Scientists express this value not as a percentage but in another, sciency way. It's 400 parts of CO2 for every million parts of air, or 400 ppm. Divide up any volume of air into a million parts, and 400 of those parts should be CO2.
That doesn't seem like much, but a tiny bit of CO2 goes a long way.
Here are four things to keep in mind as our atmosphere enters the 400 Club:
1. Don't panic
"Isn't 400 the line we're not supposed to cross?" was a common question during the last week.
Climate change is a long-term challenge. It rewards slow, steady breathing and thoughtful evidence accumulation. The significance of the global CO2 level reaching 400 is more reflective of our penchant for watching odometers turn than a harbinger of imminent catastrophe.
Scientists use official CO2 readings as mile-markers on humanity's long atmospheric road trip. We can, too. President Lyndon Johnson told Congress about CO2 pollution when it stood at about 320 ppm, in 1965. Toyota rolled out the gas-sipping Prius at 364 ppm, in 1997. Cal Ripken Jnr ended professional baseball's longest streak of consecutive games at about 365 ppm, in September 1998.
Climate science is about the journey, not every swing of the bat.
2. Panic
On the other hand, after a generation of dilly-dallying, every new CO2 level is a line we're not supposed to cross.
Single-digit rises and falls in the CO2 level aren't a big deal. The long-term trend is the big deal. There's a hubbub in Paris because policymakers have been dithering for so long that, if we don't do something soon, the world may be locked into a dangerous warming trend later this century. And meteorologically suspicious things are already happening -- the tragic flooding in Chennai, India, this month being only the most recent.
3. So where's the line? There is a line, right?
The safe zone for CO2 may be below 350 ppm, a level last seen around 1988, according to the Stockholm Resilience Centre, a leading research group. The upper end of the safe zone -- or if you prefer, the beginning of the danger zone -- is about 450 ppm.
In that sense, reaching 400 ppm means we have begun the unhappy half of our journey toward the danger zone.
"Will daily values at Mauna Loa ever fall below 400 ppm again in our lifetimes?" Ralph Keeling, the Scripps scientist who oversees CO2 monitoring, wrote in October. "I'm prepared to project that they won't ..." A little perspective -- except for the past century or so, the CO2 level hasn't been above 300, let alone 400, in at least 800,000 years.
4. What to do when the clock runs backwards
Every year, humans unearth carbon minerals and burn them, releasing almost 36 gigatonnes (billion tonnes) of CO2. Yet the Bloomberg Carbon Clock will still run backwards half the year.
Here's why. Like human beings, most trees and plants reside in the Northern Hemisphere. When they wake up in the spring, they begin to draw CO2 out of the air and fuse it into their tissue. The soil sucks it in. In autumn and winter, it soars back upward. This seasonal effect is so prominent that CO2 levels drop from about May to October.
But make no mistake -- the underlying trend has always been up.
This offers everyone a choice.
For climate deniers, it's a rhetorical gift that will allow them to defend themselves in a new and exciting way. For everybody else, be happy when the clock runs backwards. It means it's spring or summer (up north, anyway).
Keep an eye on the clock, and maybe make sure the people running the world are doing something about winding it all the way back. Then go out and play.
