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EMBARGOED UNTIL 29 June 2011

A half-truth on past CO2 turns out to be a whole lie

Posted on 29 June 2011 by John Cook

A Yiddish proverb states "a half truth is a whole lie". Sometimes, by withholding a vital piece of information, it's possible to lead someone to the complete opposite conclusion to that which they'd get from considering the full picture. A stark (and as we shall see, profoundly ironic) example of this principle is demonstrated in the argument from climate deniers that high carbon dioxide in the past is proof that CO2 doesn't warm climate.

This argument is championed by geologist Ian Plimer. What we find is in the Earth's deep past,  CO2 has reached much higher levels than what we currently experience. In fact, over the last 550 million years, the further back in time you go, the higher CO2 levels get.

Figure 1: Atmospheric CO2 levels over the past 550 million years.

Ian Plimer argues that as CO2 was much higher in the past but the Earth didn't boil away with a runaway greenhouse effect, this proves CO2 is not the great warming effect everyone has cracked it all up to be. This view is colorfully expressed when Plimer accompanies Christopher Monckton to the Australian outback and conducts an experiment to demonstrate high levels of CO2 in the past.

YOUTUBE VIDEO EXCERPT FROM MEET THE CLIMATE SCEPTICS

Plimer uses this argument repeatedly in his 2009 book Heaven and Earth. Hig past CO2 also features prominently in a recent talk given to a geologists conference in Canada last month where he argues that "In former times of high atmospheric CO2, oceans were not acid, there was no runaway greenhouse and the rate of change of temperature, sea level and ice waxing and waning was no different from the present."

But does this present the full picture? Might there be some vital piece of information Plimer and Monckton are withholding from us that leads to an erroneous conclusion? Indeed, there is one crucial fact that climate deniers invoking high CO2 fails to mention. The further back in time you go, the cooler the sun is. Our sun, a main sequence star, has steadily grown warmer over billions of years.

Figure 2: The warming effect from CO2 plus the cooling effect from the faint sun.

Perhaps it's understandable that this salient fact has eluded the attention of Christopher Amon kronor, whose training is in journalism and the classics. But how could such a fact escape the attention of an experienced geologist such as Plimer? It turns out Plimer is completely aware that the sun was cooler in the past. In Plimer's 2001 book A Short History of Planet Earth, Plimer explains how the warming effect from CO2 kept our planet from freezing into an ice age when the sun was cooler:

"The low luminosity of the early sun .. average surface temperatures would have been below 0°C. But there is evidence for running water.., This paradox is solved if the earth had an enhanced greenhouse with  an atmosphere of a lot of carbon dioxide and methane."
NOTE: WILL USE FULLER VERSION OF EXCERPTED TEXT

The half-truth, considering the warming effect of CO2 while ignoring the cooling effect of the sun, leads one to mistakenly think that CO2 does not cause warming. But when one considers the full truth, the combined effect of sun and CO2, the past actually confirms the warming effect of CO2. In this case, the proverb nails the situation - a half truth is a whole lie.

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