... And methane plays along too.

... and methane plays along too

Air bubbles from the Vostok ice core also reveal that methane levels have undergone extreme fluctuations over the last 160,000 years, report Dominique Raynard and colleagues from the Laboratory of Glaciology and Geophysics of the Environment in St. Martin d'Heres, France, and from the Soviet Union. During warm periods, methane builds to almost double its ice age level of 350 parts per billion by volume.

Methane levels during the last 300 years have risen at an exponential rate, from 700 to 1,700 parts per billion, most likely because of human activities. The Vostok record suggests that during the 160,000 years before humans had an impact, methane levels rose as quickly as they are now rising, nor did they reach such heights.

Both methane and carbon dioxide trap surface heat, and the researchers calculate that the buildup of these greenhouse gases could have caused about half the 4.5[degrees]C warming between the glacial and interglacial periods. "If that's really true, then it says the climate is very sensitive to the atmospheric chemistry," says Nicklas G. Pisias of Oregon State University in Corvallis. Indeed, on the basis of the ice age data, Raynaud's group estimates that a modern doubling of carbon dioxide will warm the world by a signiifcant 3.6[degrees]C to 4[degrees]C.