The Past is No Guide to the Future

“During the Middle Miocene … carbon dioxide levels were sustained at about 400 parts per million, which is about where we are today,” said Aradhna Tripati, UCLA assistant professor in the department of Earth and space sciences and the department of atmospheric and oceanic sciences.

As we close in on the similarities between the atmospherically carbon rich environment of the middle Miocene and today’s earth, some people might be tempted to take convenient solace in the comparison. As the reader will know from looking at other pages on this site, the middle Miocene planet was a wonderfully biodiverse place, full of life and color. S. David Webb dubbed it the era of the Clarendonian Chronofauna. For the sake of this post I’ll call it the Middle Miocene Flourish Phase (taking the biosphere as a whole into account – animals and plants). It shows what the earth can do when given just the right set of circumstances.

This begs the question, however, with the CO2 gasses we are putting in the air currently can we expect similar conditions as that wonderful Miocene time on the earth today? Sadly, no, and that’s because pretty much everything else on our present anthropogenic world is different from the middle Miocene. It’s true that atmospheric carbon should green the planet more for a while, but on our present trajectory we are set to greatly exceed the carbon of that “optimal” time. If you were to go back farther, however, to around 16 – 17 million years ago you would find the world uncomfortably hot: “The last time carbon dioxide levels were apparently as high as they are today — and were sustained at those levels — global temperatures were 5 to 10 degrees Fahrenheit higher than they are today, the sea level was approximately 75 to 120 feet higher than today, there was no permanent sea ice cap in the Arctic and very little ice on Antarctica and Greenland.” “The planet” she says, “was dramatically different.”

During that hot phase, huge volcanic activity, known as the Columbia River Flood Basalts, and other volcanic episodes, were spewing vast stores of carbon gasses into the lower atmosphere which created a greenhouse effect (think of the way a blanket holds in heat). It took hundreds of thousands of years for the earth to bring that carbon back down and the world to begin to cool.

But how else does the Anthropogenic world of today differ from that of the Miocene, such that, rather than flourish, life as it exists today will suffer as a result of the carbon we are spewing? Howard Lee, in a post at Skeptical Science, answers this important question well in his post, Why the Miocene Matters (and doesn’t) Today. With his kind permission, I reproduce his bulleted points. Note: the first point is a bit technical, but you get the gist:

• Because greenhouse forcing is a log function of CO2 concentration, the Miocene increase in radiative forcing was much smaller than the increase since the industrial era to today, let alone levels projected by the end of the century. The Miocene increase from 400 to 500 ppm is a radiative forcing increase of about 1.2Wm-2, which is much less than the modern jump from 280ppm to 400ppm today, already a radiative forcing increase of 1.9Wm-2 (it’s really 2.9Wm-2 including other greenhouse gasses) let alone 6.0 Wm-2 for the IPCC’s RCP6.0 scenario (CO2 670 ppm), and 8.5 Wm-2 for the IPCC’s RCP8.5 business-as-usual scenario (CO2 936 ppm) by the year 2100 (figures from IPCC AR5).
• Animals of the Miocene had no cities, no agriculture, no power stations, factories, roads, [fences] or rail networks. They could migrate and spread as their environment altered on a pace very slow compared to their reproduction rate.
• The Miocene saw reforestation of arid lands which may have mitigated some of the CO2 rise and warming, whereas we have been deforesting the planet for millennia, removing an important carbon sink that was available to the Miocene but not to us.
• Our oceans are currently acidifying much faster than in the Miocene or any time in the last 60 million years – a sign of how much more rapid and how far out of equilibrium with short term carbon sinks our modern climate system is.
• The MMCO global warming took place over many millennia, giving life time to adapt and migrate. Our warming is at least a thousand times faster, a rate that, if left unchecked, could well result in a climate that more resembles the end-Cretaceous or end-Permian disasters, rather than the relatively gentle MMCO.

For those interested, Howard has written a book titled Your Life as Planet Earth. I’ve not read it, but it looks quite interesting.

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