March 3, 2013–The science of 350: Scientists say that 350 parts per million CO2 in the atmosphere is the safe limit for humanity. We are already above the safe zone at our current 390ppm, and unless we rapidly return to below 350 ppm this century, we risk reaching tipping points and irreversible impacts such as the melting of the Greenland ice sheet and major methane releases from increased permafrost melt. Up until about 200 years ago our atmosphere contained about 275 parts per million of carbon dioxide, which gave human beings the environment necessary to maintain life. It provided us with just enough warmth on Earth–not too hot, not too cold! Parts per million (ppm) is just a way of measuring the concentration of different gases. It means the ratio of the number of carbon dioxide molecules to all of the molecules in the atmosphere.
Beginning in 18th century, people…