Planting forests may cool the planet more than thought

Satellite images were used by the researchers to determine the cloud cover over areas within the 30-45 degree latitudinal area. This was based on the interaction of different types and types of vegetation with the atmospheric boundary. The researchers found that clouds formed more often over forests and had a greater cooling impact on the Earth's atmosphere. The black dots are forested areas. The green dots are grasslands or other short vegetation. Shaded areas range from the cloudiest (white), to the least cloudy (brown). Credit: Amilcare PorporatoAlthough planting trees and replenishing forests is one of the most attractive natural climate solutions, the impact of trees upon atmospheric temperature is much more complicated than it seems.Scientists are unsure whether reforesting high-altitude areas like North America and Europe will make the planet more hot. Low albedo is a measure of the surface's ability reflect sunlight. Forests absorb large amounts solar radiation. Low albedo in the tropics is compensated by higher carbon dioxide uptake by dense vegetation that grows all year. However, temperate climates are more concerned about the possibility that the sun's heat trapped in the earth could offset any cooling effect forests might provide by removing carbon dioxide.Princeton University researchers have found that this concern may not be true if they overlook a critical componentcloud. The Proceedings of the National Academy of Sciences reports that the dense cloud formations associated to forested areas mean that reforestation is likely to be more effective in cooling the Earth's atmosphere."The most important thing is that no one knows whether planting trees at midlatitudes has any benefits or disadvantages because of the albedo problem," stated Amilcare Porporato (Princeton's Thomas J. Wu'94 Professor in Civil Engineering and the High Meadows Environmental Institute). We show that large areas of trees should be planted if clouds tend to form more often over forests. This is a good thing for climate reasons.Anyone who has ever felt a cloud pass above the sun on a hot summer day knows that daytime clouds can have a cooling, but temporary effect on the Earth. Clouds block the sun directly, but they also have an albedo similar to snow and ice. However, clouds are notoriously hard to study and have been largely ignored from many studies that examine the effectiveness of natural climate changes mitigation including reforestation.Porporato collaborated with Sara Cerasoli (a Princeton graduate student) and Jun Ying (an assistant professor at Nanjing University Information Science and Technology, who was previously a postdoctoral fellow within Porporato’s research group). The Carbon Mitigation Initiative, based at HMEI, supported their work.Porporato, Yin and others previously reported that climate models underestimated the cooling effect from the daily cloud cycle. The authors also suggested last year that climate change could lead to increased cloud coverage in areas such as the American Southwest, which are ideal for solar power production.Cerasoli and Porporato Yin analyzed the effect of vegetation on cloud formation at midlatitudes. They combined satellite data from cloud coverage between 2001-10 with models that relate to the interaction of plants and the atmosphere.Researchers simulated interactions between different types and levels of vegetation with the atmospheric boundary layer, which is the lowest layer in the atmosphere. It interacts directly to the Earth's surface. This was done to determine whether cloud formation is affected differently by type of vegetation. The researchers focused on areas in the 30-45 degree latitudinal area, which is roughly the subtropical to the hemiboreal zone such as the northern Midwestern United States. They considered the effects of both reforestationrestoring lost tree coverand afforestation, which entails planting forests in areas that were previously treeless, though this may come with other environmental costs.The team found that for midlatitude regions, the cooling effect of cloudsin combination with that of carbon sequestrationoutweighed the solar radiation that forested areas absorbed.Models showed that clouds formed more often over forests than in grasslands or other short-lived areas, and that this increased cloud formation had a cooling effect upon Earth's atmosphere. From satellite data, the researchers discovered that clouds tend to form earlier in afternoon in forested areas. This results in longer periods of cloud cover and less time for clouds to reflect sunlight away from Earth.These findings can be used to help create policies for allocating land for agriculturewetter and reforestation. The eastern United States and southeastern China, which are both well-suited for afforestation and reforestation, could also benefit from the development of agricultural policies. The study authors suggested that midlatitudinal forest restoration could be combined with the distribution drought-tolerant crops in areas less suited for reforestation.The authors cautioned that policy-makers must be careful when moving from science to practice. Cerasoli stated that we cannot just focus on climate change. We must also consider other factors such as biodiversity and the fact land is necessary for food production. Future studies should continue to focus on the role of clouds but should also consider specific regions and their economies.Porporato said, "The first thing to do is not make it worse." The earth system is interconnected in many ways. Because of the nature of interactions among, for instance, climate and water cycle, it is very difficult to predict how the other parts of the system might be affected by one change.The paper "Cloud cooling effects of forest afforestation & reforestation in midlatitudes" was published Aug. 9, in Proceedings of the National Academy of Sciences.Further Satellites show how forests cool the climate and increase cloud coverMore information: Sara Cerasoli and colleagues, Cloud cooling effects of forest afforestation reforestation at midlatitudes. Proceedings of the National Academy of Sciences (2021). Information from the Journal: Proceedings of the National Academy of Sciences Sara Cerasoli and colleagues, Cloud cooling effects of forest afforestation & reforestation in midlatitudes (2021). DOI: 10.1073/pnas.2026241118