date: Sat, 17 Jul 2004 11:40:38 +0100 from: Sarah Raper subject: Re:EQUILIBRIUM to: Tom Wigley Tom, Here is a thought. Seems to me ( is this right?) that we still get eventual complete melt of the glaciers even for the smallest temperature increase. Our original model did not do this. If I'm right it is another reason for tuning the original for comparison. Even with this flaw it could be that the formula works OK for a limited time domain ie 2400 but definitely not to equilibrium for small temperature changes. The 0.15 deg C comes from the Oerlemans et al 1998 paper I think. I will attach it if I can find it. As I recall it's just a number he dreamed up. cheers, On 16 Jul 2004, at 15:45, Tom Wigley wrote: > Sarah, > > Thanks again for all that useful material. I will absorb > it into the text. > > Although it will not make much difference, I guess > I should use an exponent for V in the final version > of the model. I will stick to n=1 for the analytic > results -- but n=1.2-1.4 seems necessary to be > state of the art. As far as MAGICC goes it makes > no difference since I solve the equation numerically. > > My 1880 start year comes from TAR p. 667 which > refers to 'the climate of 1865-1895'. What is confusing > is the T1990 term is relative to 'the late 19th century' > (p. 682, which I take as 1865-1895, equal near enough > to the 1880 value in MAGICC) --- but the g1990 > is relative to the glacier steady state. There is a 0.15degC > correction for the warming from this time to 1880, so the > steady state time must be well before 1880. In MAGICC > with DT2x=2.6degC the 1765-1880 warming is only > 0.1degC. > > All this is accounted for in MAGICC strictly following > the TAR, but there is still an inconsistency problem that > I noted before (and it's worse than I said before). The > AOGCM gu(1990) range is 1.5 to 2.7cm (I use the > average in MAGICC). The corresponding gs(1990) > value is pretty much the same. These must be changes > from the glacier steady state point, which must be well > before 1880 and probably before 1765. Table 11.10 > (1910-1990 data) suggests a change larger than this, > 2.4cm over 1910-1990 > . > The question is, 2.4cm from 1910 equals what from the > glacier steady state point? Even though 1910 was a > cool spot in the global temperature curve, the use of > the 0.15 correction implies that there must be positive > GSIC melt to 1880, and hence almost certainly positive > melt to 1910. So the 'observed' gs(1990) must be more > than 2.4cm. > > I guess there is nothing we can do about this. At worse > we will be about 1cm too low with gs(1990) if we stick > to the TAR AOGCM mean, and this won't affect the > longterm behaviour which must tend to V0. What is > needed is a model that runs from the steady state point > rather than 1990. Probably easy to do, but it would make > everything more messy and less directly comparable with > the TAR. > > Thwarted by Jonathan's ad hockery! > > Tom. >