Global ChangeIntersection of Nature and Culture

Can “tree sweating” buffer Europe against future heat waves?

Wednesday, February 24th, 2010

Trees can have a big impact on climate in many ways, as we have been tracking over the past few weeks here at Global Change.

Changes in species ranges, such as the shift of boreal forests northwards into barren-ground tundra, can lower albedo (reflectivity) in winter, thereby warming regional climate.

Last week, we also saw how trees can act as methane chimneys that release this greenhouse gas produced in swampy soils.

In another study, we saw that rising CO₂ in the atmosphere can cause forests to grow faster such that they become nutrient starved—especially by soil nitrogen.  This causes tree growth to slow.  Unfortunately, we saw that most models of climate change (which assume forests are removing CO₂ from the atmosphere) don’t take nutrient limitation into account, so scientists are expecting forests to soak up more CO₂ than they probably will.  This means that atmospheric CO₂ rise (and warming) will likely be worse than expected—maybe by as much as 0.5 degree by 2050 and 1 degree by the year 2100.

A further study indicated that increased deciduous forests in the Arctic can increase transpiration (water flow from soils to the atmosphere through plants).  This extra water vapor in the atmosphere might act as a greenhouse gas and cause climate in high latitudes to warm by an extra 1 degree C.

In the OnlineFirst issue of Climatic Change (this article is open access), Su-Jong Jeong and colleagues explore other possible forest impacts on climate.¹

Specifically, they focused on heat waves in Europe and asked whether forests might be able to help alleviate the impacts of them.

Here’s the idea:  In a warmer world with more CO₂, forests grow more and there is higher leaf area.  This leads to more transpiration.   When plant leaves lose water, this acts to cool the plants a lot like sweating does in animals (because the transition of less-energetic liquid water to more-energetic water vapor requires heat input, which comes from the plant).   This results in cooler trees and cooler landscapes.  Subsequent rainfall from all of this water vapor could supposedly cool landscapes further.

These authors argue that this mechanism can actually cause  forested regions in Europe to cool by 1 degree C, thereby potentially lessening the impacts of future heat waves.

Wait a minute, you might be asking, I thought you said that transpiration causes climate to warm?   That’s a good point, so let me try to clarify.  In the Arctic example above, the researchers were focusing on water vapor as a potential greenhouse gas in the atmosphere.   In the Jeong article here, they are focusing on the effects of water evaporation on the temperature of tree leaves at the surface of the earth.

It’s an interesting idea that’s not particularly new.  However, there are several potential challenges with the Jeong article that I didn’t see addressed:

• When the water vapor from trees condenses into liquid clouds and rain, this process gives off heat (because we are going from a more energetic gaseous form to a less energetic liquid form).  This warms the atmosphere wherever condensation happens, which presumably would also be Europe.
• If increased transpiration is a general feature of European forests in a warmer world, and water vapor remains persistently high in the atmosphere, it can act as a greenhouse gas like the Arctic paper above suggests.  This, too, will warm rather than cool climate.
• And, finally, heat waves are often accompanied by droughts when transpiration is generally low because trees are water stressed.  This diminishes the cooling effect and can actually cause landscapes to warm as tree leaves warm up and release heat rather than water vapor.

¹Jeong, S.J et al (in press) Potential impact of vegetation feedback on European heat waves in a 2 x CO₂ climate. Climatic Change
DOI 10.1007/s10584-010-9808-7

_____

Photo credit: http://www.flickr.com/photos/ghostparticle/ / CC BY-NC-SA 2.0