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 390ppm, 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.

There are three numbers you need to really understand global warming: 275, 400, and 350.

Since the beginning of human civilization up until about 200 years ago, our atmosphere contained about 275 parts per million of carbon dioxide. 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. 275 ppm CO₂ is a useful amount—without some CO₂ and other greenhouse gases that trap heat in our atmosphere, our planet would be too cold for humans to inhabit.

So we need some carbon in the atmosphere, but the question is how much?

Beginning in the 18th century, humans began to burn coal and gas and oil to produce energy and goods. The amount of carbon in the atmosphere began to rise, at first slowly and now more quickly. Many of the activities we do every day like turning the lights on, cooking food, or heating or cooling our homes rely on those fossil fuel energy sources that emit carbon dioxide and other heat-trapping gases into the atmosphere. We're taking millions of years worth of carbon, stored beneath the earth as fossil fuels, and releasing it into the atmosphere. By now—and this is the second number—the planet has about 400 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 CO₂concentration of 450-500 ppm. Along with increased intensity of extreme weather, such as hurricanes and blizzards, 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.

Propelled by the news of these accelerating impacts, some of the world's leading climate scientists have now revised the highest safe level of CO₂ to 350 parts per million. That's the last number you need to know, and the most important. It's the safety zone for planet earth. As James Hansen of America's National Aeronautics and Space Administration, the first scientist to warn about global warming more than two decades ago, wrote:

"If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO₂ will need to be reduced from its current 385 ppm to at most 350 ppm."

That will be a hard task, but not impossible. We need to stop taking carbon out of the ground and putting it into the air. Above all, that means we need to stop burning so much coal—and start using solar and wind energy and other such sources of renewable energy –while ensuring the Global South a fair chance to develop. If we do, then the earth’s soils and forests will slowly cycle some of that extra carbon out of the atmosphere, and eventually CO₂ concentrations will return to a safe level. By decreasing use of other fossil fuels, and improving agricultural and forestry practices around the world, scientists believe we could get back below 350 by mid-century. But the longer we remain in the danger zone—above 350—the more likely that we will see disastrous and irreversible climate impacts.

With your help, we can spread this important piece of information to our fellow citizens, communities, countries, and the world. For more in-depth information on climate science, policy, and solutions, please see our list of recommended resources below.

For more, read Bill McKibben's blog post, "The Science of 350, the Most Important Number on the Planet"