Read more here: Science of 350350 Science

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 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, 390, and 350.

For all of human history until about 200 years ago, our atmosphere contained 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 per million other 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 energy sources like coal and oil 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 390 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.

(Text from the 350 website)