One time period, in particular, is especially relevant to the discussion of rising CO2–a change between 33.5-34 million years ago (MYA) called the Eocene-Oligocene (E-O) transition.

What happened back then?  Around this time was the first appearance of consistent polar ice on Antarctica.  Before then, atmospheric CO₂ levels were high enough that Earth’s climate was a hothouse, perhaps as much as 8-10 degrees C warmer than today.  Antarctica was lush and green with forests.

The worry is that if we start approaching atmospheric levels of CO₂ similar to those before the E-O transition, we may warm the climate to a condition where polar ice us unstable.  That would be bad news because the loss of the Antarctic ice sheet would raise sea level by more than 60 meters.

This week, Paul Pearson and colleagues (who have done a lot of this great work) published a new article¹ examining the E-O transition in more detail to see if it has any clues for our modern environmental challenges.

What did they find?

• Their work suggests that there was a global atmospheric CO₂ threshold of about 750 ppm, below which ice growth on Antarctic gets triggered (less CO₂, less greenhouse effect). For reference, our current atmospheric CO₂ concentration is 389 ppm.  They do note that there is uncertainty around this estimate and that it could be as low as 450 ppm.  But 750 is their best guess.
• CO₂ declined to this level around 33.6 MYA, driving the growth of Antarctic ice sheets.  CO₂ then spiked back up to about 1100 ppm, lasting 200,000-400,000 years before finally dropping below the threshold for good around 33.2 MYA.
• Interestingly, however, the ice sheets persisted through this transient CO₂ spike.  The authors suggest that this can happen because the high reflectivity of the ice sheet reflected more sunlight, thereby keeping temperatures cool enough to prevent the loss of land ice.
• Similar to previous studies, they conclude that decreasing CO₂ over this period is what drove ice sheet growth and the formation of what we know as our modern, icy Antarctic environment.

This leads to the obvious question of why CO₂ declined in the first place.  That’s an active area of research and a longstanding debate. The current hypothesis is that the tectonic moving of continents caused the formation of massive mountains like the Himalayas around 50 MYA.  CO₂ in the atmosphere reacts with rock minerals as they weather over time, leading the to transport of carbon to the ocean, where it is turned into calcium carbonate (marine organism shells) and buried in the sediments as these organisms die.

Thus, over the past 50 million years, the earth’s atmosphere has been gradually scrubbed of its CO₂ thanks to the Himalayan Mountains.  At 33.5 MYA it dropped to low enough levels (~450-750 ppm) to form Antarctic ice. At ~2 MYA (what we call the Pleistocene), it dropped to low enough levels (~250-300 ppm) that triggered northern hemisphere ice sheets, which have advanced and retreated about 18 times.

So what’s in store for the future?  Many groups, like 350.org have been advocating for an atmospheric limit of 350 ppm CO₂.  This would help us stay under a 2 deg-C warming that would likely avert catastrophic climate changes.  If we let CO₂ rise to more than 450-750 ppm, which we will reach sometime between  2030-2070 if we continue a business as usual trajectory, we run the serious risk of putting earth’s climate into a situation that hasn’t been experienced in 33 million years–the tipping point for Antarctic ice existence.  However, as these new data suggest, we may have a buffer from the ice reflectivity that allows atmospheric CO₂ to rise above 450-750 ppm without leading to catastrophic thaw.

We probably don’t want to test this out.

¹Pearson, P. et al (2009) Atmospheric carbon dioxide through the Eocene–Oligocene climate transition. Nature 461: 1110-1113.