April 2014 was the first month in human history during which carbon dioxide in the air rose above an average 400 parts per million. The determination is based on atmospheric readings from an observatory in Mauna Loa, Hawaii. According to the Keeling Curve (named for the scientist who established the observatory), CO2 levels were around 200 ppm during ice ages, and during the warmer interglacial periods, the levels were around 280 ppm. Since the industrial revolution, heat-trapping carbon dioxide emissions have skyrocketed:

Looking ahead, if the rate of fossil-fuel burning continues to rise on a business-as-usual trajectory, such that humanity exhausts the reserves over the next few centuries, CO2 will continue to rise to levels of order 1500 ppm. The atmosphere will not return to pre-industrial levels even tens of thousands of years into the future.

Unless serious efforts are made to reduce the dependence on fossil fuels, it is clear that we are on a threshold of a new era of geologic history, one with climate very different from that of our ancestors. These curves not only demonstrate the seriousness of the global warming problem, but also illustrate the power of continuous time series to communicate and clarify the essential science.

"We're in new territory for human beings—it's been millions of years since there's been this much carbon in the atmosphere," says author Bill McKibben. "The only question now is whether the relentless rise in carbon can be matched by a relentless rise in the activism necessary to stop it." A great question, indeed.

The news coincides with the impending release of The National Climate Assessment, a 1,300-page report compiled by 300 leading scientists and experts, which flatly states that "climate change, once considered an issue for a distant future, has moved firmly into the present."

[Via Slate]