How Jo Nova doesn't get the CO2 lag

Yesterday, we looked at Jo Nova's #1 "killer blow" in her Skeptics Handbook: the tropospheric hot spot. Today, we move onto the Handbook's #2 point: the CO2 lag. When we look through the ice core record, we see that in the past, CO2 levels change after temperature change. From this, Jo Nova argues that CO2 warming is a minor force. But this doesn't give you the full picture. A number of measurements find extra CO2 is trapping heat. So the full body of evidence gives us two facts: warming causes more CO2 and more CO2 causes warming. Put these two together and you get positive feedback.

How does this work? When the Earth comes out of an ice age, the warming is initiated by changes in the Earth's orbit. As the ocean warms, the solubility of CO2 in water falls. This causes the oceans to give up more CO2 into the air. This has several effects. Firstly, the relatively weak warming from orbital changes isn't enough to take our climate out of an ice age. The extra CO2 in the atmosphere amplifies the original warming. That's the positive feedback.

Secondly, CO2 from the ocean mixes through the atmosphere, spreading the warming across the globe. Ice cores and marine sediments find that initial warming begins in Antarctica. Around 800 years later, CO2 rises and at the same time, warming spreads to the tropics and northern hemisphere (Cuffey 2001, Caillon 2003, Stott 2007).

This amplification and spreading of the warming also works in reverse when the planet enters into an ice age. A new paper published over the last week uses ocean sediments to construct a temperature record over the past 2.7 million years (Herbert et al 2010). They find that when ice sheets spread in the Northern Hemisphere, this cools the northern oceans. The result is the oceans absorb more CO2, leading to a dramatic drop in atmospheric CO2. This amplifies the cooling and spreads it across the globe.

A common misconception is that positive feedback always means runaway warming. This isn't necessarily the case. If the feedback is not too great, what happens is an amplification of initial warming with the temperatures eventually stabilising at a higher level. Think a bank account with compound interest - no bank will offer so much interest that you experience runaway interest income. Past history indicates this is the case with our climate - net positive feedback amplifies initial warming but climate settles at a higher temperature. There's a good demonstration of how this works in a past comment by Ned Flounders.

So the CO2 record is entirely consistent with the warming effect of CO2. In fact, CO2 warming explains both the dramatic changes in temperature in the Earth's past and how temperature change is able to spread from the poles to the rest of the globe. Studies comparing past co2 to temperature find a positive feedback relationship (Scheffer 2006). The CO2 lag doesn't disprove the warming effect of CO2. On the contrary, it provides evidence of a climate positive feedback.

Posted by John Cook on Sunday, 20 June, 2010

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