400. That is the symbolic milestone of concentration of CO2 in the atmosphere in parts per million. It is increasing at the rate of 2.5 PPM per year and seems to accelerate. Not in at least one million years has the planet seen such high concentration of carbon dioxide and other heat trapping gases in the atmosphere.
If you wonder what it means, this is the most direct threat to humanity and most living species that have thrived on this planet for the last 65 million years.
We cannot grow in perpetuity in a finite environment. It is time to have less and do more, and do more intelligently with what we have at our disposal.
Read more here: http://www.cnn.com/2013/05/10/us/climate-change/index.html