Climate Central: Any day now, carbon dioxide levels in the atmosphere will reach their annual peak in a cycle driven by the collective inhale and exhale of the world’s plant life. But because of the extra CO2 pumped into the air by human activities, this year’s peak will be higher than last year’s, which was higher than the year before that — a sign of the unabated emissions that are driving the Earth’s temperature ever upward.

The amount of heat-trapping carbon dioxide in the atmosphere has been measured at the observatory atop Hawaii’s Mauna Loa volcano since 1958, producing a record that shows not only the yearly maximum and minimum driven by the spring bloom and fall dieback of plants, but also the steady climb in CO2 levels every year.

The last few years have seen a spate of atmospheric CO2 milestones in the Mauna Loa record: The first measurement of CO2 above 400 parts per million (ppm) in May 2013, the first month entirely above 400 ppm in April of last year, and this year will likely see several months with an average above that level.

While 400 ppm is something of a symbolic threshold, as the amount of extra heat trapped by it versus 399 ppm is minimal, it serves to show how far carbon dioxide in the atmosphere has risen from preindustrial levels of 280 ppm. Studies have estimated that CO2 levels on Earth haven’t been this high in at least 800,000 years. [Continue reading…]