Wednesday, August 28, 2013

Yale Climate Forum stumped by simple question on sea levels

In response to the article The Inevitability of Sea-Level Rise posted at the Yale Forum on Climate Change & the Media, I asked the following simple question:
Hockey Schtick says:August 24, 2013 at 12:26 am 
Please explain why sea levels during the last interglacial were 31 feet higher than the present, and Greenland 8C warmer than the present, without anthropogenic forcing. 
What evidence suggests the current interglacial is any different?
One reply to my inquiry was posted: 
Dan Rogers says:August 25, 2013 at 1:46 pm 
We are still in the “last” ice age. It isn’t over yet. All available evidence, as cited by James Lovelock in his Gaia books, indicates that the Arctic Ocean has been ice-free during each of the previous four inter-glaciation periods the planet has experienced over the past two million years or so. So just be patient Mr. Schtick. If history is any guide, there will be further warming before the present ice age finally comes to a close, and we humans will have next to nothing to do with causing it.
From the Wikipedia entry on Ice Ages: Glaciologicallyice age implies the presence of extensive ice sheets in the northern and southern hemispheres.[1] By this definition, we are still in the ice age that began 2.6 million years ago at the start of the Pleistocene epoch, because the GreenlandArctic, and Antarctic ice sheets still exist.[2]

Related: New paper finds Greenland temperatures were ~8C warmer than the present during the last interglacial


  1. Undisputed science: There are dinosaur graves in North Dakota. Dinosaurs were cold-blooded lizards. They lived at the edge of a huge inland lake covering Minnesota. Tiny shelled creatures lived in the lake and when they died, their shells fell to the bottom, layers were compressed by later layers, and limestone formed. We have LOTS of limestone - the lake was here a LONG time. Then a meteor hit, an ice age came, and everything died.

    Logical inference: For all that to happen - for cold-blooded creatures to thrive and for the lake to remain ice-free, Minnesota winters pre-meteor must have been Much, Much warmer than now.

    Question: Why is warming back to those levels man-made badness? Why isn't it natural goodness? Why should we try to stop the planet from returning to its natural state?


  3. I'll take a stab at your question. From what I can tell, the biggest difference in conditions between the Eemian and the Holocene is the eccentricity of the earth's orbit. Presently, it is near its minimum, with a value of 0.0167. In the Eemian, it was at a (local) maximum with a value of 0.044, almost 3 times bigger. Combine this with the fact that during the Eemian, the earth's location in the precession cycle was about a half-cycle off from the present day, so that the perihelion (minimum distance from the sun) was early in the Northern Hemisphere summer instead of early in the Northern Hemisphere winter, as it is now, and you have dramatically different conditions.

    So in the present, the solar flux density at the distance of the earth's orbit varies 90 W/m2 (+/-45) over a year, so about 1320 W/m2 at the NH summer solstice, and about 1410 W/m2 at the NH winter solstice.

    In the Eemian, the solar flux density varied 250 W/m2 (+/-125), so about 1490 W/m2 at the NH summer solstice and about 1240 W/m2 at the NH winter solstice.

    So the differences in insolation at the solstices were 170 W/m2 different from the present. Given that we are trying to figure out the effects of 1.7 W/m2 of CO2-induced forcing - only 1% of this value - it is little wonder that we cannot use the Eemian to figure out what is going on in the present.

    1. Thanks for your reply.


      However, eccentricity only modifies total annual insolation by only 1–2% of the shift caused by the 21,000-year precession and 41,000-year obliquity cycles. This has led to the so-called 100,000 year problem that remains largely unsolved.

      I see your point regarding insolation at the solstices having large differences, however, total annual global solar forcing apparently only changed a small amount from eccentricity alone. [value?]

      How do you explain both Antarctica and Greenland ice core temps were higher than modern during the Eemian?

      Would you agree with the statement that current sea levels are not unprecedented, unusual, or necessarily unnatural?

      P.S. is there a good online Milankovitch cycle calculator?

  4. I don't pretend to have definitive answers, and I don't think anyone has them, but I think I can engage in some reasonable speculation. These ideas start from "mainstream" ideas on the Milankovitch cycles.

    Orbital eccentricity changes do not affect the average TOA insolation that much, but do directly affect the variation. With high eccentricity, consider the pole that has a spring/summer perihelion. You would expect higher temperatures when the sun is high in the sky, increasing the melt. It is also feasible that winter snowfall would be suppressed, but this is less obvious.

    If I have calculated correctly, the transition to the Eemian started with the perihelion in the SH spring/summer (about 5 precession cycles ago). At high eccentricity, this could have led to a lot more summer melt of the Antarctic ice sheets than in the Holocene. At the peak of the Eemian, the perihelion was moving into the NH spring/summer, which could have done similar things in the NH, especially to Greenland.

    Earlier melting of winter snow cover over North America and Asia could have led to significantly higher temperatures that over the long term could have spread globally.

    Note that I have not spent a great deal of time on this, but I would be surprised if this did not have a significant effect of some sort.

    Even if you accept AGW as a significant factor presently, you cannot get a cumulative sea-level-rise contribution of more than 3 inches so far. This is lost in the noise of sea level changes on Milankovitch scales.

    Sorry, but I have not found a good calculator for Milankovitch cycle attributes. I have just found various plots at different sites, and done a little math from there.

    1. Thanks, interesting speculations.

      How did you determine "you cannot get a cumulative sea-level-rise contribution of more than 3 inches so far"?

  5. The generally accepted sea level rise of the last century is about 20 cm (8 inches), happening at a fairly steady rate. A majority of that happened before AGW could be considered, even according to the establishment view, to have had a significant effect. So even accepting this establishment view, we do not attribute more than 2 or 3 inches to AGW effects.

  6. Temps were warmer during the last interglacial