Thanks for your responses, Colin, Albert, Ken, Alan.
The temperature variations are very spiky, with respect to the underlying forcing, if this forcing is due to Milankovitch cycles, e.g. see http://en.wikipedia.org/wiki/Image:Vostok-ice-core-petit.png And Hansen has observed that the interglacial temperatures never reached more than a fraction of a degree above today's temperature. There does seem to have been some kind of limiting effect - what I dubbed a thermostatic control. Albert thinks that at least the Milankovitch theory is weak, and prefers a geothermal explanation - however the correlation of temperature with the cycles is very strong - you can see it strongly in a Fourier analysis (as shown by Southampton Polar Institute in their presentation to the Geophysics meeting in February). So if we accept Milankovitch as the underlying forcing, why is there an apparent limit on the temperature reached during interglacial periods, and why is the peak so pointed? Any suggestion of the forcing being stopped (as Alan suggests), does not explain the limit or the peakiness. There are certainly some negative forcings which come into play, especially forestation and land erosion, but neither do these explain the limit or the peakiness. (BTW, if there were not the limit where it was, then forests would be under heat stress, as we are beginning to see today, and their negative forcing would have disappeared.) The idea that CO2 or methane suddenly disappeared, to reduce the forcing, is not consistent with the fact that the CO2 and methane levels seemed to follow the temperature, at least in some of the interglacial periods. (This was taken as evidence by some climate sceptics that CO2 wasn't implicated in our current global warming!) Albert (in his second point) raises the interesting idea that as sea levels rose, the rotting, methane producing, vegetation would have become covered by water. However, that, as an explanation for limiting temperature rise, is inconsistent with methane levels following temperature. Albert also throws doubt on the importance of the Gulf Stream. However I find it interesting that the only area to have cooled during current global warming is the North West Atlantic, presumably due to meltwater. Perhaps unfortunately for the world, this meltwater has been insufficient (some estimate by a factor of 10) to divert the Gulf Stream and allow the Arctic sea ice to reform. I have discussed the sea ice thermostat theory with a researcher at Oxford, who agreed that it was plausible. However there was insufficient evidence to prove it. If the theory is correct, then one would have expected there to have always been some Arctic sea ice remaining in summer, throughout interglacial periods. Also one would expect a sea level rise immediately preceding the peak in temperature. So the theory is testable. But is there an alternative explanation for the thermostat effect? If not, we had better be sure to keep our Arctic sea ice intact! The climate system is tending always towards positive feedback, and we have to keep that in check, otherwise global warming is very likely to spiral out of control, to well above the famous 6 degrees. BTW, I believe this is a security issue, because if we let global warming continue for more than a degree or two, and if we let sea level rise a metre or more, there will be such conflict over land resources, food and water supply that nuclear war could easily break out. Geoengineering to the rescue! Cheers from Chiswick, John ----- Original Message ----- From: "Alan Robock" <[EMAIL PROTECTED]> To: "COLIN FORREST" <[EMAIL PROTECTED]> Cc: <[EMAIL PROTECTED]>; "geoengineering" <[email protected]> Sent: Friday, October 10, 2008 2:28 PM Subject: Re: [geo] Re: What stopped global warming before? > Yes. I just want to emphasize that the system responds to forcing, and > there is no need to hypothesize some damper to stop the response. The > amplitude of the response depends on the feedbacks, but when the solar > radiation distribution changes, the system just responds. > > Alan > > Alan Robock, Professor II > Director, Meteorology Undergraduate Program > Associate Director, Center for Environmental Prediction > Department of Environmental Sciences Phone: +1-732-932-9800 x6222 > Rutgers University Fax: +1-732-932-8644 > 14 College Farm Road E-mail: [EMAIL PROTECTED] > New Brunswick, NJ 08901-8551 USA http://envsci.rutgers.edu/~robock > > > On Fri, 10 Oct 2008, COLIN FORREST wrote: > >> Hi John, >> >> Current theory is that glacial/interglacial phasing is related to the >> amount of solar insolation around 60-70 degrees north (where there is a >> lot of land surface, covered in snow) especially around May (when spring >> melt is strongest) >> >> Basically we think that the ice/snow albedo feedback is predominant on >> these timescales. >> >> Amount of solar radiation at 650 N depends on astronomical >> characteristics of the earth's orbit round the sun....tilt, eccentricity, >> and the precession of the equinoxes, together known as the Milankovich >> cycles. >> >> This phasing determines GMST, with CO2 levels lagging temperature by >> several hundred years, but adding additional positive feedback on the >> temperature. >> >> Interglacials stop when insolation in high northern latitudes in spring >> falls to low levels allowing greater snow and ice cover. >> >> Regards, Colin >> >> >> ----- Original Message ----- >> From: John Nissen >> To: geoengineering >> Cc: [EMAIL PROTECTED] ; Davies, John >> Sent: Thursday, October 09, 2008 11:21 PM >> Subject: [geo] What stopped global warming before? >> >> >> >> Hi all, >> >> Through the Ice Ages, temperatures rose sharply during interglacial >> periods, but peaked at around the temperature we have today. This >> temperature seemed to have been a natural limit. What was the thermostat >> mechanism that stopped temperatures going higher? Here are some possible >> theories: >> >> 1. Meltwater turned off the Gulf Stream, allowing the Arctic sea ice to >> grow, cooling the Arctic region with positive feedback on this cooling >> sufficient to make the global temperature fall sharply. >> >> 2. Despite greenhouse warming from water vapour (a positive feedback on >> global warming), cloud cover increased, with cloud brightening from the >> fiercer storms at sea (resulting from the global warming). This extra >> albedo was sufficient to offset the water vapour greenhouse effect and >> cool the Gulf Stream, allowing Arctic sea ice to grow. >> >> 3. Sea level rise caused pressure on coastal magma chambers, thus >> increasing volcanic activity, which had an immediate cooling effect, >> through fine dust and aerosols in the stratosphere. This again could >> have allowed Arctic sea ice to grow. >> >> 4. Any others? >> >> In each case, grow-back of the Arctic sea may have been crucial to get >> an amplification of an initial cooling. If so, the Arctic sea ice has >> been essential to the Earth's thermostat control. But today we are >> seeing this thermostat breaking in front of our eyes. That is a powerful >> argument for geoengineering to save the Arctic sea ice. Pronto. >> >> Cheers from Chiswick, >> >> John >> >> >> >> >> >> >> >> >> > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en -~----------~----~----~----~------~----~------~--~---
