Monday, February 2, 2009

Comet Impact Theory Challenged

This one of those items where the headline promises more than it actually delivers. Once again lack of evidence is hardly evidence of lack. This is about evidence supporting an increase in fires as a result of an improving climate. That is all well and good. It does not disprove the existence of a comet explosion, or an asteroid impact leading to the initiation of the climatic change.

That headline writers were extrapolating fires from the shock wave was optimistic. In fact, what evidence that existed was in the form of charcoal, and that is not evidence of burning so much as a blast of heat that toasted large areas.

If significant charcoal survived wildfires then all our soils would be terra preta. The reality is that the charcoal succumbs to fire sooner or later.

The evidence of a blast is in the rock scatter throughout the Ohio Valley. That others have suggested that a Tunguska like event would have ignited a continental wildfire is purely a speculation and as suggested, not supported. There are related charcoal deposits that are not lake sediment related and likely need to be understood.

A charring event following a shock wave passing over grassland is likely to produce a carbon layer if it did not ignite, except it would be consumed during the next season’s grassfire. The same event passing over a forest might just leave enough carbon buried in fallen trees to protect the fresh carbon layer. This tells us that a lot more data is needed, in order to put the early claims in proper perspective.

That a cosmological event took place is not in dispute although this headline claims that. The event put a lot of dust, rock and ice on a trajectory toward the Carolinas from just west of Hudson Bay. It created a shock front that flattened exposed ground but likely left huge tracts untouched.

It was a large object whose energy was largely absorbed by the crust. Much of the shock was absorbed by the ice. And it may have broken up just before impact.

It would be nice if the continent was probably burned over also, but really begs the question of why? It is amazing how a good tale runs ahead of the facts that are dramatic enough.

They did prove that a warmer climate produces more biomass and that this produces more fuel which produces more fire.

Comet Impact Theory Disproved

by Staff Writers
Bristol, UK (SPX) Jan 28, 2009

New data disproves the recent theory that a large comet exploded over North America 12,900 years ago, causing a
shock wave that travelled across North America at hundreds of kilometres per hour and triggering continent-wide wildfires.

Dr Sandy Harrison from the University of Bristol and colleagues tested the theory by examining charcoal and pollen records to assess how fire regimes in North America changed between 15 and 10,000 years ago, a time of large and rapid
climate changes.

Their results provide no evidence for continental-scale fires, but support the fact that the increase in large-scale wildfires in all regions of the world during the past decade is related to an increase in global warming.

Fire is the most ubiquitous form of landscape disturbance and has important effects on
climate through the global carbon cycle and changing atmospheric chemistry. This has triggered an interest in knowing how fire has changed in the past, and particularly how fire regimes respond to periods of major warming.

The end of the Younger Dryas, about 11,700 years ago, was an interval when the temperature of Greenland warmed by over 5 degrees C in less than a few decades. The team used 35 records of charcoal accumulation in lake sediments from sites across North America to see whether fire regimes across the continent showed any response to such rapid warming.

They found clear changes in biomass burning and fire frequency whenever climate changed abruptly, and most particularly when temperatures increased at the end of the Younger Dryas cold phase. The results published, January 26, in the Proceedings of the National Academy of Science.

Understanding whether rapid changes in climate have caused wild fires in the past will help understand whether current changes in global temperatures will cause more frequent fires at the present time. Such fires have a major impact on the economy and health of the population, as well as feeding into the increase in global warming.

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