According to recently released data from the National Oceanic and Atmospheric Administration (NOAA), the monthly average carbon dioxide concentration in the atmosphere now exceeds 400 parts per million (ppm). Although this isn't the first time that the threshold had been exceeded, with observational sites in the Arctic observing exceedances in the spring of 2012 and at NOAA's Mauna Loa Observatory in 2013, this is the first time that the average has exceeded 400 parts per million on a global basis. Carbon dioxide emissions are expected to stay above this threshold through at least May, when atmospheric concentrations of carbon dioxide typically peak.

Although the consequences of climate change are not expected to change dramatically now that carbon dioxide levels exceed 400 ppm, according to Dr. Pieter Tans, who leads NOAA's Carbon Cycle Greenhouse Gases Group, passing 400 ppm is a significant milestone as it illustrates how much human activity has contributed to carbon dioxide levels. The fact that this symbolic threshold has now been eclipsed may also bring additional pressure to bear on world leaders when they get together at the 2015 United Nations Climate Change Conference in Paris in December 2015.

Please click here to go to NOAA's Earth System Research Laboratory's website.