More Trees, Less Global Warming, Right?

A paper that is currently going through the news (for example at Scientific American) says not necessarily.

In tropical zones, forests have a significant, overall cooling effect. The soil is very wet and, so, via evotranspiration, the trees are covered by low-lying clouds that create a small albedo (power of light that is reflected by a surface). In nontropical areas, Caldeira explains, “the real significant factor is whether there’s snow on the ground in the winter.” If a forest covers a snowy expanse, “that has a strong warming influence,” he notes, because of little cloud cover resulting from less efficiency in evaporating water. The poor cloud formation coupled with the intense absorption of light by the trees “far overwhelms the cooling influence of the carbon storage,” he says.

I have to admit that I do not like that result. I like forests. And I wonder what is the effect in regions with no snowfall, like California outside the mountains? Okay, it does not look so bad, but the effect is not large either:

“In midlatitudes, we got that it was basically a wash—the carbon dioxide effects were pretty much directly balanced by the physical effects,” Caldeira says. He attributes this to the low contrast between light absorption from trees and from grass in pastures, though he notes that because there are some areas with wintry snow cover, the loss of a forest will probably have a slight, if any, cooling effect.

Although I do not like it, we have to go with reality. Planting trees outside the tropics will not leverage global warming. But that does not mean we should go and deforest all northern forests. We need trees for other reasons:

Navin Ramankutty, an assistant professor of geography and Earth system sciences at McGill University in Montreal, says this study is the first to take a comprehensive look at the consequences of deforestation on the entire world. “You can’t just blindly go ahead and reforest and that will tackle climate change,” he says, pointing out a key finding in the study. “If you think about conservation groups, they’re all talking about planting trees. We should be protecting tress for other reasons.”

Caldeira agrees, saying that protecting the forest should be part of an effort to sustain the world’s biodiversity. He also adds that the findings do not endorse clear-cutting or destroying wildlife habitats. “I think that it’s important to look at preventing climate change as a means rather than an end in itself,” he says. “Too narrow a focus on global warming and a loss of the broader focus of protecting life on this planet can lead to perverse outcomes.” Rather than looking to forests to solve the current climate crisis by capturing carbon dioxide, he suggests targeting our “energy system,” which continues to create the pollutant.

Here is the original paper, which I would really like to read, but it is behind a subscription wall. Just the abstract:

The prevention of deforestation and promotion of afforestation have often been cited as strategies to slow global warming. Deforestation releases CO₂ to the atmosphere, which exerts a warming influence on Earth’s climate. However, biophysical effects of deforestation, which include changes in land surface albedo, evapotranspiration, and cloud cover also affect climate. Here we present results from several large-scale deforestation experiments performed with a three-dimensional coupled global carbon-cycle and climate model. These simulations were performed by using a fully three-dimensional model representing physical and biogeochemical interactions among land, atmosphere, and ocean. We find that global-scale deforestation has a net cooling influence on Earth’s climate, because the warming carbon-cycle effects of deforestation are overwhelmed by the net cooling associated with changes in albedo and evapotranspiration. Latitude-specific deforestation experiments indicate that afforestation projects in the tropics would be clearly beneficial in mitigating global-scale warming, but would be counterproductive if implemented at high latitudes and would offer only marginal benefits in temperate regions. Although these results question the efficacy of mid- and high-latitude afforestation projects for climate mitigation, forests remain environmentally valuable resources for many reasons unrelated to climate.

I would guess that it also depends on the type of trees. For example conifers are darker than broad leafed trees. If the latter loose their leaves in winter, they can also expose the layer of snow below which can reflect the sunlight. Another thing is that I would expect newly planted trees to have more effect than a mature forest, since they sequester carbon from the air and store it in the wood. Older forests will have just a small net effect, because the biomass decays and releases the carbon back into the air, balancing the gain in growth. I don’t know if those questions are answered in the paper.