Scientists from NOAA's Mauna Loa Observatory in Hawaii announced Wednesday that this is the first September on record when carbon dioxide in the atmosphere has exceeded 400 parts per million, a critical metric for climate trackers.
Because plants across the Northern Hemisphere grow and absorb CO2 all summer long, early fall always has the lowest atmospheric carbon dioxide levels of the year. And this year it's a higher "lowest." So if we're measuring more than 400 parts per million in September, that means levels won't be lower at any other point this year — or ever again in our lifetime.
Here's why: Even if efforts to reduce emissions are so successful that we stop burning fossil fuels and put an end to deforestation (a scenario so unlikely nobody is proposing it), there is no feasible way of removing the huge amounts of carbon we've already released into the atmosphere. This means the 400 parts per million are here to stay, far into the foreseeable future.
Historical data shows just how fast this has happened. Scientists say we have to go back 2 million years to find similarly high concentrations of atmospheric CO2. For at least 400,000 years, levels fluctuated between 170 and 290 parts per million. Then, the Industrial Revolution happened, and since 1950 atmospheric carbon has shot up from about 310 parts per million to the over 400, level we see today. And it will only go higher, scientists predict.
While there might not be a huge difference between the effects of a carbon concentration of 399 parts per million vs. 401, the 400 parts per million mark has become an important symbolic threshold, and it's already been crossed multiple times this year. March, for example, was the first month where all daily measurements exceeded that number, and in May, scientists clocked the first 400 parts per million measurement ever in Antarctica, the place with the lowest carbon concentrations on Earth.
As NASA writes, we are crossing into the Anthropocene era — when the planet will be very different from the one our ancestors knew. Just how different we'll have to wait and see.