date: Sun, 18 Jul 2004 15:24:13 +0100 from: Sarah Raper subject: Re: more GSIC to: Tom Wigley Make sure you use the right index: for example 1/1.36 = 0.735 in A=V**0.735, or V=A**1.36 On 18 Jul 2004, at 03:42, Tom Wigley wrote: > Sarah, > > You are right that all the GSIC will melt even for zero warming > from today. Our old model was better in that regard because it > had a different eventual melt for different warmings. But this is a > tricky thing for me even to conceptualize. My intuition says that > if we stabilized temperature at today's level then not all of the > GSIC ice would melt -- but how much would remain? It also > seems reasonable that there is some warming amount that would > ensure that virtually all the GSIC ice would melt --- 3degC, > 5degC, ??? > > So the next step would be to try to get some realism here, but I > really have no idea what would be realistic. I will be able to do this, but not this summer. For glaciers I have 1 deg distributions for ELA and for glacier altitudinal range, so I can use these together to work equilibrium volumes for different deltaT. All ice gone when ELA is at top of glacier. For icecaps it will need some modeling because max altitude of the ice cap declines as the ice cap melts ie there is a feedback. It is a strange thing but the modeling of ice cap involves just about the opposite of modelling of glaciers. The glaciers shrink up the mountain whereas the ice caps shrink down - I will assume a flat bed (1st order). Thus I should be able to get the numbers we need for the original model which I still like conceptually. But you are right, we are not yet in a position to do it. I think we should flag the problem in the paper and maybe point to future where we will do it. So it is an extended model for use to about 2400 only..... > > However, I think what we have is fine out to 2400 for most cases. > For zero warming from now, sea level rises at about 5cm/century > initially and then more slowly later -- so it would take 1000 years > or more for all the GSIC ice to melt. We never get far enough > for things to look silly. This melt rate (%cm/century) is proportional > to (0.15 + T(1990)), so the 0.15 has an important effect. Climate > sensitivity has an effect too (in MAGICC) since this influences > T(1990) -- a model artifact. (The TAR says that the 0.15 comes > from Zuo and Oerlemans, which I have added to the refs.) > > So this leads to a question -- at present the GSIC model, > embedded in MAGICC, uses T(1990) from MAGICC. Perhaps > it would be better to use an observed value (like the 0.65 I use > in the text example)? For most cases this would have little effect, > but it has a noticeable effect for cases with very little post-1990 > warming. Arguing against this, to add another add hoc correction > like this would just make things messier. I'm just thinking out loud > here. Shouldn't you use a value which gives an equivalent result to Gregory? > > As it happens, the analytic result I gave was for zero warming > from 1990, not (as the text implied) for linear warming. The linear > warming result is quite complicated -- but all I needed to illustrate > the points made in the paper was/is the zero warming case. I > have corrected this. > > I have also done the A proportional to V**n case, and dumped > the n=1 results. The refs you gave were just what I needed, and > I have cited these. n.NE.1 has little effect out to 2100, but does > lead to greater melt by 2400. I have revised the text to cover all > this, but need to re-do the diagrams. I hope to send this new > version to you tomorrow. I think it is a significant improvement > even though the results are pretty much the same. Some of the > bibliographic holes are filled -- I can imagine that someone like > Oerlemans would criticize anything that didn't liberally cite his > work (so there are now more Oerlemans refs). > Don't go overboard, the more you cite him the more likely he will get it to review. > Best wishes, > Tom. >