Carbon dioxide concentrations in the atmosphere topped 400 parts per million in the Northern Hemisphere for the entire month of April, according to the World Meteorological Organization.

It is the first time in modern times that such a level was reached for an entire month in an entire hemisphere.

Scientists say carbon dioxide levels are a leading cause of climate change, and the 400 ppm threshold is a highly symbolic number.

Carbon dioxide concentrations are believed to have been around 240 ppm at the dawn of the industrial revolution 200 years ago, according to the Intergovernmental Panel on Climate Change. The IPCC said in April that levels would have to stay below 450 ppm in order to avoid the most catastrophic impacts of climate change.

The 400 ppm-level was first reached two years ago in the Arctic, where it is now the norm. Outside the poles, the level was first reached, albeit briefly, a little over a year ago, at the Mauna Loa Observatory in Hawaii.

The WMO now says it expects average levels worldwide to rise above 400 ppm some time in the next two years and stay there.

Michel Jarraud, secretary general of the WMO, which is an agency of the United Nations, said the new benchmark should “serve as yet another wakeup call.”

United Nations climate talks restart June 4 in Bonn, with negotiations scheduled to help set the groundwork for a global agreement to confront climate change next year.


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