
[T]here are areas in the Amazon in which a repeated cycle of deforestation, regrowth and fire has led to a landscape that is highly degraded. It’s an impoverished kind of savannah. I think that’s what the forests of the future will look like if climate change is not checked.
So even if we did stop cutting down trees, the changing climate means that the forests will go up in smoke.
To address climate change, we need to reduce greenhouse gas emissions. Deforestation is one source of greenhouse gas emissions, but what proportion of total emissions comes from deforestation?
In her New York Times article, Unger repeats a common mistake:
“Deforestation accounts for about 20 percent of global emissions of carbon dioxide.”
Researchers from Winrock International and Woods Hole Research Center came up with a figure of 10% based on data from between 2000 and 2005. Since then emissions from fossil fuels have increased, meaning that deforestation now accounts for an even lower percentage of global emissions.
This graph from the Global Carbon Project illustrates the proportion of greenhouse gas emissions that come from land-use change since 1960:
In 2013, the figure was 8%.
Nadine Unger has written a response to the criticism. She provides sources for statements she made in the New York Times article and clarifies that,
Protection and restoration of tropical and other forests is essential regardless of their effects on climate. Currently, global climate modeling is not sufficiently advanced to predict reliably the effects of changing forests on the global average surface temperature.
The primary key to solving the global climate problem is the transformation of our energy system into one that does not use the sky as a waste dump for our greenhouse gas pollution.
Exactly. To address climate change we need to leave fossil fuels in the ground.