New ideas about how changing vegetation at high latitudes can cause climate warming to accelerate
Thursday, January 7th, 2010
Most people have probably heard about positive feedbacks at high latitudes and why they matter:
- Warming thaws sea ice, and the decrease in the reflective ice surface causes more solar radiation to be absorbed by the oceans, thereby causing even more warming.
- Warming is causing the northward march of treeline. As boreal conifers in Canada, Alaska, and Siberia migrate into barren tundra, their dark foliage towers above bright snow in winter, causing the landscapes to darken, absorb more sunlight, and warm up, thereby enhancing regional warming.
A new study1 in the Early Edition of the Proceedings of the National Academy of Sciences (open access), indicates that other effects of forest changes might also matter.
Specifically, boreal forests and tundra may become more dominated by deciduous trees (ones that drop their leaves in autumn), which are usually found in warmer regions. What happens if we have a future Arctic dominated by these species?
Using a set of ecosystem and climate models, Abigail Swann and colleagues determined that a rise in deciduous forests would cause an increase in water vapor to the atmosphere (deciduous trees transpire—lose water through their leaves—more than conifers). This makes the atmosphere in the Arctic more laden with water vapor, which is a good greenhouse gas. This warming, in turn, induces further sea ice and snow loss, causing warming to happen more quickly. But wait, there’s more: Warmer, ice-free oceans also release more water vapor to the atmosphere, causing greenhouse warming to increase even more.
How big an effect? About 1 degree C in the Arctic, equivalent to increasing the atmospheric CO2 about 100 ppm in the atmosphere. They found that these changes in water vapor have about the same impact as the changes in reflectivity caused by the color of forest foliage overtopping snow in the tundra.
Things like this are reasons why when warming starts, it can accelerate faster than we think.
1Swann, A. (in press) Changes in Arctic vegetation amplify high-latitude warming through the greenhouse effect. Proceedings of the National Academy of Sciences
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Photo credit: Phil Camill (from my Flickr collection) http://www.flickr.com/photos/pcamill/ / CC BY-NC-ND 2.0
Tags: trees and forests
Posted in: biodiversity science, climate change science, polar ice | 1 Comment »
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