Science Monday, July 23, 2012

Climate similar to past; fires’ severity increases

Today’s hot, dry climate alone is not enough to cause the massive megafires seen in the southwestern U.S. over the past decade, according to researchers. Similar climates have existed at times over the past 1,500 years, but the type and severity of today’s fires have not, leading researchers to conclude current fires are influenced by human activities over the past century.

Earlier research had found that fires during the so-called Little Ice Age, a period of cool and wet climate from about 1600 to the mid-1800s, occurred frequently but burned the forest floors rather than the canopy crowns. The new research, published in May, analyzed 1,500 years of data from tree rings, fire scars and statistical models to estimate fire activity in ponderosa pine forests in the southern Colorado Plateau during a period known as the Medieval Warm Period, from about 800 to 1300, when the climate was similar to today’s.

They found that the weather patterns driving fires in the two periods were similar, and that the trend of occasional, low-intensity surface fires that clear out underbrush should have continued through today.

Instead, human management of forests, including fire suppression and grazing, has led to fewer fires in general over the past century and, recently, more frequent severe fires that destroy whole forests rather than just underbrush.

The researchers, from the University of Arizona and Southern Methodist University, said the findings suggest that while Southwest forests have been able to withstand climatic changes in the past, they may not be able to do so now given their current conditions. They said changes in forest management may be needed to remedy that.

■ After a study last year found elevated methane levels in groundwater wells near gas wells in northeastern Pennsylvania and upstate New York, some scientists questioned the implications of those findings, saying that methane could not possibly have traveled from the 2,500-meter deep shale formations where the gas is extracted to 90-meter deep aquifers.

Now, that possibility is back on the table after a study earlier this month found that natural pathways through which gas can eventually travel up to the aquifers do exist.

In studying three aquifers and more than 400 groundwater samples in the region around the Marcellus shale formation, the researchers found that some samples contained salts that are unique to the shale thousands of feet below, leading them to conclude they must have migrated toward the surface over thousands of years.

This long time period means that it is unlikely the fluids used in hydraulic fracturing, in which a mix of chemicals is injected deep into the ground to free gas from rock formations, will migrate up fast enough to contaminate groundwater, the researchers said in the study, published in the Proceedings of the National Academy of Sciences.

But they did note that these pathways between shale formations and groundwater wells could provide shortcuts for gases like methane to contaminate those wells, and may have implications for the increasingly common practice of fracking since it casts doubt on the idea that the physical distance between gas deposits and wells is enough to protect those water supplies.


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