Warming in the Arctic? Blame the Snow. The Dirty Snow, That Is. - Inglês

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Carbon dioxide and other greenhouse gases get most of the blame for climate change. And rightly so, as they are by far the most dominant influence globally.

But don’t forget dirt.

Building on research from earlier in the decade, scientists at the University of California, Irvine, have calculated the warming effect of soot falling on Arctic snow. And the result may come as a surprise: in the Arctic, dirty snow may be responsible for more warming than greenhouse gases.

“It turns out that nothing is more efficient at increasing the Earth’s temperature than soot in snow,” said Charles S. Zender, a professor of Earth system science at the university.

Soot, from coal burning, inefficient cookstoves and forest and agricultural fires, falls in the Arctic, darkening snow. Dark colors absorb more sunlight than lighter colors and radiate more heat in return.

Previous studies had looked at this basic effect and found that it contributed to regional warming. “But they had probably underestimated the impact,” Dr. Zender said.

The new work, published in The Journal of Geophysical Research, takes a more detailed look at the effects of soot. Among other things, Dr. Zender said, it accelerates the “aging” of snow, as fine crystals turn into larger grains and the snow turns darker. (City dwellers are quite familiar with this phenomenon, as a bright white blanket turns to gray within days.)

With this and other effects built into their model, the researchers found that over the past 200 years, dirty snow has been responsible for 0.5 to 1.5 degrees Celsius of warming in the Arctic. Over all, the region has warmed 1.6 degrees in that time.

Dr. Zender said that several steps could be taken to reduce the impact. One is a matter of timing. If farmers must burn fields, for example, it’s better to do it in the late summer and fall, when the impact on the snow cover will be less.

More efficient cookstoves for villagers across Asia would also help. By burning cleaner, they would produce less soot. Conversely they would produce more carbon dioxide, “but in this case emitting CO2 is the lesser of two evils,” Dr. Zender said.

Froggy Went A-Raftin’ From South America, Millions of Years Ago

Scientists at Penn State have penciled in the branches of the evolutionary tree of a major group of frogs, the eleutherodactylines, in the process making a surprising discovery: the species dispersed from South America to Central America and the Caribbean over water, not land.

There are roughly 800 species of eleutherodactylines, small frogs that, unlike many frogs found in temperate climates, lay their eggs on land. Scientists have not known how the species got from South America to the West Indies and Latin America and Mexico, but the prevailing theory was that they traveled across an ancient land mass between North and South America. Then about 70 or 80 million years ago this land mass moved eastward, taking some frogs with it (into what is now the Caribbean) and isolating the frogs to the north.

But the new research, a genetic analysis of 300 species directed by S. Blair Hedges, a Penn State professor, shows that the Caribbean frogs diverged from a common South American ancestor much later than that — in a single event 47 million to 29 million years ago. And the eleutherodactylines in Latin America and Mexico diverged from a South American ancestor 42 million to 31 million years ago.

The findings, reported in The Proceedings of the National Academy of Sciences, suggest that in both cases frogs from South America traveled across the water, probably hitching a ride on floating vegetation.

After a Major Wildfire, It Seems, Some Forests Are Better Left Alone

What to do with a forest after an extensive fire? The answer has often been to harvest the dead trees (in part to try to make up for economic losses caused by the fire) and plant seedlings to re-establish the forest.

But there has been a growing debate as to whether salvage logging and replanting is the best post-fire approach. A study in southwestern Oregon suggests one reason it might not be: areas that are managed in this way tend to burn more severely the next time the forest catches fire.

Jonathan R. Thompson of Oregon State University and colleagues studied an area in the Siskiyou Mountains near the coast. The area burned in 1987, and parts of it were logged and replanted with conifers. In August 2002 the area burned again in a huge blaze known as the Biscuit Fire.

Using satellite images and forest management data, the researchers created a model to calculate the severity of fire in randomly selected plots. Their findings are published in The Proceedings of the National Academy of Sciences.

In general, areas that burned severely in 1987 burned severely again 15 years later. But among these, the researchers found, areas that were left unmanaged had less devastation.

Salvage logging leaves behind debris like treetops and branches that are a ready source of fuel, as are young conifer plantations, with their uniform growth. In unmanaged areas, the researchers say, regrowth is more diverse, and there are likely to be gaps that reduce fire severity.

June 12, 2007


FONTE: http://www.nytimes.com/2007/06/12/science/12obse1.html?ref=science

How do they weigh greenhouse gas emissions?

While it is possible to weigh a quantity of gas, by comparing the weight of an evacuated container to one filled at a known pressure, climate scientists do not rely on direct measurements. Instead, they use estimates based on the molecular weight of carbon dioxide; the weights of other greenhouse gases are converted to their greenhouse impact as compared with that of a ton of carbon dioxide.

Carbon dioxide, the benchmark greenhouse gas implicated in global warming, has a molecule containing one carbon atom and two oxygen atoms. The CO2 output from burning a quantity of coal or oil is known. Depending on the fuel, the carbon dioxide can weigh almost three times as much as the fuel, because of the addition of oxygen from the air.

The Carbon Dioxide Information Analysis Center of the Department of Energy does the official estimates of carbon dioxide emissions for vehicles in the United States.

The center points out that the calculations involve numerous assumptions and vary by source. The center’s own estimates use a model called Greet (Greenhouse Gases, Regulated Emissions and Energy Use in Transportation), developed for the Department of Energy by the Argonne National Laboratory.