An Inconvenient Truth: Ice Ages And Global Warming

I haven’t seen Al Gore’s movie “An Inconvenient Truth”, although it is high on my agenda to see. Here is a blog post on this movie on “Blog or Die”. The author brings out an interesting point based on the chart of the Ice-Ages and the level of Oxygen/CO2.

The movie presents many facts and draws conclusions. I want to separate out what is fact from what I have questions about.

Fact List:

Carbon dioxide is a greenhouse gas. That means that CO₂ tends to allow light through, but not heat. That’s a little like when you leave your car windows up on a sunny day. The light streams in through the windows, but the heat doesn’t radiate out through glass as well.

There is twice as much CO₂ in the atmosphere than there has been at any time in the last 600,000 years. We know this because there are ice core samples taken in the Antarctic. Ice is deposited in recognizable annual layers, and schentists can measure the CO₂ in the various layers.

There is a strong correlation between the amount of CO₂ in the atmosphere and the global temperature. The global temperature can be inferred through the ratio of two isotopes of Oxygen in the ice cores. One can see by looking at a time-series chart that CO₂ in the atmosphere tracks temperature.

The earth is getting warmer and ice is melting. I’ll talk more about that below.

Ok, that’s it for facts from the movie. Now for some analysis.

The earth is getting warmer. I borrowed my kid’s college geology book. It has a nice chart based on core samples from the Indian Ocean, showing the temperature pver 500,000 years inferred from Oxygen isotope ratios.

Image from The Earth Through Time Eighth Edition by Harold Levin, John Wiley & Sons Publishers, 2006.

There is a clear pattern of ice ages every 100,000 years or so. You remember ice ages? Huge areas of North America all covered with ice? Well that ice is melting. It’s been melting for ten thousand years. In fact, after every ice age (see chart), there follows a relatively rapid temperature rise. If you look at today’s high temperatures, it looks pretty much like 130,000 years ago. Now here’s the interesting part: after the last ice age, it got a bit hotter than it is today!

So here’s one place where the CO₂ / temperature correlation breaks down. There’s twice as much CO₂ in the atmosphere today than 130,000 years ago, but it was a bit hotter then.