• Critical Number of atmospheric concentration of CO2 (350  parts per million), that is required to not be crossed in the long term.

Two years ago, after leading climatologists observed rapid ice melt in the Arctic and other frightening signs of climate change, they issued a series of studies showing that the planet faced both human and natural disaster if atmospheric concentrations of CO2 remained above 350 parts per million.

Dr. James Hansen, of NASA, the United States' space agency, has been researching global warming longer than just about anyone else. He was the first to publicly testify before the U.S. Congress, in June of 1988, that global warming was real. He and his colleagues have used both real-world observation, computer simulation, and mountains of data about ancient climates to calculate what constitutes dangerous quantities of carbon in the atmosphere. The Bush Administration has tried to keep Hansen and his team from speaking publicly, but their analysis has been widely praised by other scientists, and by experts like Nobel Prize winner Al Gore.

Right now, mostly because we’ve burned so much fossil fuel,
the atmospheric concentration of CO2 is 390 ppm

What does it take to reach this goal ?

It means building solar arrays instead of coal plants, it means planting trees instead of clear-cutting rainforests, it means increasing efficiency and decreasing our waste to name some of the things that has to be done.