On exercises and fires

this is an exercise i wrote a couple of months ago... it's not about astronomy or physics, for a change. this time it's about earth system science, and i didn't really know much about it before reading the paper. but still, it's an example of how i write, and pretty interesting too. so, here it is:

Not All Fires Warm the World

Planting trees over vast areas of the planet may help to slow climate change, but what if then forest fires heat the world again? Now researchers say there is no need to worry, as northern forest fires may actually contribute to cooling the climate, at least on decade-long time scales.

Speculations about the role of boreal forests, the mainly evergreen woods found at high northern latitudes, in mitigating the effects of man-made global warming have raised the question of forest fires and their possible intensification due to climate changes. Previous studies focussed mostly on the conspicuous quantities of carbon dioxide and other greenhouse gases emitted during fires, highlighting a possible degeneration into a self-feeding loop. However, there is more than greenhouse gases involved in the complex interactions of forest fires with their landscape and the atmosphere.

A new study led by Earth system scientist James Randerson of the University of California, Irvine, sheds new light on the subject through an in-depth analysis of the 1999 Donnelly Flats fire, which destroyed about 7600 hectares of black spruce in interior Alaska.

Besides monitoring greenhouse gases and aerosol emissions, the team investigated the different amount of sunlight reflected by forests and burned landscapes — the so-called albedo. The bare terrain left behind by a fire reflects much more light than the dark forest’s canopy, especially when snow is lying on the ground. A higher reflecting power results in cooling and, when summed together with all other contributions, it alleviates the warming effect caused by other agents. And there is more: over the 80 years following the fire event, it even reverts the score.

Soot produced in the fire and deposited on surrounding snow and sea ice is also a player in the ‘albedo game’, this time reducing the reflecting power of otherwise almost white surfaces. Its contribution increases warming, but becomes negligible after the first year. The loss of canopy, instead, is a long-term phenomenon, as vegetation needs several decades to recover to pre-fire conditions.

Randerson and his team observed the burn perimeter over five years, and complemented these measurements with satellite data surveying the light-reflecting properties of various areas in interior Alaska where fires occurred at several different epochs in the past century.

This extensive study of the overall impact of boreal forest fires on the temperature of both northern regions and the whole planet bears the reassuring finding that fires do not contribute to the Earth’s warming if we consider long enough time scales. It will be interesting to compare these results with studies of Siberian larch forests, and, even further, to apply a similar approach to temperate and tropical ecosystems.

-----------

This story is based on "The Impact of Boreal Forest Fire on Climate Warming", by J.T. Randerson et al., published on Science on 17 November 2006 (Vol. 314. no. 5802, pp. 1130 - 1132).

The photo of the Donnelly Flats Fire, near Delta Junction, Alaska (June 13-20th 1999) is courtesy of Tom Lucas, Delta News Web.