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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

What the science says...

During the Ordovician, solar output was much lower than current levels. Consequently, CO2 levels only needed to fall below 3000 parts per million for glaciation to be possible. The latest CO2 data calculated from sediment cores show that CO2 level

Older scientific papers inferred very high CO₂ levels in the Ordovician, generating a paradox of a cold climate during a time of high greenhouse gas levels. But recent work has shown that atmospheric CO₂ was much lower than the myth claims, and it kept falling through the Ordovician. It was less than 8 times preindustrial values towards the end (see the graph below), which may sound very high, but with a 4% fainter sun back then and with a large continent over the South Pole, it was low enough to trigger a major continental ice sheet.

The Ordovician was a time of mountain building (the Taconic/Caledonian orogeny) and violent ashy volcanic eruptions as the continents of Laurentia, Baltica and Avalonia began to collide. Mountain building, lots of fresh volcanic ash and erosion tend to accelerate the weathering of silicate rocks, which draws down CO₂ from the atmosphere, cooling the planet on a timeframe of hundreds of thousands to millions of years. And indeed, strontium isotopes confirm a large increase in the contribution of weathered volcanic rocks into ocean waters between about 470 and 450 million years ago. Neodymium isotopes (a proxy for ancient sea level change) show that ice sheets were in place in the late Ordovician.

The latter half of the Ordovician also saw the development of Earth's earliest plant-dominated terrestrial biosphere. Those early moss-like plants accelerated rock weathering rates, simultaneously drawing down CO₂ and supplying nutrients like phosphorous to the oceans, which fertilized plankton activity, which further reduced CO₂as their carbon-rich remains sank to the sea bed. The climate cooled so much that it crossed a "tipping point" 444 million years ago, triggering the Hirnantian Glaciation, which was so severe it resulted in one of the biggest mass extinctions since animals first evolved. For more on that see this article.

So, far from presenting a paradox, late Ordovician CO₂ levels are entirely consistent with a cool climate and glaciation. Moreover the geological, geochemical and fossil evidence all consistently show that a big drawdown of CO₂ drove that cooling, proving again that CO₂ is the principle control knob on climates both ancient and modern.

Falling CO2 and temperatures in the Ordovician

Cooling climate before the Hirnantian Mass Extinction. Cyan horizontal band is the Hirnantian Stage. Redrawn from Armstrong & Harper 2014. ¹³C-derived CO2 range from Pancost et al 2013, Plant spore first appearance simplified from Edwards et al 2014 and Rubinstein et al 2010.

Intermediate rebuttal written by howardlee

Update July 2015:

Updated on 2015-10-23 by pattimer.

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