With a recent British court case critiquing An Inconvenient Truth and the news that Al Gore just won the Nobel Peace Prize, the attacks on Gore and his slideshow have stepped up in recent times. A common criticism is his use of the CO2/temperature record to show that in the past, CO2 caused temperature increase. A close look at ice core records finds that CO2 actually lags temperature. In fact, a study came out just a few weeks ago (Stott 2007) that confirms CO2 increases around 1000 years after temperature rise. This raises an important question - does temperature rise cause CO2 rise or the other way around? The answer is both.
The dominant signal in the temperature record (the white line in the above figure) is a 100,000 year cycle where long ice ages are broken by short warm periods called interglacials. This cycle coincides with a change in Earth's orbit as it evolves from a more circular orbit to a more elliptical orbit. When springtime insolation (incoming sunlight) increases in the southern hemisphere, this causes temperature to rise in the south. The warming is amplified as retreating Antarctic ice means less sunlight is reflected back into space.
As the southern oceans warm, they give up more CO2 to the atmosphere as the solubility of CO2 in water falls with rising temperature. The CO2 mixes through the atmosphere, amplifying and spreading the warming to the tropics and northern hemisphere. This is why warming in the southern hemisphere precedes warming in the northern hemisphere (Caillon 2003). This is confirmed by marine cores that show tropical temperatures lag southern warming by ~1000 years (Stott 2007). CO2 warming also explains how the relatively weak forcing from orbital cycles can bring the planet out of an ice age.
So where does that leave Al Gore? What he says in An Inconvenient Truth is this:
"The relationship is very complicated but there is one relationship that is far more powerful than all the others and it is this - when there is more carbon dioxide, the temperature gets warmer because it traps more heat from the sun inside."
This statement, while an oversimplification, is essentially correct. A more accurate and informative statement would've been
"A change in Earth's orbit warmed the southern oceans which released more CO2 into the atmosphere. The extra CO2 trapped more heat from the sun and amplified the warming. It also mixed through the atmosphere, spreading the warming to the tropics and northern hemisphere"
Of course, the audience may have dozed off by the end of the explanation and slept through all the pretty pictures of polar bears and glaciers.
Note - I've posted more info as well as links to many peer reviewed studies on this topic on our CO2 lags temperature page.
Posted by John Cook on Friday, 12 October, 2007
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