Earlier this year, the concentration of carbon dioxide in the atmosphere reached 400 parts per million (ppm). The last time there was that much CO₂ in our atmosphere was three million years ago, when sea levels were 24 meters higher than they are today. Now sea levels are rising again.
Last September, Arctic sea ice covered the smallest area ever recorded. All but one of the 10 warmest years since 1880, when global records began to be kept, have occurred in the 21st century.
Some climate scientists believe that 400 ppm of CO₂ in the atmosphere is already enough to take us past the tipping point at which we risk a climate catastrophe that will turn billions of people into refugees. They say that we need to get the amount of atmospheric CO₂ back down to 350 ppm. That figure lies behind the name taken by 350.org, a grassroots movement with volunteers in 188 countries trying to solve the problem of climate change.