AGW part #4 – The History of Temperature

To determine how much the earth has warmed by man’s influence we need to have a good temperature and CO2 record from history. After all you can’t claim that we are destroying the planet if temperature varies up and down all the time. It turns out this is not particularly easy to do. The field of climate reconstruction is called paleoclimatology and its conclusions are as slippery and often contested as archeology.

Let’s start with CO2 ice core data. Ice cores are taken from areas where scientists believe the ice has lasted for hundreds thousands of years. The ice is formed from snow which piles up and compacts down to ice trapping microbubbles of air inside. Summer to winter layer counting gives a date for each layer when layers are compressed and become too thin less accurate radiocarbon dating and highly accurate volcanic ash layers are used. By analyzing the CO2 concentration the scientists can plot a history of CO2. A second value you can take from the ice core is the temperature per layer. This is derrived by a calculation based on the ratio of oxygen isotopes in the cores. It’s a bit complicated to describe here.

Here is a plot of temperature and CO2 based on ice core data.

From a Nature publication

The first thing you notice is that CO2 in red tracks well with the temperature variation. At first glance this seems like proof that CO2 causes global warming but if you look very closely the CO2 spikes occur after the temperature rise. Numerous studies have since been done and as far as I can tell it has become generally accepted that the CO2 increased approximately 1000-4000 years after each rise in temperature.

This means that CO2 in this case was the result of temperature rise, not the cause. This does not, however prove or disprove CO2 global warming. It does mean that when Al Gore presented this in his movie, with a time shifted CO2 pattern to make them align, that it should not be trusted.

Historic temperature has risen well above where it is today by this data, and it was clearly due to natural causes (again no conclusion). I notice 4 spikes in temperature which have a steep upslope (end of ice ages) and peak followed by a drop. We are clearly at the peak of the end of an ice age and facing a fairly severe drop in temperature in the next 1000-2000 years if nature has anything to say about it.

This graph scares the crap out of me because most of the time the earth is about 6to8 degrees C colder than it is now. If you had to choose between global warming of a couple of degrees or cooling of 6 to 8 you would be smart to choose warming.

This, however is not the end story for temperature.

There are many issues with the accuracy of this method of temperature reconstruction. I will discuss those in Part 5