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地球暖化科學研究報告
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地球有碳循環，那有甲烷循環嗎？
地球的循環系統自然會平衡碳濃度，只要我們停止傷害海洋並復育海洋、停止濫墾森林並種樹、停止排放二氧化碳，地球媽媽自然會調整循環系統，恢復整個地球生態的碳中和，那麼甲烷有沒有循環系統？什麼東西會吸收甲烷呢？會不會甲烷停在空中很久很久都不會消失？
地球：我們可愛的家 環境物理學教授尼古拉斯葛魯柏：討論地球的自然循環（一）
http://www.suprememastertv.com/tw/bbs/board.php?bo_table=download_tw&wr_id=3930&goto_url=&url=link2
地球：我們可愛的家 環境物理學教授尼古拉斯葛魯柏：討論地球的自然循環（二）
http://www.suprememastertv.com/tw/bbs/board.php?bo_table=download_tw&wr_id=3959&goto_url=&url=link2
甲烷從海底永凍層帶冒出？
Methane Bubbling Up From Undersea Permafrost?
Mason Inman in San Francisco, California for National Geographic News
December 19, 2008
The East Siberian Sea is bubbling with methane, a powerful greenhouse gas, being released from underwater reserves, according to a recent expedition.
東西伯利亞海正在冒出甲烷﹣一種強效的溫室氣體，正從海底貯存庫釋出，根據探險隊所述。
This could be a sign that global warming is thawing underwater permafrost, which is releasing methane that has been locked away for many thousands of years.
這可能是全球暖化正在融解海底永凍層的徵候﹣釋放已封存數千年之久的甲烷。
If these methane emissions from the Arctic speed up, it could cause "really serious climate consequences," said expedition member Igor Semiletov of the University of Alaska, Fairbanks.
如果這些來自北極的甲烷加快排放的話，它可能造成“非常嚴重的氣候變化”，一個探險員如是說。
(Related: "Global Warming Feedback Loop Caused by Methane, Scientists Say" [August 29, 2006].)
Semiletov and colleagues have traveled along the Siberian coast—this year they covered 13,000 miles (22,000 kilometers)—while monitoring methane concentrations in the air and observing the seas.
"According to our data, more than 50 percent of the Arctic Siberian shelf is serving as a source of methane to the atmosphere," Semiletov said.
This vast shelf is about 750,000 square miles (2 million square kilometers)—about the same size as Greenland or Mexico—and about 80 percent of it is covered with permafrost, Semiletov said.
He presented the findings from his group at an American Geophysical Union meeting in San Francisco this week.
Not-So-Permanent Permafrost
Permafrost is basically dirt that's been permanently frozen for hundreds or thousands of years, much of it since the last ice age.
Sea levels back then near the Siberian coast were about 325 feet (100 meters) lower than today, and the exposed ground froze solid down to 1,600 to 2,300 feet (500 to 700 meters) deep.
Over the past 10,000 years, sea levels rose to cover some of this permafrost, and in recent years those seas have seen increases in average temperatures.
"As a result, sub-sea permafrost has warmed up to minus 1 degree Celsius [30 degrees Fahrenheit]," Semiletov said. "It's very, very close to the thawing point."
Underneath the permafrost are stores of methane, the same as the natural gas people use for cooking and heating.
There are also methane hydrates, a solid that forms when methane and water mix in cold temperatures. The hydrates release gas as they warm.
"It was assumed that these stores of methane have not been leaking, because the sub-sea permafrost served as a lid keeping hydrates and natural gas in place," Semiletov said.
But now global warming may be starting to release these stores of methane into the atmosphere.
Drastic Increase
Regions farther from the Equator generally are experiencing more warming, and the Arctic is warming fastest of all.
"Springtime air temperatures on the East Siberian Arctic shelf [have] increased up to 5 degrees Celsius [9 degrees Fahrenheit]," Semiletov said. "It's a hot spot."
In comparison, the world as a whole has warmed about 1.25 degrees Fahrenheit (0.7 degrees Celsius) since pre-industrial times.
If abrupt methane release became widespread, it could create a feedback loop that would lead to even more drastic global warming.
"Our early observations in 1994 to 1999 didn't reveal a widespread enhanced dissolved methane concentration" along the Siberian coast, Semiletov said.
"With this newly obtained data, we suggest an increase of methane release from the East Siberian Arctic shelf," he said.
"We have obtained a drastic increase of air methane in some sites—sometimes up to four times higher than the background [global average]."
Vladimir Romanovsky, a permafrost expert also at the University of Alaska, Fairbanks, says the study is worrying.
"It has very serious implications for changes in greenhouse gases," Romanovsky said, adding that the releases described should be monitored more closely.
"It could be very important, but we still need some numbers to see how big [of a problem] it is."
Carolyn Ruppel is a geophysicist with the U.S. Geological Survey who is based in Woods Hole, Massachusetts.
"The nature of the shallow Arctic continental shelves means that they have the potential to release significant methane now and into the future," said Ruppel, who was not involved in the study.
But there are a number of places this methane could be coming from, she said.
"Quantifying the contribution of each potential methane source will be a major challenge."
http://news.nationalgeographic.com/news/2008/12/081219-methane-siberia.html?source=rss
Global methane levels move upward again
Dani Cooper ABC 16 December 2008
Methane levels in the atmosphere have started to rise after almost eight years of near-zero growth, an international study says.
And the increase may lead to an acceleration of global warming, the scientists behind the study warn.
Dr Paul Fraser of the CSIRO, who co-authored the study that appears in Geophysical Research Letters, says samples taken from 12 stations across a global network showed a trend upwards in methane levels.
"After seven years [of zero growth] methane has started to rise again to growth rates of the early 1990s," Fraser says.
The chief research scientist in the Division of Marine and Atmospheric Research says the increase in methane could lead to an acceleration in global warming.
"If methane concentrations continue to grow at the current rate then it will be once again the second-most important greenhouse gas to control after CO2 over the next few decades," he says.
Fraser says methane accounts for about 20% of all greenhouse gases since the industrial revolution.
More potent
It is 20 times more potent than carbon dioxide as a greenhouse gas and comes from sources such as natural wetlands, rice fields, livestock, fires, coal mines and natural gas reticulation.
Emissions have been balanced over the past decade by natural sinks that absorb the gas and through oxidation into the atmosphere.
Fraser says sources of methane have been growing with drainage of tropical wetlands for agricultural use and increased fossil fuel use.
However, the destruction of natural tropical wetlands removes an important sink from the delicate equation.
"[The result is] in the past year the total sources have overwhelmed the total sinks, and methane has again started to rise," Fraser says.
This imbalance has resulted in several million tonnes of extra methane in the atmosphere, the paper says.
The study, which also involved the Massachusetts Institute of Technology, Scripps Institution of Oceanography and the University of Bristol, says the rise in methane concentrations is due in part to increased methane releases in the high latitudes of the Northern Hemisphere.
Fraser says this could be a one-off anomaly due to a very hot Northern Hemisphere summer in 2007 that would have warmed the wetlands of the tundra increasing methane production.
However if it is a long-term trend then it is a worrying development.
Surprisingly the researchers found the methane levels rose simultaneously at all measurement locations across the globe.
The researchers say further research is needed to understand why this happened.
The Intergovernmental Panel on Climate Change has identified the need to understand causes of the variations of methane growth rates as a priority area of research.
Fraser says scientists have only a basic understanding of methane variations and continuing high-quality observations are required particularly in tropical and boreal locations.
http://www.abc.net.au/science/articles/2008/12/16/2447483.htm?site=science&topic=latest
Methane, Potent Greenhouse Gas, Flowing Into The Atmosphere From Tundra Much Faster Than Expected
ScienceDaily (Dec. 11, 2008) — Much more methane gas is being emitted into the atmosphere from the tundra in northeast Greenland than previous studies have shown. New figures reveal that large amounts of greenhouse gases are being emitted into the atmosphere, not just during the warm summer months, but also during the colder autumn months.
更多來自格陵蘭東北方的凍原地帶的甲烷正排放到大氣層，且超過先前研究所示，新圖表顯示大量溫室氣體正排放到大氣層，不只是在暖和的夏天月，也在冰冷秋冬月期排放。
Naturally, this raises new questions concerning our understanding of the Earth’s climate system. Scientists at the University of Copenhagen in collaboration with scientists from Lund University in Sweden and the National Environmental Research Institute (DMU), Aarhus University, recently presented new and surprising figures in the scientific journal, Nature.
Methane is a highly efficient greenhouse gas, and the majority of methane in the atmosphere stems from emissions from tundra areas around the world, a significant part of which come from the polar areas. So far, scientists have believed that the tundra emits most of methane gas into the atmosphere during the warm months. However, new results show that nature has a trick up its sleeve. Scientists have learned that the onset of freezing during the long autumn months also forces enormous amounts of greenhouses gases out of the tundra. The readings were made at the Zackenberg research station in northeast Greenland.
“Actually, methane emissions in September and October 2007 matched the total methane emissions during the three warm summer months,” says Charlotte Sigsgaard, research assistant at the Department of Geography & Geology, University of Copenhagen.
The figures are very surprising, and our scientists were pleased that the extended measuring period, which was enabled by the International Polar Year in 2007, has offered new input for our general understanding of the climate system and, in particular, the drastic climate changes in the Arctic regions.
“Monitoring in -20°C can be somewhat problematic, but in this case it was fantastic and quite surprising to monitor how methane emissions from the tundra suddenly increased drastically in connection with the onset of freezing at Zackenberg,” says Charlotte Sigsgaard and adds that the new figures emphasise the importance of intensifying monitoring activities in the high arctic regions.
Charlotte Sigsgaard spent a couple of cold months last autumn in the field at Zackenberg, daily monitoring, operating and using the equipment for collecting samples from the tundra. Her work may now have given science new and previously unknown pieces for the big climate puzzle.
http://www.sciencedaily.com/releases/2008/12/081210133814.htm
Rising methane levels are cause for concern
Unexplained methane rises raise concerns that our understanding of its role in the global carbon cycle is inadequate, writes Chris Goodall.
對無法理解為何甲烷會上升的關心﹣瞭解它在全球碳循環中所扮演的角色是不及格的。
From Carbon Commentary, part of the Guardian Environment Network
10 November 2008
We seem to know less about methane emissions than we thought. After a decade of stability, methane concentrations in the atmosphere have been rising strongly in the last 18 months.
Early research work suggested that this rise was concentrated in the northern latitudes of the northern hemisphere and was consistent with greater emissions from decaying organic matter in melting permafrost or from the melting of Arctic sea ice.
Now this result has been called into question by the publication of a new study showing the concentrations of methane are rising almost everywhere. Since methane takes some time to diffuse around the globe, the later work suggests that the rise in methane may not be directly due to enhanced emissions from biological sources. ．．．新公佈的研究顯示甲烷濃度幾乎到處都在上升。
Methane is the second most important greenhouse gas, producing about 20% of the radiative forcing of all the main gases. Its concentration in the atmosphere has risen about two and half times since the industrial revolution to about 1750 parts per billion.
Although it is present in very much smaller concentrations than CO2, each molecule has a more powerful global warming effect. It also lasts much less long in the atmosphere, typically reacting with the hydroxyl radical (*OH) to form carbon dioxide and water. The average life of a molecule of methane in the atmosphere is about 8 years compared to about a century for carbon dioxide.
甲烷分子在大氣層平均生命約８年．．．。
The growth rate of methane concentrations in the atmosphere slowed in the second half of the last century. The period between 1999 and early 2007 showed virtually no increase, leading to optimism that methane emissions were under control.
Deep coal mines were an important source and much of the industry was closed down in the northern hemisphere in the 1980s and 1990s. Rice farming practices, which floods vegetated areas, allowing plant matter to rot anaerobically and produce methane, were changed in some parts of Asia to reduce emissions. Natural gas is largely methane and pipeline leaks were also reduced.
The recent rise surprised many researchers. In the past, methane concentrations have tended to increase in periods of marked El Niño (high sea temperatures in the eastern Pacific), resulting in greater dieback of vegetation and more methane production from the rotting plant matter.
But the last year or so has been a period of lower than average temperatures in the Pacific (La Niña rather than El Niño). So this isn't a good explanation for the sudden jump of about 10 parts per billion, or about 0.5% increase in the average concentrations.
Dr Rebecca Fisher of Royal Holloway College, University of London, published work with colleagues early this year showing that the rise in methane was particularly great in the Arctic. Some measuring stations saw increases of twice the average global rise. Since methane takes time to diffuse around the world, this suggested a regionally specific source. It could be from the sea or from rotting vegetation exposed by melting.
Dr Fisher's work was, in a sense, comforting. It suggested we might have an explanation for why methane concentrations were rising. In particular, she showed that increased methane concentrations were associated with rises in the percentage of gas containing the lighter carbon isotope, C12, which is associated with emissions from methane-producing bacteria. It looked as though we could be reasonably confident that at least part of the source of increased emissions was rotting plant matter.
More recent work has dented this belief. MIT scientists have just published work with scientists from Australia and elsewhere that shows that the rise in methane levels has been quite uniform across the globe. This shouldn't happen if methane was produced by plant sources, since there is far more organic matter in the northern hemisphere. Concentrations should be temporarily higher in the north in the months and years it takes methane to spread uniformly across the globe.
The MIT team speculate that the rise in methane may be a function of decreasing concentrations of hydroxyl, the scavenger radical that mops up methane, perhaps as well as increasing emissions. But we don't yet have good monitoring of *OH concentrations and it will be some time before we are able to tell if this hypothesis is correct or, indeed, what is causing this change.
The scientific debate about the cause of increased methane is important because it suggests that we do not yet have a good model for what determines changes in concentrations.
One of the primary worries about global warming is that it will eventually trigger the eruption of untold millions of tonnes of methane from deep sea water. (This is usually known as the 'clathrate gun' hypothesis.) The gas is currently locked into a stable bond with the extremely cold waters in the deep oceans.
Continued world temperature increases will eventually cause the methane to burst from its chemical locks within the cold liquid and rise to the surface. This probably happened at times of rapid warming in the far-distant past.
The fact that we cannot immediately know today why the methane rise is occurring, and whether it is likely to continue, raises worries that our understanding of methane's role in the global carbon cycle is simply not very good.
http://www.guardian.co.uk/environment/2008/nov/10/network-climate-change
地球的循環系統自然會平衡碳濃度，只要我們停止傷害海洋並復育海洋、停止濫墾森林並種樹、停止排放二氧化碳，地球媽媽自然會調整循環系統，恢復整個地球生態的碳中和，那麼甲烷有沒有循環系統？什麼東西會吸收甲烷呢？會不會甲烷停在空中很久很久都不會消失？
http://www.suprememastertv.com/tw/bbs/board.php?bo_table=download_tw&wr_id=3930&goto_url=&url=link2
地球：我們可愛的家 環境物理學教授尼古拉斯葛魯柏：討論地球的自然循環（二）
http://www.suprememastertv.com/tw/bbs/board.php?bo_table=download_tw&wr_id=3959&goto_url=&url=link2
甲烷從海底永凍層帶冒出？
Methane Bubbling Up From Undersea Permafrost?
Mason Inman in San Francisco, California for National Geographic News
December 19, 2008
The East Siberian Sea is bubbling with methane, a powerful greenhouse gas, being released from underwater reserves, according to a recent expedition.
東西伯利亞海正在冒出甲烷﹣一種強效的溫室氣體，正從海底貯存庫釋出，根據探險隊所述。
This could be a sign that global warming is thawing underwater permafrost, which is releasing methane that has been locked away for many thousands of years.
這可能是全球暖化正在融解海底永凍層的徵候﹣釋放已封存數千年之久的甲烷。
If these methane emissions from the Arctic speed up, it could cause "really serious climate consequences," said expedition member Igor Semiletov of the University of Alaska, Fairbanks.
如果這些來自北極的甲烷加快排放的話，它可能造成“非常嚴重的氣候變化”，一個探險員如是說。
(Related: "Global Warming Feedback Loop Caused by Methane, Scientists Say" [August 29, 2006].)
 |
The sea is also bubbling with methane, a powerful greenhouse gas, being released from underwater reserves, a Russian team announced during a December 2008 meeting. The discovery could be a sign that global warming is thawing underwater permafrost, which is releasing methane that has been locked away for many thousands of years. Images courtesy NASA |
"According to our data, more than 50 percent of the Arctic Siberian shelf is serving as a source of methane to the atmosphere," Semiletov said.
This vast shelf is about 750,000 square miles (2 million square kilometers)—about the same size as Greenland or Mexico—and about 80 percent of it is covered with permafrost, Semiletov said.
He presented the findings from his group at an American Geophysical Union meeting in San Francisco this week.
Not-So-Permanent Permafrost
Permafrost is basically dirt that's been permanently frozen for hundreds or thousands of years, much of it since the last ice age.
Sea levels back then near the Siberian coast were about 325 feet (100 meters) lower than today, and the exposed ground froze solid down to 1,600 to 2,300 feet (500 to 700 meters) deep.
Over the past 10,000 years, sea levels rose to cover some of this permafrost, and in recent years those seas have seen increases in average temperatures.
"As a result, sub-sea permafrost has warmed up to minus 1 degree Celsius [30 degrees Fahrenheit]," Semiletov said. "It's very, very close to the thawing point."
Underneath the permafrost are stores of methane, the same as the natural gas people use for cooking and heating.
There are also methane hydrates, a solid that forms when methane and water mix in cold temperatures. The hydrates release gas as they warm.
"It was assumed that these stores of methane have not been leaking, because the sub-sea permafrost served as a lid keeping hydrates and natural gas in place," Semiletov said.
But now global warming may be starting to release these stores of methane into the atmosphere.
Drastic Increase
Regions farther from the Equator generally are experiencing more warming, and the Arctic is warming fastest of all.
"Springtime air temperatures on the East Siberian Arctic shelf [have] increased up to 5 degrees Celsius [9 degrees Fahrenheit]," Semiletov said. "It's a hot spot."
In comparison, the world as a whole has warmed about 1.25 degrees Fahrenheit (0.7 degrees Celsius) since pre-industrial times.
If abrupt methane release became widespread, it could create a feedback loop that would lead to even more drastic global warming.
"Our early observations in 1994 to 1999 didn't reveal a widespread enhanced dissolved methane concentration" along the Siberian coast, Semiletov said.
"With this newly obtained data, we suggest an increase of methane release from the East Siberian Arctic shelf," he said.
"We have obtained a drastic increase of air methane in some sites—sometimes up to four times higher than the background [global average]."
Vladimir Romanovsky, a permafrost expert also at the University of Alaska, Fairbanks, says the study is worrying.
"It has very serious implications for changes in greenhouse gases," Romanovsky said, adding that the releases described should be monitored more closely.
"It could be very important, but we still need some numbers to see how big [of a problem] it is."
Carolyn Ruppel is a geophysicist with the U.S. Geological Survey who is based in Woods Hole, Massachusetts.
"The nature of the shallow Arctic continental shelves means that they have the potential to release significant methane now and into the future," said Ruppel, who was not involved in the study.
But there are a number of places this methane could be coming from, she said.
"Quantifying the contribution of each potential methane source will be a major challenge."
http://news.nationalgeographic.com/news/2008/12/081219-methane-siberia.html?source=rss
Global methane levels move upward again
Dani Cooper ABC 16 December 2008
 |
| Researchers believe that recent methane increases are in part due to releases in the high latitudes of the Northern Hemisphere (Source: iStockphoto) |
And the increase may lead to an acceleration of global warming, the scientists behind the study warn.
Dr Paul Fraser of the CSIRO, who co-authored the study that appears in Geophysical Research Letters, says samples taken from 12 stations across a global network showed a trend upwards in methane levels.
"After seven years [of zero growth] methane has started to rise again to growth rates of the early 1990s," Fraser says.
The chief research scientist in the Division of Marine and Atmospheric Research says the increase in methane could lead to an acceleration in global warming.
"If methane concentrations continue to grow at the current rate then it will be once again the second-most important greenhouse gas to control after CO2 over the next few decades," he says.
Fraser says methane accounts for about 20% of all greenhouse gases since the industrial revolution.
More potent
It is 20 times more potent than carbon dioxide as a greenhouse gas and comes from sources such as natural wetlands, rice fields, livestock, fires, coal mines and natural gas reticulation.
Emissions have been balanced over the past decade by natural sinks that absorb the gas and through oxidation into the atmosphere.
Fraser says sources of methane have been growing with drainage of tropical wetlands for agricultural use and increased fossil fuel use.
However, the destruction of natural tropical wetlands removes an important sink from the delicate equation.
"[The result is] in the past year the total sources have overwhelmed the total sinks, and methane has again started to rise," Fraser says.
This imbalance has resulted in several million tonnes of extra methane in the atmosphere, the paper says.
The study, which also involved the Massachusetts Institute of Technology, Scripps Institution of Oceanography and the University of Bristol, says the rise in methane concentrations is due in part to increased methane releases in the high latitudes of the Northern Hemisphere.
Fraser says this could be a one-off anomaly due to a very hot Northern Hemisphere summer in 2007 that would have warmed the wetlands of the tundra increasing methane production.
However if it is a long-term trend then it is a worrying development.
Surprisingly the researchers found the methane levels rose simultaneously at all measurement locations across the globe.
The researchers say further research is needed to understand why this happened.
The Intergovernmental Panel on Climate Change has identified the need to understand causes of the variations of methane growth rates as a priority area of research.
Fraser says scientists have only a basic understanding of methane variations and continuing high-quality observations are required particularly in tropical and boreal locations.
http://www.abc.net.au/science/articles/2008/12/16/2447483.htm?site=science&topic=latest
Methane, Potent Greenhouse Gas, Flowing Into The Atmosphere From Tundra Much Faster Than Expected
ScienceDaily (Dec. 11, 2008) — Much more methane gas is being emitted into the atmosphere from the tundra in northeast Greenland than previous studies have shown. New figures reveal that large amounts of greenhouse gases are being emitted into the atmosphere, not just during the warm summer months, but also during the colder autumn months.
更多來自格陵蘭東北方的凍原地帶的甲烷正排放到大氣層，且超過先前研究所示，新圖表顯示大量溫室氣體正排放到大氣層，不只是在暖和的夏天月，也在冰冷秋冬月期排放。
Naturally, this raises new questions concerning our understanding of the Earth’s climate system. Scientists at the University of Copenhagen in collaboration with scientists from Lund University in Sweden and the National Environmental Research Institute (DMU), Aarhus University, recently presented new and surprising figures in the scientific journal, Nature.
Methane is a highly efficient greenhouse gas, and the majority of methane in the atmosphere stems from emissions from tundra areas around the world, a significant part of which come from the polar areas. So far, scientists have believed that the tundra emits most of methane gas into the atmosphere during the warm months. However, new results show that nature has a trick up its sleeve. Scientists have learned that the onset of freezing during the long autumn months also forces enormous amounts of greenhouses gases out of the tundra. The readings were made at the Zackenberg research station in northeast Greenland.
“Actually, methane emissions in September and October 2007 matched the total methane emissions during the three warm summer months,” says Charlotte Sigsgaard, research assistant at the Department of Geography & Geology, University of Copenhagen.
The figures are very surprising, and our scientists were pleased that the extended measuring period, which was enabled by the International Polar Year in 2007, has offered new input for our general understanding of the climate system and, in particular, the drastic climate changes in the Arctic regions.
“Monitoring in -20°C can be somewhat problematic, but in this case it was fantastic and quite surprising to monitor how methane emissions from the tundra suddenly increased drastically in connection with the onset of freezing at Zackenberg,” says Charlotte Sigsgaard and adds that the new figures emphasise the importance of intensifying monitoring activities in the high arctic regions.
Charlotte Sigsgaard spent a couple of cold months last autumn in the field at Zackenberg, daily monitoring, operating and using the equipment for collecting samples from the tundra. Her work may now have given science new and previously unknown pieces for the big climate puzzle.
http://www.sciencedaily.com/releases/2008/12/081210133814.htm
Rising methane levels are cause for concern
Unexplained methane rises raise concerns that our understanding of its role in the global carbon cycle is inadequate, writes Chris Goodall.
對無法理解為何甲烷會上升的關心﹣瞭解它在全球碳循環中所扮演的角色是不及格的。
From Carbon Commentary, part of the Guardian Environment Network
10 November 2008
 |
| Methane concentrations at the Mauna Loa observatory in Hawaii. The grey data points are preliminary. Graphic: NOAA. |
Early research work suggested that this rise was concentrated in the northern latitudes of the northern hemisphere and was consistent with greater emissions from decaying organic matter in melting permafrost or from the melting of Arctic sea ice.
Now this result has been called into question by the publication of a new study showing the concentrations of methane are rising almost everywhere. Since methane takes some time to diffuse around the globe, the later work suggests that the rise in methane may not be directly due to enhanced emissions from biological sources. ．．．新公佈的研究顯示甲烷濃度幾乎到處都在上升。
Methane is the second most important greenhouse gas, producing about 20% of the radiative forcing of all the main gases. Its concentration in the atmosphere has risen about two and half times since the industrial revolution to about 1750 parts per billion.
Although it is present in very much smaller concentrations than CO2, each molecule has a more powerful global warming effect. It also lasts much less long in the atmosphere, typically reacting with the hydroxyl radical (*OH) to form carbon dioxide and water. The average life of a molecule of methane in the atmosphere is about 8 years compared to about a century for carbon dioxide.
甲烷分子在大氣層平均生命約８年．．．。
The growth rate of methane concentrations in the atmosphere slowed in the second half of the last century. The period between 1999 and early 2007 showed virtually no increase, leading to optimism that methane emissions were under control.
Deep coal mines were an important source and much of the industry was closed down in the northern hemisphere in the 1980s and 1990s. Rice farming practices, which floods vegetated areas, allowing plant matter to rot anaerobically and produce methane, were changed in some parts of Asia to reduce emissions. Natural gas is largely methane and pipeline leaks were also reduced.
The recent rise surprised many researchers. In the past, methane concentrations have tended to increase in periods of marked El Niño (high sea temperatures in the eastern Pacific), resulting in greater dieback of vegetation and more methane production from the rotting plant matter.
But the last year or so has been a period of lower than average temperatures in the Pacific (La Niña rather than El Niño). So this isn't a good explanation for the sudden jump of about 10 parts per billion, or about 0.5% increase in the average concentrations.
Dr Rebecca Fisher of Royal Holloway College, University of London, published work with colleagues early this year showing that the rise in methane was particularly great in the Arctic. Some measuring stations saw increases of twice the average global rise. Since methane takes time to diffuse around the world, this suggested a regionally specific source. It could be from the sea or from rotting vegetation exposed by melting.
Dr Fisher's work was, in a sense, comforting. It suggested we might have an explanation for why methane concentrations were rising. In particular, she showed that increased methane concentrations were associated with rises in the percentage of gas containing the lighter carbon isotope, C12, which is associated with emissions from methane-producing bacteria. It looked as though we could be reasonably confident that at least part of the source of increased emissions was rotting plant matter.
More recent work has dented this belief. MIT scientists have just published work with scientists from Australia and elsewhere that shows that the rise in methane levels has been quite uniform across the globe. This shouldn't happen if methane was produced by plant sources, since there is far more organic matter in the northern hemisphere. Concentrations should be temporarily higher in the north in the months and years it takes methane to spread uniformly across the globe.
The MIT team speculate that the rise in methane may be a function of decreasing concentrations of hydroxyl, the scavenger radical that mops up methane, perhaps as well as increasing emissions. But we don't yet have good monitoring of *OH concentrations and it will be some time before we are able to tell if this hypothesis is correct or, indeed, what is causing this change.
The scientific debate about the cause of increased methane is important because it suggests that we do not yet have a good model for what determines changes in concentrations.
One of the primary worries about global warming is that it will eventually trigger the eruption of untold millions of tonnes of methane from deep sea water. (This is usually known as the 'clathrate gun' hypothesis.) The gas is currently locked into a stable bond with the extremely cold waters in the deep oceans.
Continued world temperature increases will eventually cause the methane to burst from its chemical locks within the cold liquid and rise to the surface. This probably happened at times of rapid warming in the far-distant past.
The fact that we cannot immediately know today why the methane rise is occurring, and whether it is likely to continue, raises worries that our understanding of methane's role in the global carbon cycle is simply not very good.
http://www.guardian.co.uk/environment/2008/nov/10/network-climate-change
