John, If we think that the amount of carbon in the ocean+atmosphere+biosphere on million year time scales is determined by relations between volcanism and silicate rock weathering (i.e., amount adjusts such that weathering rates balance degassing rates), then one explanation of the bounds on glacial interglacial CO2 is that there is a finite amount of carbon to slosh around the system and there are basically two semi-stable sates, one with more carbon in the ocean and one with less.
If you add more carbon to the system, it is possible that you might still have two stable states on the tens of kyr time scale, one with more and one with less carbon in the ocean, but both of these states would likely have higher atmospheric CO2 content. On the other hand, it is possible that the addition of CO2 results in only one stable state (evidence of strong oscillations in the climate system seems to be mostly at colder time periods). Nobody is really sure, of course. Temperature limits are probably a bit more complex than CO2 limits since temperature limits depend on CO2, other greenhouse gases, albedo changes, etc. Best, Ken ___________________________________________________ Ken Caldeira Carnegie Institution Dept of Global Ecology 260 Panama Street, Stanford, CA 94305 USA [email protected] http://dge.stanford.edu/DGE/CIWDGE/labs/caldeiralab +1 650 704 7212; fax: +1 650 462 5968 On Mon, Dec 7, 2009 at 9:18 AM, John Nissen <[email protected]> wrote: > > Hi Ken, > > Unfortunately I had to simplify the analogy somewhat - into just brakes and > throttle. But the oscillations I refer to are the coming and going of the > glacial periods over the past 2.5 million years or so. I believe the > temperature records, inasmuch as we have them, show that the maximum > temperatures at the end of each period of deglaciation are remarkably > similar to one another, the minima less so. The global temperature today is > only slightly below the maximum at the end of the Eemian warming, when > sea-level was 4-6 metres higher (maximum 2 metres from Greenland, rest from > Antarctic) [1]. > > Here's a quote from an article on the Pleistocene temperature [2]: > > "Analysis of the d18O record indicates two basis [sic] climate states, one > glacial and one interglacial. Evidence for these bistable climate state is > further provided by oxygen isotope analysis of numerous ice cores. Figure > 5.10 shows a d18O profile along the Camp Century (Greenland) ice core > (Dansgaard *et al*., 1984) for the last 130,000 years. The record clearly > reveals the last major interglacial period at about 120 thousand years (Ka) > and the ensuing glaciation." > > Cheers, > > John > > [1] http://en.wikipedia.org/wiki/Eemian > > [2] http://www.global-climate-change.org.uk/5-3-1.php > > --- > > > Ken Caldeira wrote: > > This idea of two stable states has been pushed recently by Jim Lovelock, > however I think there is very little evidence to support such a view. > > Just about every model result indicates monotonically and more-or-less > continuously increasing effect with increasing doses of CO2 (or changes in > sunlight). > > [Of course, the models are a gross simplification of reality.] > > There are a few sources of metastability (eg, large ice sheets) but I do > not think that these sources of metastability govern the overall behavior of > the system on sub-millennial time scales. > > > ___________________________________________________ > Ken Caldeira > > Carnegie Institution Dept of Global Ecology > 260 Panama Street, Stanford, CA 94305 USA > > [email protected] > http://dge.stanford.edu/DGE/CIWDGE/labs/caldeiralab > +1 650 704 7212; fax: +1 650 462 5968 > > > > > On Mon, Dec 7, 2009 at 6:22 AM, John Nissen <[email protected]> wrote: > >> >> Hi all, >> >> I've a simple picture of the situation. Think of the Earth system as a >> bus, with automatic speed control mechanism to keep the bus between two >> speeds - a top speed and a bottom speed. When the bus gets to the top speed >> the brakes automatically come on and the throttle starts closing. Then when >> it has slowed to the bottom speed the brakes automatically come off and the >> throttle starts opening again. This mechanism has worked for over a million >> years, keeping the bus running between higher and lower speeds, without the >> driver doing anything. >> >> [*Global temperature has oscillated between a top and bottom temperature >> over the course of the Ice Ages. The Arctic sea ice is probably involved in >> preventing the temperature going above the top limit. CO2 may be involved >> in preventing the temperature going below the bottom limit*.] >> >> Then, about 8000 years ago, when the bus was near its top speed, the >> driver took over control. Although the brakes were starting to come on, the >> throttle was kept open by the driver by just the right amount to keep the >> bus going at the higher speed. This kept the passengers extremely happy! >> They thought this situation would carry on for ever, but this proved to be >> wishful thinking. >> >> [*By amazing chance, mankind's inadvertent emissions of CO2 and methane >> have kept global temperature, and hence sea level, pretty constant for about >> 8000 years, thus allowing the emergence of civilisation.*] >> >> Then suddenly, a hundred years ago, the driver started opening the >> throttle more than it had ever been before, whilst at the same time taking >> off the break. Surprise, surprise, the bus started to accelerate. Some >> passengers refused to see the danger - they were enjoying the speed, and >> were happy for the driver to open the throttle even more. Other passengers >> saw the danger. The passengers broke into argument, and eventually agreed >> on a compromise to gradually stop opening the throttle, although it was >> realised the bus could reach a dangerous speed. >> >> [*Mankind has injected a colossal pulse of CO2 in the atmosphere, which >> continues to grow, and will continue to grow under any Copenhagen >> agreement. Global warming is an inevitable consequence of the excess CO2 >> (although other gases contribute, both positively and negatively).*] >> >> Nobody wanted to point out that, at this speed, the bus would become >> unstable and almost certainly crash off the road putting everybody's life at >> risk. Thus it was actually in everybody's interest to close the throttle >> and apply the brakes straight away. The compromise they had reached just >> gave a false sense of security. >> >> [*Nobody seems willing to point out that our present path, akin to >> business as usual, could lead to the collapse of our civilisation, e.g. >> through famine. Any deal at Copenhagen is liable to engender a false sense >> of security.*] >> >> Soon after the compromise had been reached, the bus, already travelling >> dangerously fast, skidded on some ice, which the driver saw too late, hit a >> tree and burst into flames, killing all on board. >> >> [*Tipping points have been ignored. The Arctic sea ice could disappear >> quite rapidly, with potentially lethal consequences from methane release >> and/or Greenland ice sheet disintegration. The Amazon rainforest is also at >> risk of drought, leading to die off and burn.*] >> >> Sadly this fate was not inevitable. Unknown to the driver, there were >> emergency brakes that could have slowed the bus, even while the throttle was >> open. But the driver had been told that these emergency brakes were too >> dangerous to use, so was not prepared to use them. When he saw the ice, he >> tried to apply the emergency brakes but it was too late. >> >> [*The only reasonable chance of saving the Arctic sea ice is through the >> application of geoengineering to cool the region. The chance of >> geoengineering successfully diminishes as positive feedbacks build up in the >> Arctic. The more we delay geoengineering, the more we risk passing a point >> of no return, so that a gruesome fate becomes inevitable, possibly even for >> most people alive today.*] >> >> Cheers, >> >> 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]<geoengineering%[email protected]> >> . >> For more options, visit this group at >> http://groups.google.com/group/geoengineering?hl=en. >> > > -- 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.
