350 parts per million is what many scientists, climate experts, and progressive national governments are now saying is the safe upper limit for CO2 in our atmosphere. Accelerating arctic warming and other early climate impacts have led scientists to conclude that we are already above the safe zone at our current 392ppm, and that unless we are able to 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.Parts per million is simply a way of measuring the concentration of different gases, and means the ratio of the number of carbon dioxide molecules to all of the molecules in the atmosphere. the planet has 392 parts per million CO2 – and this number is rising by about 2 parts per million every year.
Scientists are now saying that's too much – that number is higher than any time seen in the recorded history of our planet—and we're already beginning to see disastrous impacts on people and places all over the world. Glaciers everywhere are melting and disappearing fast—and they are a source of drinking water for hundreds of millions of people. Mosquitoes, who like a warmer world, are spreading into lots of new places, and bringing malaria and dengue fever with them. Drought is becoming much more common, making food harder to grow in many places. Sea levels have begun to rise, and scientists warn that they could go up as much as several meters this century. If that happens, many of the world's cities, island nations, and farmland will be underwater. The oceans are growing more acidic because of the CO2 they are absorbing, which makes it harder for animals like corals and clams to build and maintain their shells and skeletons. Coral reefs could start dissolving at an atmospheric CO2 concentration of 450-500 ppm. These impacts are combining to exacerbate conflicts and security issues in already resource-strapped regions.
The Arctic is sending us perhaps the clearest message that climate change is occurring much more rapidly than scientists previously thought. In the summer of 2007, sea ice was roughly 39% below the summer average for 1979-2000, a loss of area equal to nearly five United Kingdoms.
Finally, the world's leading climate scientists have now revised the highest safe level of CO2 to 350 parts per million.