"The amount of carbon dioxide in the Earth's atmosphere has exceeded 402 parts per million (ppm) during the past two days of observations, which is higher than at any time in at least the past 800,000 years, according to readings from monitoring equipment on a mountaintop in Hawaii. Carbon dioxide, or CO2, is the most important long-lived greenhouse gas responsible for manmade global warming, and it is building up in the atmosphere due to the burning of fossil fuels such as coal, oil and natural gas.
Once emitted, a single molecule of carbon dioxide can remain aloft for hundreds of years, which means that the effects of today's industrial activities will be felt for the next several centuries, if not thousands of years. Carbon dioxide and other greenhouse gases, such as methane, warm the planet by absorbing and redirecting outgoing solar radiation that would otherwise escape back into space.
In 2013, atmospheric levels of carbon dioxide briefly hit 400 ppm for the first time in mid-May, but this year that symbolic threshold has been crossed even earlier. This means it is more likely that the annual peak, which typically occurs in mid-to-late May, will climb further above 400 ppm for the first time."