Thursday, 29 November 2012

Pine Beetles at it again - causing warming this time!

Yale Environment 360 tells us that Pine Beetle Attacks Cause Temperature Rise in Canadian Forests. Is it the beating of all those tiny wings creating heat? Their black carapaces (the shiny bits) absorbing more heat from the sun? No it's because trees naturally cool their environment. More beetles, fewer trees; fewer trees less cooling; less cooling, higher temperatures. Simple really, or so the researchers would have us believe. I just had to put down my dog-eared copy of "Only 100 days left to save the World - 101 things you didn't know about Global Warming" and read on.
The decimation of trees by mountain pine beetles in British Columbia has caused air temperatures in affected areas to climb by an average of 1 degree Celsius during the summer months, according to a new study. In an analysis of satellite and forest data collected between 1999 and 2010, scientists from the University of Toronto and University of California, Berkeley calculated that areas hit hardest by widespread pine beetle infestations have experienced even sharper temperature increases of several degrees Celsius, as regions are increasingly deprived of the natural cooling effect of trees. Since the evaporation of water through leaves prevents some of the sun’s radiation from heating the ground surface, the widespread loss of trees causes the temperature increases, said Holly Maness, a researcher at UC Berkeley and co-author of the study, published in the journal Nature Geoscience. And because warming temperatures and milder winters have helped pine beetles to flourish, these infestations are creating a feedback effect that is making the forests even more vulnerable. According to scientists, mountain beetles have affected 66,000 square miles in British Columbia, or 20 percent of the province’s total area.
That's pretty straightforward and convincing isn't it? What possible criticisms could I have? Well, for a start there's the chicken and egg business. (What's he talking about? - ed.). Did the pine beetles cause the warming or did the warming encourage the pine beetles to do what pine beetles do best (reproduce and eat, not necessarily in that order)? I ask only because "Global warming threatens pine forests, forcing federal officials to shift strategy" which I'd interpret as chicken -> egg whereas this paper seems to be claiming egg -> chicken -> bigger egg -> bigger chicken (feedback).

The thing that caught my eye was the apparently plausible warming mechanism outlined by Holly Maness - "Since the evaporation of water through leaves prevents some of the sun’s radiation from heating the ground surface, the widespread loss of trees causes the temperature increases". Now call me a sceptic, but I understood, as do many (most?) scientists and authors, that it was evapotranspiration wot dun it? Also, it's the leaves themselves which shade the ground below (simple really), and green chlorophyll absorbs sunlight, especially ultraviolet and infrared - the leaves are darker than grass, it's called the albedo effect. In a nutshell (how appropriate!)
Forests also influence local climate. Dependant on the latitude forest influences the temperature in a region: in the tropics forest have a net cooling effect through evapotranspiration while at higher altitudes, mainly boreal forests, there is a net warming effect because the relative dark colour of the canopies absorbs warmth from the sun (albedo effect).
When water evaporates through leaf pores, it absorbs heat (a lot of heat) thus cooling the leaves and the tree (and forest) canopy. Holly seems to think otherwise. It's true that the evaporated water vapour would absorb some short-wave infra-red from the Sun, but there's not a great deal of that radiation left by the time sunlight reaches the treetops; most is absorbed on its way through the atmosphere - by water vapour - lots more water vapour than the trees expire. Secondly, water vapour is the most abundant greenhouse gas (hasn't Holly heard that?), and absorption and emission of upward radiation from the forest canopy would certainly outweigh the small absorption of solar infra-red by the evaporated water vapour.

just a minute - surely the paper itself can't possibly say this can it?
The present mountain pine beetle infestation in forests in British Columbia ranks among the largest ecological disturbances recorded in Canada so far. These recent outbreaks are thought to have been favoured by large-scale climatic shifts, and may foreshadow outbreaks of a similar magnitude in North American forests over the coming decades. The associated forest dieback could result in substantial shifts in evapotranspiration and albedo, thereby altering the local surface energy balance, and in turn regional temperature and climate. Here we quantify the impact of the Canadian pine beetle disturbance on the local summertime surface energy budget, using measurements of evapotranspiration, albedo and surface temperature, obtained primarily through remote sensing. 
We show that over the 170,000 km2 of affected forest, the typical decrease in summertime evapotranspiration is 19%. Changes to the absorbed short-wave flux are negligible, in comparison. As a result, outgoing sensible and radiative heat fluxes increased by 8% and 1%, respectively, corresponding to a typical increase in surface temperature of 1°C. These changes are comparable to those observed for other types of disturbance, such as wildfire, and may have secondary consequences for climate, including modifications to circulation, cloud cover and precipitation.
That would seem to set the record straight. Perhaps the article's author and one of the co-authors should have read the paper, or at least the abstract.

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