甲烷的危机

Deep trouble.
Methane bubbles from a tar-covered oil seep in the Santa Barbara Channel.

背景资料

以往我们一听到温室效应，第一直觉都会联想到二氧化碳。但是越来越多的研究指出，甲烷对温室效应的影响，是超乎我们之前所想像的。加州大学专门研究古老海洋地质学家Tessa Hill等从观察圣巴巴拉市(Santa Barbara)周围海洋底部沉积物之后发现了这个问题。

从海床渗漏出来的石油可能会比人类使用内燃机所造成的气候变迁还要严重，因为这些石油含有丰富的温室气体－甲烷。他们发现当全球气候变迁的同时，海底石油和甲烷外泄的情形也跟着活跃起来。证据是大约在11,000 还有15,000年前的两次主要暖化时期，沙土中油溚的含量比平常还多了3倍之多。这个现象显示说在这个时期中，有3倍的石油从岩石的缝隙中外泄出来，同时也有3倍的甲烷跟着外泄出来到大气中。

甲烷除了是造成温室效应的气体之外，还要注意到它对大气所带来其它方面的危险。一般而言，像二氧化氮和一氧化碳这种有害气体，在大气中会和氢氧根结合而转换成其它的分子。但是如果在短时间内释放出太多的甲烷气体，会增加甲烷分子和这些有害气体结合的机会，而大大地减低了大气原本自行清除污染物的能力。

存在于海洋底层的可燃冰－水合甲烷，是天然气和水结合在一起的固体化合物，外形与冰相似。由于含有大量甲烷等可燃气体，因此极易燃烧。水合甲烷一方面是人类未来能源的新希望，但另一方面也可能为人类带来一些危机。大量从海底冒出的甲烷除了会启动全球气候暖化的机制之外，也能够摧毁原本稳定的海底地层，而在近海可能引起大陆架坍塌。因此，将来当人类计划开采这些可燃冰矿的同时，也必须考虑到水合甲烷可能带来的危险。

Marine Methane Heats Things Up

By Julie Rehmeyer
ScienceNOW Daily News
28 August 2006

Bedrock below the ocean bottom keeps a lid on oil reservoirs, but it's not an impermeable cap. Small cracks allow petroleum and methane to bubble to the surface. Once there, the petroleum oxidizes and turns to tar, which sinks. Meanwhile, the methane drifts into the atmosphere, where it makes up about 15% of the total amount of the gas sent skyward by natural sources such as wetlands and melting tundra. Humans contribute slightly more than all natural sources combined.

But does undersea methane make up a larger piece of the pie during periods of global warming? Paleo-oceanographers Tessa Hill of the University of California (UC), Davis, James Kennett of UC Santa Barbara, and collaborators attacked the question by looking at tar deposits from sediment cores taken off the Santa Barbara coast. They found 3 times more tar mixed into the sand from the last two major warming periods, 11,000 and 15,000 years ago, than was seen on average. This suggests that 3 times more oil was released from seeps during those periods, and 3 times more methane along with it, the team reports online this week in Proceeding of the National Academy of Sciences. The team suggests that global warming may have first melted undersea methane ice, disturbing the sea floor and opening new cracks in the oil reservoirs.

"This is a source of methane that we might have assumed in the past was stable," says Hill. "As it turns out, it's very sensitive to climate change. I would anticipate that it would be sensitive to climate change in the future as well." If methane was released similarly from all the other marine reservoirs worldwide, it would account for nearly half the increase in atmospheric methane during those warming periods, says Hill, who believes this methane may have driven further warming. The researchers acknowledge, however, that global warming would probably affect different petroleum deposits differently, so such a simultaneous release is unlikely; further research will be needed at other oil seeps around the world, they say.

The evidence from Santa Barbara is "beautiful," says Jérôme Chappellaz, an atmospheric scientist at the Laboratoire de Glaciologie et Géophysique de l'Environnement in St Martin d'Hères Cedex, France. But he cautions against extrapolating to all marine oil reservoirs around the world. Evidence from carbon testing of ice core samples points away from marine methane sources and toward wetlands and melting tundra, he says.