Madrid, 10 (European Press)
Reforestation is one of the simplest and most attractive natural climate solutions, but the effect of trees on atmospheric temperature is more complex than it appears.
One question scientists are asking is whether reforesting places at mid-latitudes such as North America or Europe could actually warm the planet. Forests absorb large amounts of solar radiation as a result of reduced albedo, which is a measure of a surface’s ability to reflect sunlight.
In the tropics, low albedo is compensated by increased carbon dioxide uptake by dense vegetation throughout the year. But in temperate climates, the concern is that the heat entrapped by the sun could counteract any cooling effects that forests could provide by removing carbon dioxide from the atmosphere.
But a new study by Princeton University researchers finds that these concerns may be missing a crucial ingredient: drag. They report in the Proceedings of the National Academy of Sciences that denser cloud formations associated with forest areas mean that reforestation will likely be more effective in cooling the Earth’s atmosphere than previously thought.
Corresponding author said Amilker Purpurato, professor of civil and environmental engineering at Princeton University and the High Meadows Environment Institute (HMEI). “We show that if we consider that clouds tend to form more frequently over forest areas, planting trees in large areas is beneficial and should be done for climatic purposes.”
As anyone who feels a cloud passing over the sun on a hot day knows, diurnal clouds have a cooling, albeit fleeting, effect on Earth. In addition to directly blocking the sun’s rays, the clouds have a high albedo that resembles ice and snow. However, clouds are known to be very difficult to study and have been largely left out of many studies looking at the efficacy of mitigating natural climate change, including reforestation, Purburato said.
To consider reforestation in the context of cloud cover, Porporato worked with lead author Sara Cerasoli, a Princeton graduate student in civil and environmental engineering, and Jun Ying, an assistant professor at the University of Information Science and Technology in Nanjing, who was previously a postdoc. Fellow of the Porporato Research Group. His work has been supported by the HMEI-based Carbon Mitigation Initiative.
Purburato and Yen previously reported that climate models underestimate the cooling effect of the daily cloud cycle. They also reported last year that climate change could increase daily cloud cover in arid regions such as the southwestern United States, which are currently ideal for solar energy production.
For the latter study, Cerasoli, Porporato and Yin investigated the influence of vegetation on cloud formation in mid-latitudes by integrating satellite cloud cover data from 2001 to 2010 with models related to the interaction between plants and the atmosphere.
The researchers modeled interactions between different types of vegetation and the atmospheric boundary layer, which is the lowest layer of the atmosphere and interacts directly with the Earth’s surface, to determine whether cloud formation is affected differently by vegetation type. They focused on regions in the latitudinal range of 30 to 45 degrees, roughly from the subtropics to the semi-arid regions such as the North Midwest of the United States. They considered the effects of both reforestation (restoring lost tree cover) and afforestation, which involves planting forests in previously treeless areas, although this may have other environmental costs.
The team found that for mid-latitudes, the cooling effect of clouds, in addition to sequestering carbon, outweighs the solar radiation absorbed by forest areas.
Models showed that clouds form more often over forest areas than over grasslands and other areas with little vegetation, and that this improved cloud formation had a cooling effect on Earth’s atmosphere. Los investigadores observaron a partir de los datos satelitales que las nubes también tienden a formarse más temprano en la tarde sobre áreas boscosas, lo que resulta en una solar mayor duración de la cobertura de nubes tie más tien de la má s ground.
The results can help establish policies for allocating land for reforestation and agriculture; Wetter mid-latitudes such as the eastern United States or southeastern China are suitable for reforestation and afforestation, but are also attractive for cultivation. The study authors report that one approach is to combine mid-latitude reforestation with distribution of drought-tolerant crops in areas less suitable for reforestation.
However, the authors urged caution when making the leap from science to policy. “Not only can we consider climate change, but we must also take into account other factors, such as biodiversity and the fact that land is also essential for food production,” Cerasoli said. “Future studies should continue to look at the role of clouds, but should focus on more specific regions and take their economies into account.”