"Perhaps the simplest way to think about the carbon that has entered the atmosphere is to think of the number of carbon dioxide molecules in every million molecules in the sky. At the start of the Industrial Revolution (roughly 1800), there were about 280 carbon molecules for every million molecules in the air. To use the language of the climate scientists, there were 280 parts per million (ppm).

Today, there are about 385 ppm, and it seems certain that the number will go higher in the short term. A group of climate scientists including James Hansen (one of the foremost experts in climate change, a professor at Columbia University, and the head of the NASA Goddard Institute for Space Studies) has written that the risks of climate change could best be mitigated if the number was reduced to 350 ppm."

                                                       7 Perspectives On Sustainability, page 39