On Monday (26th of May 2014) the World Meteorological Association announced in a press release that Carbon Dioxide levels in the earths atmosphere passed the important barrier of 400 parts per million. In a press release they stated,
“For the first time, monthly concentrations of carbon dioxide (CO2) in the atmosphere topped 400 parts per million (ppm) in April throughout the northern hemisphere. This threshold is of symbolic and scientific significance and reinforces evidence that the burning of fossil fuels and other human activities are responsible for the continuing increase in heat-trapping greenhouse gases warming our planet.”
You can find the full press release here.