Carbon dioxide levels reach a new milestone

UP, UP and away. Parts of the planet have seen levels of carbon dioxide rise above 400 parts per million for the first time. Although it's largely symbolic, the milestone is a stark reminder of humanity's powerful influence on the atmosphere.

"During the month of April, the mean was over 400 ppm for the first time, throughout the Arctic," says Pieter Tans of the US National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colorado. CO₂ levels reach an annual peak around April as the gas is released by respiration, and then fall over the summer as plants suck it up. As a result, the 2012 average will be a little lower, at about 393 ppm.

Nevertheless, Tans says global levels will top 400 ppm in a few years.

The Arctic is not the only place seeing record levels. The Japan Meteorological Agency has reported levels above 400 ppm for both March and April at a monitoring station in Ofunato, according to local media.

Despite its psychological significance, there's nothing to suggest 400 ppm is a major threshold in the climate system, according to Tans. In fact, we don't know what a safe level of CO₂ would be.

The campaign group 350.org wants levels reduced to 350 ppm, but Tans says that is arbitrary. The safe level could be 380 ppm, or 320 ppm - we just don't know. As a result, any growth in CO₂ increases the risk of catastrophic climate change.

"We're playing a very dangerous game," Tans says.

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