Nature
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New evidence reveals trees have a net cooling effect over a much larger area of Earth than previously suspected, once cloud-formation is factored in.
It's not news that trees pull carbon dioxide from the atmosphere, helping to stop the Earth getting too hot. However, trees are darker than the grasslands they replace, and therefore also contribute to the capturing of extra heat. Climate scientists feared this effect may undo the benefits of reforestation in certain locations.
A paper in Proceedings of the National Academy of Sciences looks beyond the carbon capturing capacity of newly planted forests to consider other effects they have on global temperatures. In particular, the authors note, forests also lead to increased cloud cover, enhancing their cooling effects. Those who advocate major tree planting efforts were right all along, but not necessarily for the reason they thought.
The IPCC's 6th assessment report released this week has highlighted the urgency to transform energy systems to slash emissions of methane and carbon dioxide. However, it also makes it clear that isn't enough – we need to draw existing CO2 out of the atmosphere. Planting forests is the most obvious, and possibly most important, method known.
The role of clouds in setting global temperatures is not straightforward. Thick, long-hanging clouds reflect a lot of sunlight back to space, providing a powerful cooling force. However, some high clouds trap rising heat, particularly at night, making them part of the greenhouse effect, not its solution. Consequently, in the context of rising temperatures, increasing cloud cover is not always desirable.
However, the clouds induced by forests are the sort we need. “We show that if one considers that clouds tend to form more frequently over forested areas, then planting trees over large areas is advantageous and should be done for climate purposes." senior author, Princeton's Professor Amilcare Porporato, said in a statement.
To reach this conclusion, researchers used satellite records of 2001-2010 cloud cover at latitudes of 30-45, comparing conditions in areas where forests had been replanted or planted for the first time. Not only do forests attract more cloud cover, they found, but clouds form above them earlier in the day, increasing the amount of sunlight they reflect.
The reflectiveness (albedo) of the Earth's surface plays an important part in both local and global climate, most evidently in the way reduced snow-cover warms the planet by leading to greater absorption of sunlight.
Climate scientists are satisfied that in the tropics, any forest-induced warming is overwhelmed by the carbon they store. However, trees grow slower at high latitudes, and disrupt snow cover. It is thought increased tree-cover in the far north has a net warming effect, compared to grazed grasslands. Mid-latitudes, covering much of North America and Eurasia, have been considered uncertain – particularly since most trees there lose their leaves in winter, temporarily returning a substantial portion of their carbon to the atmosphere.
Researchers on the new study are confident the cloud-generating effect of forests at the latitudes they studied is so large that forests here are overwhelmingly beneficial for the climate. There are also plenty of other advantages of reafforestation, such as the habitat the forests offer to endangered species. Nevertheless, the authors acknowledge the competition that can take place with other land-uses, particularly agriculture.
