Pine beetles contributing to climate change, study says
A mountain pine beetle crawls out of a ponderosa pine tree while another, right, remains in its hole in Green Mountain Falls, Colo., Aug. 24, 2006. (The Gazette, Hunter McRae)
Published Sunday, November 25, 2012 1:55PM EST
EDMONTON -- Newly published research suggests mountain pine beetles have become so widespread that they're not just benefiting from global warming, they're starting to contribute to it.
"The effects of climate change cascade," said Holly Maness, whose paper was published Sunday in the journal Nature Geoscience.
"Previous studies have shown that climate change allowed the beetle to flourish. But our work shows that beetle infestations in turn feed back into climate."
Scientists have concluded that the gradually warming climate has allowed the tree-killing beetle to spread into forests it used to be frozen out of. The report quotes figures suggesting that over the last decade, the bugs have spread over about 20 per cent of the total area of British Columbia, making it one of the largest ecological disturbances ever recorded.
Maness, an earth scientist working at the University of Toronto, decided to study how turning about 170,000 square kilometres of green forest into grey, leafless stands of dead trees would affect the regional climate.
Using temperature data from satellites, she and her team concluded that beetle-ravaged forests were, on average, one degree warmer during the summer than healthy forests.
The reason? Tree sweat, or rather, the lack thereof.
"Trees sweat to help cool themselves in the same way that humans do," said Maness.
"When you kill a tree, it's going to stop sweating. That means that solar radiation that was previously spent evaporating water from these trees is now going into heating the surface."
The amount of water released by trees isn't as easily measured as surface temperature, but Maness's paper uses an average of three different proxies to estimate it. When she calculated the amount of energy that was no longer going into evaporation from tree leaves, the result was very close to what it would take to create the observed temperature change.
"Solar energy can either evaporate water or it can heat the surface, so if you measure both of those things and they give results that are consistent with each other, you have a fair amount of confidence the result is right."
Maness acknowledges her study raises more questions than it answers. What's all that heat collected in the ground going to do?
"We've shown that the surface temperature increases that we've seen are sufficient to drive changes in cloud cover and precipitation, but it needs to be directly measured."
Earlier snow melt is another possibility that brings with it potential changes in groundwater and spring runoff patterns. Maness found that some changes are similar to what happens in areas scorched by forest fires, but the beetle impacts are much larger and leave a more complex pattern of dead and living trees.
Maness is willing to suggest that beetle-caused warming won't allow the bugs to spread more quickly. The effect is limited to areas already infested.
"It would surprise me if it accelerated their ability to claim more forests."
But she suggested the temperature impact she uncovered contains a warning.
"When you disrupt the delicate balance imposed by a stable climate, then you can have this cascade of effects. Unfortunately, it's a lot easier to explain these effects in hindsight.
"In thinking about climate adaptation and mitigation strategies, it's going to be essential to keep these uncertainties in mind and work to minimize our perturbations in climate."