Support Our Work
Editorial viewpoints from Climate Central's writers and editors.

400 ppm: A Milestone that Means Everything, and Nothing

By Michael D. Lemonick

I’m not big on taking note of milestones. They’re artificial, and usually meaningless, but people get all worked up about them anyway. I don’t like to stay up late on New Year’s Eve, for example, because Dec. 31 is a purely arbitrary date. Nothing real actually begins the next day, but we all pretend otherwise. I have similar feelings about the first day of spring, the temperature reaching 100° as opposed to 99° and all sorts of other magic-sounding dates and numbers that don’t have any real significance.

But since no law says I have to be consistent, I’m going to take note of a milestone that happened some time in the past couple of months, and which was reported last week by NOAA. For the first time in recorded history, and almost certainly for much longer than that, the atmosphere’s concentration of carbon dioxide, or CO2, has nipped above 400 parts per million in at least one part of the world. Monitoring stations in Alaska, northern Europe, and Asia have all noted readings above that level during this past spring.

In one sense, this isn’t all that important. There’s no meaningful difference between 399 ppm and 400, and the current world average is more like 393. Even in the Arctic, scientists know the CO2 level will drop back below 400 this summer, as trees in the Northern Hemisphere suck carbon dioxide back out of the atmosphere (you can see the annual ups and downs as trees start growing in the spring and go into hibernation in the fall). We won’t get to a world average of 400 for several years yet.

Climate scientists generally agree with all that, but suggest that the 400 ppm milestone is important anyway, for symbolic reasons. “It's just a reminder to everybody that we haven't fixed this and we're still in trouble,” Jim Butler at NOAA’s Earth System Research Lab told the Associated Press.

I don’t know if the reminder will do much good, given the seeming indifference with which policymakers have responded to earlier CO2 milestones. I think reaching 400 parts per million anywhere in the world is crucially important for an entirely different reason: however much CO2  is in the atmosphere today is the minimum level we’re going to have to live with for the indefinite future. Once carbon dioxide is swirling around in the stratosphere above us, it will stay there for hundreds and hundreds of years. It’s as though you gained the most weight in your life, and knew you’d never weigh even a single pound less, ever.

CO2 does eventually get pulled back out of the atmosphere by natural processes, but that happens very slowly. Climate scientists like to compare the atmosphere to a bathtub half-full of water, with a very slow drain and a slowly trickling faucet. If the drain and the trickle are balanced, the water level never changes — just as the trickle of natural CO2 into the atmosphere and the drainage into trees, carbonate rocks and other places have been in balance for at least 2,000 years, and probably more. Atmospheric CO2  hovered at around 270-290 parts per million that whole time, and the climate stayed more or less stable.

Recent monthly mean CO2 measured at Mauna Loa Observatory in Hawaii. Credit: NOAA.

Since the wholesale burning of fossil fuels began with the Industrial Revolution, however, CO2 levels have been climbing. The faucet has been opened wider, but the drain is still very slow. And even if we manage to cut emissions significantly — something that’s not looking likely anytime soon — the faucet will still open wider than it was in pre-industrial times. CO2 levels will continue to climb, just not as fast.

So once we get to 400 (or 425 or 450 or 500 or whatever), that’s where we’ll be for the foreseeable future. The elevated temperatures caused by this crucial heat-trapping gas will be with us indefinitely as well. The world’s glaciers and ice caps will continue to melt — just think of the difference between putting a small block of ice in a hot oven for 10 seconds and putting it in there for an hour. The oceans will continue to warm. As a result of both the melting and warming, sea level will continue to rise. Scientists expect the oceans to be perhaps 3 feet higher by 2100, but it won’t stop then (which means, by the way, that 2100 is another meaningless milestone).

And all of that’s going to be true even if we cut back drastically on emissions. If we don’t, then every new CO2 milestone — 500 ppm, 800, 964, whatever number you choose — will be the new the level of climate-changing pollution the planet will have to deal with at the very minimum from that time onward.

Maybe that’s another reason for me to ignore milestones from now on. They’re either meaningless, or highly depressing. Or, as in this case, both at once.

« Commentary

« Commentary


My Chernobyl In November 1995 Climate Central's Shari Bell traveled to Chernobyl in the Ukrainian republic. Here's her story in photos.

View Gallery