Tuesday, May 1, 2012

Carbon Dioxide and the Last Ice Age

One of the pieces of evidence climate change deniers like to pull out is data showing the global temperature at the end of the last ice age increased prior to the increase in CO2 levels. I remember hearing a lecture by one the chief deniers, Richard Lindzen, where he pointed at a graph showing temperature rise before CO2 rise and saying that cause is suppose to precede effect. Pretty clever and it was effective with the audience. I looked around and noticed how many people were nodding their heads in agreement. This piece of data has been difficult, but not impossible to explain. But it is remarkable that deniers will say this one piece of evidence is conclusive while ignoring all other evidence. They are like a gleeful little kid that thinks he has found a treasure.

Now, they are finding out it was false gold all along..

One of the problems we have with figuring out what happened long ago is the data is sparse and difficult to get. We cannot just open an old almanac and look up the temperatures and CO2 levels for 20,000 years ago. We have to painstakingly compile the data by taking multiple samples at places all over the world. We can't rely on one kind of evidence, we have to collect many independent lines of evidence. Tree rings are great, but not enough. We need more. So, we collect sediments and drill ice cores in ice sheets. Then, we have to take these physical samples to the laboratory and make very fine and detailed measurements. Then we have to analyze the data and figure out what it means. And, we have to compare all of these different lines of evidence to ensure they are consistent with each other. If they aren't, we have to go back and find out why. All of this takes a great deal of time.

What that means is over time the data base gets better.

A new study used improved data to show the temperature at the end of the last ice age didn't lead the CO2 rise, it actually lagged the rise in CO2 levels. Take a look. The green line is global temperatures. The red line is the Antarctica temperature. The blue dots are measurements of CO2. (Credit: Shakun et al. Nature, 484, 49, 2012) :


With a more detailed database we can see the evidence now shows the level of CO2 in the atmosphere was clearly rising before the temperature was rising. A rising level of CO2 was the principle driver in the end of the last ice age.

The deniers ignored all of the other evidence before. I'm betting they will ignore this evidence as well. But, there's this thing about science. It's right; whether you want to believe or not.

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