Siberian Lakes Disappearing
Lakes in northern Siberia are shrinking and disappearing at a steady rate as the Arctic climate warms and permafrost thaws, and none of the lakes have been refilling.
Using satellite photos of north-central Siberia taken from 1972 to 1998, researchers found that the number of large lakes in a 220,000-square-mile region had declined by 1,170, or more than 11 percent. Each of the lakes studied was larger than 100 acres.
During the 26-year period, total lake coverage in that area decreased by more than 6 percent, even though precipitation increased slightly.
Writing in Friday's issue of the journal Science, the researchers concluded that "the ultimate effect of continued climate warming on high-latitude, permafrost-controlled lakes and wetlands may well be their widespread disappearance." Of the almost 11,000 Siberian lakes that shrank significantly, the researchers found, 125 lakes disappeared entirely and were refilled with vegetation.
Permafrost is ground ice that generally does not melt throughout the year. There are, however, gradations of permafrost, ranging from conditions where the soil remains ice cold from the surface on down all the time, to situations where some surface permafrost melts and where nearby areas experience thawing during warm spells.
The researchers said that although the Siberian region they studied saw an overall and substantial decline in the number of lakes, the more northern reaches of "continuous" permafrost experienced an increase. The reason, the researchers hypothesized, is that as permafrost warms, it slumps and collapses -- creating depressions (thermokarst) where water collects as lakes. But if the warming continues, they said, the thermokarst in the northern reaches will go through the same shrinking process now seen at the more southern lakes and will gradually disappear.
-- Marc Kaufman
Asian Settlers a Small Group
A Rutgers University geneticist studying the original migration of colonists from northern China to the New World more than 10,000 years ago has determined that these first settlers numbered as few as 70 people.
Evolutionary geneticist Jody Hey devised a complex model to describe how one population can split into two, then fueled it with data from nine genetic sequences common to both Native Americans and northern Asians.
What he found was a surprisingly small "effective," or childbearing age, population of about 70 individuals, who broke away from an ancestral Asian community of 9,000 to cross the Bering Strait land bridge to the Americas about 14,000 years ago.
"It suggests something on the order of a tribal group," Hey said in a telephone interview last week. "So few people may be a little bit surprising, but there hasn't been much information out there. To the extent that there is conflict, the conflict is with scholars' intuition."
Hey, reporting in the June issue of the journal PLoS (Public Library of Science) Biology, said he used genetic data from northern Asia and from Native American populations from the United States southward to the Andes and the Amazon basin.
He said the peak probability for time of settlement based on his calculations was 7,000 to 8,000 years ago, but "the peak is very broad" and extends back to 13,000 or 14,000 years ago, which corresponds roughly to estimates based on archaeological records.
"My analysis is consistent with other estimates," he said, even though a fierce argument rages among archaeologists who date the arrival of the first immigrants anywhere from 11,000 to 14,000 years ago, or even earlier.
-- Guy Gugliotta
Martian Meteor Seen on Film
The panoramic camera on the Mars rover Spirit was pointed east a little before dawn on March 7, 2004, when a streak of light passed from left to right low on the horizon, flaring for perhaps 15 seconds. Then it was gone.
"We actually talked about it when we saw the exposure," said Texas A&M University planetary scientist Mark T. Lemmon, in charge of the panoramic camera for the NASA Mars mission. "We thought it was either a meteor or a satellite -- maybe Viking 2, which we haven't heard from in 20 years."
But in research reported in the journal Nature last week, Lemmon joined a team of scientists led by Franck Selsis of the Lyon Astronomical Research Center in France to conclude that Spirit had quite likely captured the first-ever image of a Martian meteor.
Meteor showers can occur when a planet passes close to the orbit of a comet, which leaves dust in its wake as it courses through the heavens. A shower results from particles "grazing" the planet's atmosphere in a streak of light familiar to any sky watcher on Earth.
"We probably would have seen more if we had been looking earlier -- around 2 a.m. is best," Lemmon said. But the team was still able to show that the meteor came from the orbit of the comet Wiseman-Skiff. The team said showers probably occurred regularly whenever Mars approached the comet's orbit.
Lemmon said satellites illuminated by the sun can also provide a similar light streak, but the NASA scientists discounted all the satellites orbiting Mars, including Viking 2, launched in 1975 and shut down in 1978.
-- Guy Gugliotta