More about the Eocene study discussed in Friday’s letter. I went ahead and dug into the full report as much as I could over the weekend and came away with some revisions and additions to my original comments. This is all worth doing because the information in the report is actually quite sensational once you see it in a full perspective. The authors made no attempt to make it sound sensational, nor were they aiming for any kind of publicity. They come across simply as a group of Earth System scientists doing everyday communication with their colleagues about why a particular new model describing the late Eocene climate was superior to older models in a number of ways. They know it is not a subject the public cares deeply about, nor does the public need to at this point. Or should we? Here is my new set of impressions:
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1. If today we achieve a doubling of both CO2 and methane from their pre-industrial levels (280 ppm and 671 ppb), accompanied by other well-known adjustments and feedbacks, and are unable to bring them back down again for an extended period of time, the level of radiative forcing that results will be high enough to create a situation where eventually both the Greenland and Antarctic ice sheets would no longer exist. That was the situation at the very end of the Eocene, but with the climate then trending cooler, and having reached a point just before the Antarctic ice sheet was able to form. We have already raised CO2 to 414 ppm, which is logarithmically more than half way to a double, and methane to 1866 ppb, where it has doubled once and gone more than half-way to a second double. They are both moving, much faster than we like.
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2. Just doubling those two gases would, according to the study, be enough to raise global surface air temperatures by an average of 3.2C with only a small delay. This figure is right in line with what we hear from scientists every day, apart from those troublesome new estimates that go to 4C or more, and that by itself is good news if it holds. That much heating would in fact not do away completely with the big ice sheets but would start the ball rolling, the early stages of which we have already seen happening. Some further assistance in the warming process would be required to wipe out all the ice, and that is where Earth System scientists quietly come into the picture. They have stories to tell about how the surface of the earth, the atmosphere and oceans would all experience changes that on balance add more warming to air temperatures. A lot more, it seems.
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3. On Friday I remarked about how the study Abstract spoke of average global temperature increases of 5 to 7 degrees C over pre-industrial figures at the end of the Eocene, but I had it wrong. That was the increase attributed to just the Earth System changes that were generated in advanced stages along the way, in addition to the 3.2C added through changes of the radiation sort, for a total increase of 9.5C or more when the ice sheets are absent. Most of that extra boost is attributed to geographical changes that result from all the meltwater that, while being produced, is able to add 240 feet or so to global sea level along the way. Land surface area is thereby considerably reduced and so is the average elevation of land surfaces. There are numerous albedo changes that favor warmth, including a total lack of any ice and snow effects, and several more. (Many of the relevant details are attributed to the findings of other Eocene studies that are only referenced in this one, and are not especially new or controversial.)
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4. Some of the overall effect is the result of the global surface itself warming much more evenly than before from the equator to the poles. That would result in many differences in patterns of air pressure, wind speed, wind direction, jet stream movement and strength and so on. I think it would also reduce the ability of water vapor to have the same strong greenhouse effect as it does today, for lack of the leverage it now has to bring a great amount of warming to regions that are very cold, if and when those regions should no longer exist. (Water vapor increases cannot add much warming to regions that are already very warm because the needed amounts are great, and if produced they get rained out so easily.) This could explain why the model found that doubling the major greenhouse gases during the “hothouse” era should have less immediate warming impact than they do today, simply because water vapor, the big feedback partner whose effects are customarily accounted for in combination with those of the CO2 forcing, would effectively become a weaker partner.
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5. The study does not tell us how long it could take to melt down the great ice sheets if we now complete the CO2 double. It would not be millions of years. Would it even be 1000? We need to get an answer. We also need to keep a close eye of methane, which is way ahead of the game, and getting no accounting credit for any of the large current water vapor feedback, as properly deserved.
–Again, the full report is found at https://www.clim-past-discuss.net/cp-2020-29/cp-2020-29.pdf
Carl