In Siberia, vast quantities of methane, an extremely potent greenhouse gas, are locked up in what used to be called “permafrost” – regions in which it was assumed that the ground was permanently frozen. But areas that used to be frozen are now thawing, releasing methane and thus contributing to further warming – and to further thawing, which releases more methane.
而另一个循环则昭示了类似的危险。在塞尔维亚，大量的沼气（一种对温室效应影响极大的气体）都被封存在曾被称为“永久冻土”的地区，因为人们相信这些地区的土地都将永久处于封冻状态。但如今这些土地开始解冻并释放出沼气，并因此而导致进一步暖化——以及进一步解冻和更多沼气的释放。