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longchamp tote

Published: Thursday 29 August, 2013

longchamp tote longchamp tote ´╗┐Fossils of forgotten fires

A world warmed by the humaninduced greenhouse effect holds many perils. Sea levels will rise, patterns of crop growth may change, rain will flood some regions, others will go parched. Many believe that a change in climate will also affect the incidence of forest fires, presenting grave hazards to both human communities and ecosystems.

Now Sarah Millspaugh and colleagues from the University of Oregon, USA, show that the incidence of forest fires in the Yellowstone National Park region has indeed risen and declined in concert with climate fluctuations over the past 17,000 years.

Clearly, a dry summer leaves a forest at greater risk from fire. The longerterm connection between clim longchamp tote ate and fire is n longchamp tote ot so obvious. A region that experiences a prolonged episode of dryness as some parts of the USA have since the end of the last ice age around 12,000 years ago would naturally be expected to suffer more fires. In the days before widespread human habitation, such fires would result mostly from lightning strikes, so their incidence might also depend on how common thunderstorms are in such conditions. Computer simulations of a world warmed by greenhouse gases have suggested that lightning strikes might be more frequent.

Longterm changes in climate are also likely to change the type of vegetation covering a region. Droughthardy species would, for example, become more widespread. This could also affect the susceptibility to natural fires. Such ambiguities have plagued attempts to deduce how fire incidence has varied with climate over centuries or millennia.

Millspaughs team has deduced the history of fire frequency in the Yellowstone region by prospecting for charcoal the remains of burned wood in the sediments of Cygnet Lake, in the middle of the park, as they report in the journal Geology1. Because charcoal is basically raw carbon, it can be radiocarbondated. The variations in charcoal content of the sediments over time can then be compared against records of temperature or rainfall constructed from other features of the geological record.

The great virtue of Cygnet Lake is that studies of ancient pollen in the sediments have revealed no major shifts in vegetation type there since at least the end of the ice age. The fire record is therefore uncomplicated by other factors.

The incidence of large fires increased from about four per thousand years during the end of the ice age to about fifteen per millennium about 10,000 years ago, the researchers find. As fires became more frequent, they became smaller, perhaps because there was less time for the forest to recover in between.

From about 7,000 years ago, the frequency began to decline to its current rate of around two or three each millennium. Over this period, the amount of sunshine falling on the Yellowstone region during the height of summer rose and fell in parallel, being greatest about ten millennia ago. This increase, which is thought to have intensified summer drought in the northwestern USA, is due to slow, rhythmic changes in the nature of the Earths orbit around the Sun. Such changes are believed to cause the cycle of ice ages.

These findings support the contention that a drier climate carries a higher risk of forest fire and indicate how much greater the risk can be. But the researchers point out that the changes in climate since the ice longchamp tote age ended are not perfectly analogous to the changes anticipated in a world warmed by humanmade greenhouse gases. Also, in highly managed areas like Yellowstone, human intervention is likely to exert a strong influence on the size and frequency of fires. longchamp tote