With all the news now being dominated by the coronavirus, this will be a good day to talk about something else, and the farther away the better. How about the Eocene period, which lasted from 56 to 34 million years ago? It is one of the most interesting eras because it began with some of the hottest temperatures in history and ended with a very rapid cooling episode that caused the Antarctic ice sheet to emerge. Scientists are keenly interested in the latter segment for the simple reason that everything we can learn about it should be helpful toward bettering our understanding of the way climate change progresses and what kind of future climate we may be heading into. The more recent Pliocene period, which offers an even better model with more certain matches, has so far gotten most of that kind of attention. The late part of the Eocene, in reversal, represents a potentially next next big step leading in the direction of a true “hothouse” state of climate for our planet. You can start by checking out a picture of the Eocene in its entirety from Wikipedia at this link:
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A new study has come to my attention which has not yet been published in its accepting journal (Climate of the Past) but has been made available online for discussion purposes. It appears to be a high-quality work, offering a representation of the most up-to-date evidence, modelling and interpretation of the last 8 million years of the Eocene, divided into two halves separated by the exceptionally sharp break that led to a major cooling of the climate. That break can be studied as an example of our own immediate period but operating in reverse. In fact scientists have already learned quite a bit about how things changed and what caused the changes. You can read the Abstract of the study at this link— https://www.clim-past-discuss.net/cp-2020-29/ and then I will give you my own idea of what was most interesting about its content.
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1. The best estimates for CO2 concentration are around highs of 1200 ppm for the 4 million years prior to the break and lows near 500 ppm for the 4 million years that came after. The average temperature for the entire period is estimated at 5 to 7C above pre-industrial, or 4 to 6C higher than today, and would be more than that at the highest end.
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2. The absence of ice sheets and differences in geographical elevation meant that polar regions, while much warmer than today, were relatively not as much different in warmth compared with lower latitudes as they are today. In other words they did much more of the cooling at that time, just as today they are doing a larger part of the warming, at least, so far, in the Arctic.
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3. The combination of heavier clouds and more water vapor, which are classified as feedbacks rather than forcings, had a relatively stronger effect on temperature than forcings at that time, as compared with the situation today. On balance they had a tendency to hold back the gains when temperatures were the warmest during the Eocene, thereafter tending more toward resisting the strength of the cooling period, when a reduction in greenhouse gases was taking center stage. That could be an advantage for us in the years ahead.
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4. Overall, the rate of climate sensitivity is taken to be a bit more than 3.0C for each doubling of both CO2 and methane concentrations, which is not much different from the common estimates that are used today. We will not reach a CO2 double until we hit 560 ppm, which leaves some breathing room over our current 413 parts. With methane the story is different, because here we have already doubled the concentration since 1750 and are well on the way to a second double. I believe methane, all by itself, or with no feedbacks counted, adds about half as much as CO2 to global temperatures per doubling when CO2 is also treated in isolation and not beefed up with feedbacks like it usually is. That means it has contributed more than CO2 to today’s temperature increases since 1750. (See CL #1618, Feb. 20, for more on that subject.)
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If you really feel ambitious, and want to avoid crowds this weekend, the new discussion paper on the Eocene climate transition is fully available at https://www.clim-past-discuss.net/cp-2020-29/
Carl