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<blockquote>Why do Sea Level Changes Always Stop at about the Same Heights?</blockquote>
They don't. The first figure that should be there is a sea level reconstruction. There are several. For example Spratt et al., 2016.
In general sea level is proportional to temperature. The question is: why does the planet stops warming and cooling where it does?
The answer is primarily due to orbital conditions. Interglacials are warmer or cooler depending on orbital configuration. Melting the polar ice-sheets isn't trivial as the more North the melting gets, the harder it is to further melt the ice-sheets, as the yearly energy received from the Sun falls fast. That's why polar ice is so resilient and that is why sea levels don't go much above present.
On the other side you have a similar limit. Obliquity decrease takes insolation from high latitudes and puts it in the tropics, so as the ice advances South, the tropics receive more energy from the Sun. So it becomes more and more difficult to extend the ice-sheets South as the energy increase is higher and higher. This puts a limit to how cold the planet can get, and it limits also how low the sea-level can get.
Another factor is that as continental ice-sheets grow they become more unstable. When they reach a size equivalent to a 120-m sea-level drop they are so unstable that the next high obliquity period will cause them to collapse producing an interglacial. This is Didier Paillard's hypothesis from 1998 (Nature 391, 378), that is generally accepted, as it explains why after ice-sheets become that large an interglacial follows.
But vegetating gets its carbon from CO2 in the atmosphere, not from the soils. They drop litter which increases the mass of carbon on the forest floor and in the soils. There is much more carbon in the biosphere in warm climates than in cold climates.
Furthermore, as GMST declines, the planet gets drier, forests give way to grasslands then to deserts. The amount of carbon in the biosphere decreases.
So the high concentration of CO2 in the atmosphere and the 5 times higher mass of carbon in the biosphere during the ETM (50 Ma ago) must have come from somewhere. I suggest it is now dissolved in the oceans and tied up in carbonate deposits.
Javier, Thank you. Yours is a much clearer explanation than mine above.
Javier: "Obliquity decrease takes insolation from high latitudes and puts it in the tropics, so as the ice advances South, the tropics receive more energy from the Sun."
WR: to add: Colder conditions also result in less water vapor in the air. Following simple logic: less water vapor results in less clouds also over the tropics that enhance the effect of the higher tropical insolation. Extra absorption of sun energy by the oceans will constrain the slowing down of sea surface temperatures. A balancing mechanism, reacting on the change in orbital circumstances.
A check on the sea levels over the past several glacial cycles (500ka) given in the ref) shows that sea levels actually do stop at around 0 and 120m. The warming and cooling are processes that are based on the Milankovitch cycles, but with feedback mechanisms that stop the process at the stated levels. Topography is involved and this appears to relate to evaporation (water vapour) being a main driving force.
I assume that colder and drier air with less water vapoiur (higher density) would have a slightly higher T gradient, thus the 195m estimate would be closer to 2 C. This is often overlooked: for example, reconstructions of the forest temperatures in S Brazil have shown that Ts were some 5-7 C cooler OVER PRESENT LAND. Taking the exposed coastline of -100-120 m, we can reduce this to around 3-5 C - the equivalent TODAY of a hill of 400 -500 m. The Atlantic Rain Forest grows happily at +800 m
Yep. Eocene was warmer, as was the Cretaceous. However, it is very possible that this was due to increased ait density. See Journal of Experimental Biology 2018 Cannell Giant Dragonflies.
In the Permian patm would have been about 1.6 bar for these bugs to function.
Re Carbon budget
This is difficult to follow - what is easier to check is the measurement of changes in carbon in forests. Burning thousands of ha of the Amazon, replanting tropical rainforest, etc. What seems to have been overlooked in the vast amount of carbon put into the atmosphere by the drowing (in just a few thousand years) of these forests. The ball park figures show that this was significant and lags sea-level rise. This offers an explanation for the "lag" in CO2 re T.
This is an interesting post, but it boils down to yet another epicycle composed to justify CO2 as the thermostat. CO2 is a lousy thermostat. Taking the analogy of a room, if you set CO2 to warm your feet, it will also cool the air at the ceiling.
Moberg and Grinstad show some increase in SLR. Of course, if those geology records have spatial coverage as poor as most other paleo reconstruction studies, what does it prove. Every year more evidence surfaces that some warming occurred in more and more regions. If people in 100 years only look at Huston & Dean, Parker and Watson and some others, they could ask what Acceleration in SLR during the Modern Warm Period. What warm period. If the looked at Sydney and many other records of tidal gauges they could ask the same thing.
There are too many uncertainties and very incomplete information of the past to have the level of confidence imputed by some.
I found your article very informative for the simple reason that it gives me more areas to think about that I had never considered before. Thanks.
There seems to be an assumption here that the "multi-decadal signal" represents internal variability. I would suggest you try applying this procedure to the CMIP5 mean. You'll find the same pattern.
"There are too many uncertainties and very incomplete information of the past to have the level of confidence imputed by some."
This may be true, therefore I stressed the limitations of the described method: "A postulate is included: There are no other contributions than described in the introduction section, perhaps longer lasting, so that we can’t identify them." The time window of the application was also described: 1950...2016. I can't answer your questions reagarding the MWP because this is beyond the scope of the mainpost.
To the CMIP5 mean? This shows nothing else than the modeled forced GMST IMO because all variationes introduced by single models and runs will be cancelled out.
Yes, I understand. Thanks for your post.
I read the above and I exlained the above in much clearer terms and the one item you left out is RADIAT HEAT IS REFLECTED BY WATER.
Yes, exactly. Which would suggest that the multi-decadal variability pattern identified here is forced, not internal variability.
Nice post. Had not thought about the ‘limit set points’. Opens up some new lines for further self education. And, is also direct mechanism support for the dust/albedo theory for more rapid deglaciation than glaciation.
IF the pattern would be replicated in the CMIP5 mean:
I made the same operation with the same areas described in the mainpost.
Which suggests that the pattern is indeed internal variability.
To the rapid deglaciation theory can also be added the bigger surface area of tropical waters. Tropical waters are the main absorbers of Sun energy.
Besides, in circumstances of higher salinity of the sea surface some broad undeep waters could produce warm intermediate water - which will not be the case everywhere, due to lower densities because of higher rainfall and/or river runoff.
Where warmer intermediate water wells up, the 'surface start temperature' will be raised.
Frank Bosse, thank you for the essay.
No, I mean the residuals in the global average. Apply the Tamino filter (minus ENSO) to the CMIP5 mean global average, regress against forcing and find the residuals of that regression.
Matthew, it was my pleasure albeit the one or other disputant here was not so happy with it.