This March, global levels of CO2 passed 400 parts per million.

Although short-term local measurements of 400 ppm have been recorded previously, this marks the first time since record keeping began that CO2 levels were above 400 ppm globally for a month.

Already we’re seeing the deadly effects of climate change in the form of rising seas, monster storms, wildfires, and extreme weather of all kinds. Passing 400 ppm is an ominous sign of what might come next — and it sends a clear signal that the world must act ahead of the UN climate talks in Paris this December.