Climate sensitivity is the equilibrium temperature change in response to changes of the radiative forcing. Therefore climate sensitivity depends on the initial climate state, but potentially can be accurately inferred from precise palaeoclimate data. Slow climate feedbacks, especially changes of ice sheet size and atmospheric CO2, amplify the total Earth system sensitivity by an amount that depends on the time scale considered. (wikipedia)
Check this article:
Making sense of palaeoclimate sensitivity :
http://www.nature.com/nature/journal/v491/n7426/abs/nature11574.html
Climate sensitivity is important but the dominant effects are associated with potential feedback mechanisms. Simple models suggest delta T is 1.5 degree C increase with just radiative forcing, but that could be as large as 4-5 C with strong positive cloud feedbacks - of course cloud feedback could be negative as well - so the range of climate change outcomes, in my opinion, is dominated by the overall amplitude of the various feedbacks.
In addition, there is a potential large scale albedo feedback due to changing snow and ice cover. That feedback cycle is shown in the attached figure:
Most uncertainty will be occurred for rainfall simulated from GCMs. So you have to be very careful when you use downscale rainfall from GCMs. You have to calibrate with reliable observed rainfall with appropriate techniques.
Yes but that IPCC summary doesn't consider the role of feedbacks and how they change the probability density function (PDF) for future scenarios. They discuss well the uncertainty between forcing and temperature sensitivity and interplay with natural climate variability. The PDF you derive from those uncertainties is generally broadened by feedbacks. I attach an example of this from a few years back but the point remains that the standard IPCC PDF (the gray shaded area) is always too narow relative to what happens when you include feedbacks
G Bothun's reply about feedbacks neglects to mention that a large number of feedbacks are already included in the GCM's - e.g., clouds, water vapor. Adding consideration for these feedbacks would double their effect. Of course, one can still question the adequacy of their representation in the models, and wonder - as does Varias - about the long term effects of slow feedbacksnot examined by GCM's.
IMHO, one important effect that may be suppressed by GCM's is sudden climate change, such as the onset and termination of the Younger Dryas, caused by nonlinear feedbacks. These processes were often filtered out in the early days of GCM development because they were regarded as undesirable, spurious numerical effects. Now that it has been recognized that some of these effects are real and of great importance, the long tails they add to the PDF's of future climate scenarios need to be reexamined with new modelling methods.
Eric
as for younger-dryas type sudden changes: I presume you mean sudden changes in the
Atlantic Mer. Overt Circul but triggered by what? the Laurentide Ices Sield is no more
Harry,
You're quite right that the disappearance of the Laurentide Ice Shield means that the particular mechanism that most likely triggered the Younger Dryas is not operative, today. However, I'm referring to sudden climate change of any cause, including but not limited to the Atlantic Mer. Overt Circul.
Nonlinear modelling experiments by myself and a number of others (North, Ghil, Verbitsky, and others) suggest that there may be other triggers of similarly sudden, extreme events, and even that they can appear with no identifiable trigger in nonlinear dynamical systems. G. Bothun apparently concurs that such possibilities have not yet been adequately treated. Unfortunately, models of this type have relatively poor predictive value, but I believe that they can provide valuable answers to crucial questions about credibility and likelihood of sudden climate change.
Eric
One example of a non-linear feedback that I am actually (personally and scientifically) worried about is the accelerating release of methane from permafrost. That has a very large potential amplitude for climate change.
There are also some potential non-linear feedbacks associated with changes in the behavior of the MJO (madden-julian oscillation and subsequent kelvin wave acitivity) .
Bottom line is that earth systems couple to themselves in deep and complex ways and the whole system may be "chaotic" .
http://www.math.nyu.edu/faculty/majda/pdfFiles/2014%20updates/Submitted%20accepted/Realistic%20initiation%20and%20dynamics%20of%20the%20Madded-Julian%20Oscilliation%20in%20a%20coarse%20Ajaya%20Boualem.pdf
is an interesting dynamical /chaotic treatment of the MJO
Eric
Could you summarise the mechanisms of those non-linear effects you studied? Or else provide the references in you RG-page
dr Bothun
The methane-hydrate item is being discussed in the IPCCC-reports and I just went thru it because of a national hype about a press-release last week. It seems that this would only occur at a very late stage of the warming and not a process that needs any measures to be taken this century
There are two basic aspects/sources of methane-hydrate releases - ocean bottom or coast line bottom deposits that are currently confined by the pressure burden of the mostly cold ocean water. The second source is permafrost release mostly in Siberia - the actual source amount in that deposit is rather unknown. The first source, I agree is not vulnerable at this time, however, at least I believe the second source is vulnerable now with some data to support that belief.
Ever since the summer of 2010/11 - there is increasing evidence of methane release.
The following links are a news story and a somewhat scientific article.
http://www.nbcnews.com/science/environment/siberian-holes-could-be-visible-effect-global-warming-experts-say-n174331
http://www.newscientist.com/article/dn23205-major-methane-release-is-almost-inevitable.html#.U_pCsmOTHzE
@ dr Bothun
Citation of the last paragraphs of the NewScientist article with my emphasis in capital letters:
As for the methane that could be released into the atmosphere, Schuur estimates that emissions will be equivalent to between 160 and 290 billion tonnes of carbon dioxide.
That sounds like a lot, but is little compared to the vast amount humans are likely to emit, says Lenton. "The signal's going to be SWAMPED by FOSSIL fuel [emissions]."
He says the dangers of the permafrost greenhouse gases have been OVERHYPED, particularly as much of the methane will be converted to carbon dioxide by microbes in the soil, leading to a slower warming effect.
Schuur agrees with Lenton that the methane emissions are "NOT A RUN-AWAY EFFECT but an additional source that is not accounted in current climate models".
Could be, the future will tell.
The mircophysics/biology I think is key here - agreed that if it has a release into the soil with some residence time instead of direct sublimation in the atmosphere, then clearly the soil/bacteria as a buffer is a large effect - this must depend strongly on the depth of the permafrost layer.
AT a revised GWP of 35 to 1 - its not' clear that the signal will be SWAMPED byfossil emissions.
The current CO2e level is about 475 ppm with CO2 = 400 and CH4 ~ 60; so currently CH4 augmentation is a 15% effect relative to CO2.
@ dr Bothun
I merely literally cited the expert interviewed in the article. I presume that he is most knowledgeable on this subject and he plainly downplaying the issue
most probably they are far from rational estimations, I have my own aproach, they diverge from it
https://www.researchgate.net/profile/Dimis_Poulos
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