That graph is slightly wrong. That horizontal axis is the age of the ice, but the trapped air is younger than the ice. How does that happen? And why does it matter?
It happens because ice sheets start out as snow, and snow is light and fluffy stuff where air can freely flow in and out. And in Antarctica, where it’s always below freezing, the snow stays fluffy until it’s crushed into solid ice by the sheer weight of snow on top.
That takes a surprising amount of snow and time. You can watch the snow convert to ice by looking at a shallow ice core with a magnifier, and you’ll see that it’s not until you go about 200 feet down that the snow is squished enough to trap air into little separate bubbles.
So why is this important? It matters because the air can be newer than the ice! If you look, say, 50 feet down, the ice may be a thousand years old, but the air pockets are still connected together and connected to the surface, so the air down there is still fresh. It may have been breathed by a penguin just last year.
That’s why it’s a misleading graph. The horizontal axis should really be labeled ”Age of Ice”, and not just “Age”. Because there are two ages: the age of the ice is older than the age of the air bubbles and the CO2 that it contains.
- As a personal note, this plot disturbed me, when I first saw it. I grew up in New England, and the, snow usually melts and freezes into ice rapidly, in just a few days. But things are different in Antarctica: there’s a almost never any melting…
- Bear in mind that there are lots of ice cores these days. Vostok is only one of many. One of the things we’ve learned from comparing ice cores is that the apparent lag between CO2 and temperature varies from one core to another.
- Bear in mind that the temperature of Antarctica isn’t quite the same as the average temperature of the Earth; each region responds a bit differently. For instance in the last few years, we’ve seen the Arctic warm much faster than the Earth as a whole.
It’s Actually More Complex than That.
I’ve told you only half the story. While it’s certainly true that CO2 causes the climate to heat up, it’s also true that a change in the temperature also causes a change the CO2 level. For instance, if it gets cold enough to have an ice age and giant ice sheets, the vegetation changes, which changes the amount of carbon stored in the soil, and that changes the amount of CO2 in the atmosphere.
Additionally, less CO2 dissolves in the ocean when the water is warmer, so as the Earth warms, the oceans aren’t as good at pulling CO2 from the atmosphere. So, you can imagine a feedback loop, where CO2 makes the Earth warmer, and that releases more CO2, which makes the Earth even warmer. (Fortunately, each time you go around the loop, the effect gets smaller.)
So, you can start global warming either by adding heat, or by adding CO2. If you start by adding heat, there’s a tendency for the CO2 level to follow; and if you start by adding CO2, then warming will follow.