[naturenews] from [nature.com]
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Published online 10 June 2010 | Nature | doi:10.1038/news.2010.292
News
Global warming's impact on Asia's rivers overblown
｛地球温暖化の影響論、アジアの河川を誇大問題化｝
Freshwater flow dominated by monsoon rains rather than glacier run-off.
｛淡水の流量は、氷河からの流出よりもモンスーンの雨量によって左右される｝
By Richard A. Lovett
{Meltwater from glaciers makes a large contribution to the Indus river but not to all Asian rivers.}World Pictures/Photoshot}
Although global warming is expected to shrink glaciers in the Himalayas and other high mountains in Central Asia, the declining ice will have less overall impact on the region's water supplies than previously believed, a study concludes.
It's an important finding, says Richard Armstrong, a climatologist at the US National Snow and Ice Data Center in Boulder, Colorado, who notes that the Intergovernmental Panel on Climate Change had previously predicted dire restrictions on water supplies in Asia. "There clearly were some misunderstandings," he says.
The researchers behind the latest study began by calculating the importance of meltwater in the overall hydrology of five rivers: the Indus, the Ganges and the Brahmaputra in India, Pakistan and Bangladesh, and the Yellow River and the Yangtze in China1. The authors found that meltwater is most important to the Indus, with a contribution roughly 1.5 times that from lowland rains. In the Brahmaputra, meltwater flow is equivalent to only one-quarter of the volume supplied by lowland rainfall, and, in the other rivers, it forms no more than one-tenth of the input.
Furthermore, the study found that in the Indus and Ganges basins, glacial ice contributes only about 40% of the total meltwater, with the rest coming from seasonal snows. In the other three rivers its contribution is even lower.
High and dry?
That's important, says Walter Immerzeel, a hydrologist at FutureWater in Wageningen, The Netherlands, and lead author of the study1, because Asian rivers are fed by three sources: rain, snow melt and melting glaciers.
The first two are driven by current weather patterns, because rains fall either as water or as snow that will later melt. The last is a carry-over from the build-up of glaciers in prior centuries. As the glaciers shrink, their contribution will also decline until the glaciers have either melted entirely, or stabilized at smaller sizes.
{“The glaciers are tiny, compared with the monsoon.”}
Climate change will therefore have two effects, Immerzeel says. One will be to reduce the contribution of glaciers to total run-off. The other will be to change weather patterns, including rain and snowfall. Combining these and looking at averages from five climate models, Immerzeel and colleagues concluded that the change in upstream water inputs will range from a decrease of 19.6% for the Brahmaputra to a 9.5% increase for the Yellow River. The latter, he notes, is due to increased winter rains. "The Yellow River depends only marginally on meltwater," he says, "and, on average, the models project an increase in winter precipitation in the Yellow River basin."
What this means, Armstrong says, is that river flows are dominated by seasonal rains. "The glaciers are tiny, compared with the monsoon," he says.
Nevertheless, the study concludes that climate change will reduce water supplies enough that by 2050, declines in irrigation water are likely to reduce the number of people the region's agriculture can support by about 60 million — 4.5% of the region's present population.
Model uncertainty
One caveat, Immerzeel notes, is that climate models don't fare well at simulating the effect of warming on Asian rainfall. "There's still a lot of research going into the effect of climate change on the behaviour of the monsoon," he says.
Further refinements will also come from improved mapping of the area's glaciers, something that Armstrong's team has recently started, using remote-sensing data from satellites. That's an important next step, he says, although he adds, "I don't think we'll have a substantially different result."
The findings are important for policy-makers, says Jeffrey Kargel, a glaciologist at the University of Arizona in Tucson. "This paper adds to mounting evidence that the Indus Basin [between India and Pakistan] is particularly vulnerable to climate change," says Kargel. "This is a matter that obviously concerns India and Pakistan very much."
"The two nations must talk to one another and see that it is in their mutual best interests to arrive at an equitable means of sharing and utilizing water," he adds.
References
1. Immerzeel, W. W. , van Beek, L. P. H. & Bierkens, M. F. P. Science 328, 1382-1385 (2010).
[nature.com > naturenews]
Published online 10 June 2010 | Nature | doi:10.1038/news.2010.290
News
The Sun as comet snatcher
｛太陽、誘拐犯？｝
Most of the Solar System's comets may have been stolen from other stars.
｛太陽系の彗星のほとんどが、他の系外の星から侵入したものか？｝
By Lucas Laursen
{{Comets in the Sun's Oort cloud may be stolen goods.}
DR SETH SHOSTAK / SCIENCE PHOTO LIBRARY}
New simulations suggest that the Sun may have captured more than its fair share of comets from the primordial star-forming soup. The study, led by Harold Levison of the Southwest Research Institute in Boulder, Colorado, seeks to account for the abundance of comets in the outer reaches of the Solar System.
Our Solar System's comets spend most of their time between roughly 5,000 and 100,000 times further away from the Sun than the Earth, in a little-understood region beyond the planets known as the Oort cloud. Occasionally, some zip past the inner Solar System, and a rare few, such as Halley's Comet, return on a regular basis. But the origin of even the most well known comets is something of a mystery.
An influential model of how the Solar System formed predicts that around 6 billion comets in the Oort cloud are home-grown1. But some astronomers estimate that there are as many as 400 billion comets surrounding the Solar System — a discrepancy that researchers have struggled to explain.
Now the Levison study suggests that these mystery comets may actually have formed around other stars during the first moments of star formation. "Our Sun is a relatively heavy star," explains Ramon Brasser, a co-author of the study, which appears online in Science today2. When material such as gas, dust and ice began to find gravitational dancing partners, our Sun may have been massive enough to skim spare comets from its more lightweight neighbours.
Stolen goods
Levinson and colleagues are not the first to suggest that some comets may be from beyond the Solar System. A team examined the possibility in a 1990 study, but concluded that the Sun's pull was not sufficient to attract enough comets3. "They did not have the computing power to do the simulation we have done," Brasser says.
Brasser and his colleagues built a computer model in which many stars form near one another in a stellar cluster. In the simulation, each star spawns planetary objects including comets, some of which settle into orbits occupying an extended scattered disk around the star and others of which are ejected into the wider gas cloud enveloping the cluster. About 3 million years into the simulation, the gas surrounding the newly formed stars collapses into each solar system, and most of the free-floating comets find homes around one of the stars. During subsequent flybys with other stars, the simulation shows the Sun snags enough comets to account for its present collection.
However, running such a detailed simulation required the astronomers to make many assumptions about solar system formation, which introduce large uncertainties into the picture. "The most vulnerable assumption is that extended scattered disks would actually exist at the early time considered," says Hans Rickman of Uppsala Astronomical Observatory in Sweden. Extended scattered disks, a hypothetical home for comets in orbit around stars, have not been directly observed, nor do theorists agree on how they form, Rickman says. In one recent model, the disk does not form until 1 billion years after the beginning of a solar system.
Brasser says the simulation also had to assume that all stars have the same number of comets because solid numbers are not available. "People will have difficulty with this assumption," he admits. And for lack of more information, the authors write in the paper that they assumed that other solar systems have a distribution of large planets (whose mass influence cometary orbits) similar to our own.
Even the more widely accepted estimates of the number of comets in the Oort cloud could be too high, Rickman says, and a later formation of the cloud could make it easier to fill with home-grown comets. All the uncertainty, he says, "makes me think there does not have to be any problem at all".
References
1. Levison, H. F. & Duncan, M. J. Icarus 127, 13 (1997).
2. Levison, H. F. , Duncan, M. J. , Brasser, R. & Kaufmann, D. E. Science published online, doi: 10.1126/science.1187535 (10 June 2010).
3. Zheng, J-Q. , Valtonen, M. J. & Valtaoja, L. Celest. Mech. Dyn. Astr. 49, 265 (1990).
[nature.com > naturenews]
Published online 10 June 2010 | Nature | doi:10.1038/news.2010.292
News
Global warming's impact on Asia's rivers overblown
｛地球温暖化の影響論、アジアの河川を誇大問題化｝
Freshwater flow dominated by monsoon rains rather than glacier run-off.
｛淡水の流量は、氷河からの流出よりもモンスーンの雨量によって左右される｝
By Richard A. Lovett
{Meltwater from glaciers makes a large contribution to the Indus river but not to all Asian rivers.}World Pictures/Photoshot}
Although global warming is expected to shrink glaciers in the Himalayas and other high mountains in Central Asia, the declining ice will have less overall impact on the region's water supplies than previously believed, a study concludes.
It's an important finding, says Richard Armstrong, a climatologist at the US National Snow and Ice Data Center in Boulder, Colorado, who notes that the Intergovernmental Panel on Climate Change had previously predicted dire restrictions on water supplies in Asia. "There clearly were some misunderstandings," he says.
The researchers behind the latest study began by calculating the importance of meltwater in the overall hydrology of five rivers: the Indus, the Ganges and the Brahmaputra in India, Pakistan and Bangladesh, and the Yellow River and the Yangtze in China1. The authors found that meltwater is most important to the Indus, with a contribution roughly 1.5 times that from lowland rains. In the Brahmaputra, meltwater flow is equivalent to only one-quarter of the volume supplied by lowland rainfall, and, in the other rivers, it forms no more than one-tenth of the input.
Furthermore, the study found that in the Indus and Ganges basins, glacial ice contributes only about 40% of the total meltwater, with the rest coming from seasonal snows. In the other three rivers its contribution is even lower.
High and dry?
That's important, says Walter Immerzeel, a hydrologist at FutureWater in Wageningen, The Netherlands, and lead author of the study1, because Asian rivers are fed by three sources: rain, snow melt and melting glaciers.
The first two are driven by current weather patterns, because rains fall either as water or as snow that will later melt. The last is a carry-over from the build-up of glaciers in prior centuries. As the glaciers shrink, their contribution will also decline until the glaciers have either melted entirely, or stabilized at smaller sizes.
{“The glaciers are tiny, compared with the monsoon.”}
Climate change will therefore have two effects, Immerzeel says. One will be to reduce the contribution of glaciers to total run-off. The other will be to change weather patterns, including rain and snowfall. Combining these and looking at averages from five climate models, Immerzeel and colleagues concluded that the change in upstream water inputs will range from a decrease of 19.6% for the Brahmaputra to a 9.5% increase for the Yellow River. The latter, he notes, is due to increased winter rains. "The Yellow River depends only marginally on meltwater," he says, "and, on average, the models project an increase in winter precipitation in the Yellow River basin."
What this means, Armstrong says, is that river flows are dominated by seasonal rains. "The glaciers are tiny, compared with the monsoon," he says.
Nevertheless, the study concludes that climate change will reduce water supplies enough that by 2050, declines in irrigation water are likely to reduce the number of people the region's agriculture can support by about 60 million — 4.5% of the region's present population.
Model uncertainty
One caveat, Immerzeel notes, is that climate models don't fare well at simulating the effect of warming on Asian rainfall. "There's still a lot of research going into the effect of climate change on the behaviour of the monsoon," he says.
Further refinements will also come from improved mapping of the area's glaciers, something that Armstrong's team has recently started, using remote-sensing data from satellites. That's an important next step, he says, although he adds, "I don't think we'll have a substantially different result."
The findings are important for policy-makers, says Jeffrey Kargel, a glaciologist at the University of Arizona in Tucson. "This paper adds to mounting evidence that the Indus Basin [between India and Pakistan] is particularly vulnerable to climate change," says Kargel. "This is a matter that obviously concerns India and Pakistan very much."
"The two nations must talk to one another and see that it is in their mutual best interests to arrive at an equitable means of sharing and utilizing water," he adds.
References
1. Immerzeel, W. W. , van Beek, L. P. H. & Bierkens, M. F. P. Science 328, 1382-1385 (2010).
[nature.com > naturenews]
Published online 10 June 2010 | Nature | doi:10.1038/news.2010.290
News
The Sun as comet snatcher
｛太陽、誘拐犯？｝
Most of the Solar System's comets may have been stolen from other stars.
｛太陽系の彗星のほとんどが、他の系外の星から侵入したものか？｝
By Lucas Laursen
{{Comets in the Sun's Oort cloud may be stolen goods.}
DR SETH SHOSTAK / SCIENCE PHOTO LIBRARY}
New simulations suggest that the Sun may have captured more than its fair share of comets from the primordial star-forming soup. The study, led by Harold Levison of the Southwest Research Institute in Boulder, Colorado, seeks to account for the abundance of comets in the outer reaches of the Solar System.
Our Solar System's comets spend most of their time between roughly 5,000 and 100,000 times further away from the Sun than the Earth, in a little-understood region beyond the planets known as the Oort cloud. Occasionally, some zip past the inner Solar System, and a rare few, such as Halley's Comet, return on a regular basis. But the origin of even the most well known comets is something of a mystery.
An influential model of how the Solar System formed predicts that around 6 billion comets in the Oort cloud are home-grown1. But some astronomers estimate that there are as many as 400 billion comets surrounding the Solar System — a discrepancy that researchers have struggled to explain.
Now the Levison study suggests that these mystery comets may actually have formed around other stars during the first moments of star formation. "Our Sun is a relatively heavy star," explains Ramon Brasser, a co-author of the study, which appears online in Science today2. When material such as gas, dust and ice began to find gravitational dancing partners, our Sun may have been massive enough to skim spare comets from its more lightweight neighbours.
Stolen goods
Levinson and colleagues are not the first to suggest that some comets may be from beyond the Solar System. A team examined the possibility in a 1990 study, but concluded that the Sun's pull was not sufficient to attract enough comets3. "They did not have the computing power to do the simulation we have done," Brasser says.
Brasser and his colleagues built a computer model in which many stars form near one another in a stellar cluster. In the simulation, each star spawns planetary objects including comets, some of which settle into orbits occupying an extended scattered disk around the star and others of which are ejected into the wider gas cloud enveloping the cluster. About 3 million years into the simulation, the gas surrounding the newly formed stars collapses into each solar system, and most of the free-floating comets find homes around one of the stars. During subsequent flybys with other stars, the simulation shows the Sun snags enough comets to account for its present collection.
However, running such a detailed simulation required the astronomers to make many assumptions about solar system formation, which introduce large uncertainties into the picture. "The most vulnerable assumption is that extended scattered disks would actually exist at the early time considered," says Hans Rickman of Uppsala Astronomical Observatory in Sweden. Extended scattered disks, a hypothetical home for comets in orbit around stars, have not been directly observed, nor do theorists agree on how they form, Rickman says. In one recent model, the disk does not form until 1 billion years after the beginning of a solar system.
Brasser says the simulation also had to assume that all stars have the same number of comets because solid numbers are not available. "People will have difficulty with this assumption," he admits. And for lack of more information, the authors write in the paper that they assumed that other solar systems have a distribution of large planets (whose mass influence cometary orbits) similar to our own.
Even the more widely accepted estimates of the number of comets in the Oort cloud could be too high, Rickman says, and a later formation of the cloud could make it easier to fill with home-grown comets. All the uncertainty, he says, "makes me think there does not have to be any problem at all".
References
1. Levison, H. F. & Duncan, M. J. Icarus 127, 13 (1997).
2. Levison, H. F. , Duncan, M. J. , Brasser, R. & Kaufmann, D. E. Science published online, doi: 10.1126/science.1187535 (10 June 2010).
3. Zheng, J-Q. , Valtonen, M. J. & Valtaoja, L. Celest. Mech. Dyn. Astr. 49, 265 (1990).
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