Planting Forests at Mid-Latitudes Could Help Cool Planet

A new paper suggests that climate models underestimate the cooling effect of planting forests at mid-latitudes. Published August 9 in the scientific journal Proceedings of the National Academy of Sciences, the paper states that planting trees in North America and Europe could cool the planet more than previously thought. 

Why Scientists Question the Cooling Effect of Trees

We all know that planting trees is an important strategy in taking carbon from the atmosphere and tackling the climate crisis. Identifying where to plant trees, and the impacts of planting those trees in a particular location, however, is not always as straightforward as it may first appear. One question scientists have been asking is whether reforesting mid-latitude locations such as North America and Europe could actually make our planet warmer. 

Forests absorb a lot of solar radiation, since they reflect less sun (have a low albedo). In tropical regions, low albedo (and the additional heat) is offset by the higher uptake of carbon dioxide by the dense year-round vegetation. In temperate climates, the concern is that additional heat trapped by forests with low albedo could counteract cooling effects from sequestration. 

Clouds Are an Overlooked Component

This new study from Princeton University has found that the low albedo of forests may be less of an issue than previously imagined, because the predictions may have overlooked one crucial component—clouds. 

Clouds are notoriously difficult to study and have been largely discounted from many of the studies that have looked into afforestation, reforestation, and natural climate change mitigation in the past. Clouds, however, have a cooling, if transitory, effect on the Earth. They directly block the sun, but also have a high albedo, similar to ice and snow. They reflect more sunlight and therefore have a cooling effect. 

Clouds form more frequently over areas of forest than they do over grasslands and other areas with short vegetation. This study found that clouds tend to form earlier in the afternoon over forested areas, which means clouds are in place for longer and have more time to reflect solar radiation away from the Earth. 

When this is taken into account, the cooling effect from the clouds, in combination with the carbon sequestration of the forests themselves, outweighed the solar radiation absorbed by forests. 

Looking Into Clouds

Study co-author Amilcare Porporato, professor of Civil and Environmental Engineering at Princeton University, worked with lead author Sara Cerasoli, a Princeton graduate student, and Jun Ying of Nanjing University with support from the Carbon Mitigation Initiative to investigate the influence of cloud formation in mid-latitude regions. 

Porporato and Yin had previously reported that climate models underestimate the cooling effect of the daily cloud cycle. They also reported last year that climate change could result in increased daily cloud cover in arid regions like the American Southwest. 

For this latest study, the team looked into the issue by combining satellite data of cloud coverage from 2001 to 2010 with models relating to the interaction between plants and the atmosphere. They modeled interactions between different types of vegetation and the atmospheric boundary layer—the lowest layer of the atmosphere, which interacts with the surface of the planet. Focusing on the 30- to 45-degree latitudinal range, they determined the cooling effects of afforestation and reforestation. 

The team's findings could be helpful to those developing policy and allocating land for reforestation and agriculture. The study authors noted that one useful approach could be to pair mid-latitudinal reforestation with the distribution of drought-tolerant crops for regions less suited to reforestation, but they urged caution when leaping from science to policy. Many different factors, not just climate change, must be taken into account.

Cerasoli said, "Future studies should continue to consider the role of clouds, but should focus on more specific regions and take their economies into account." Porporato went on to caution that our first consideration should be not to make things worse. He pointed to the interconnectedness of all of Earth's cycles and systems and the complexity of interactions between them. He noted that when one thing is changed, it can be very difficult to predict how other elements will be affected. 

As we previously reported, European rainfall will be boosted by planting more trees, but this could bring negative impacts, in addition to positive ones. This goes to show just how important it is to take a careful, thoughtful approach.