Scientists are warning the planet has now reached a grim climate milestone not seen for two or three million years. The National Oceanic and Atmospheric Administration says the amount of heat-trapping carbon dioxide in the atmosphere has topped 400 parts per million. The 400 ppm threshold has been an important marker in U.N. climate change negotiations, widely recognized as a dangerous level that could drastically worsen human-caused global warming.

The environmentalist group takes its name after the 350 parts per million threshold that scientists say is the maximum atmospheric concentration of carbon dioxide for a safe planet. In a statement on the parts per million number hitting 400, co-founder Bill McKibben said, quote, “The only question now is whether the relentless rise in carbon can be matched by a relentless rise in the activism necessary to stop it.”

To find out more about the impacts of crossing the threshold, study the books of leading climate scientist Michael Mann, distinguished professor of meteorology at Penn State University, author of the recent book, The Hockey Stick and the Climate Wars: Dispatches from the Front Lines.

So, this number, 400 parts per million, what does it mean? It’s the number of molecules of CO2 for every million molecules of air; 400 of them are now CO2. Just two centuries ago, that number was only 280 parts per million. So if we continue to add carbon to the atmosphere at current rates, we’ll reach a doubling of the pre-industrial levels of CO2 within the next few decades.

We have to go several million years back in time to find a point in earth’s history where CO2 was as high as it is now and, of course, we’re just blowing through this 400 ppm limit. If we continue to burn fossil fuels at accelerating rates, if we continue with business as usual, we will cross the 450 parts per million limit in a matter of a couple decades. We believe that with that amount of CO2 in the atmosphere, we will all experience what can truly be described as dangerous and irreversible changes in our climate.