> Is someone actually claiming as much as 35% uncertainty in > _global_ emissions?
They cite the AR4 draft as follows: "The net result of these combined regional reductions and increases leads to uncertainty in whether the global SO2 has increased or decreased since the 1980s" and "The most recent study (Stern, 2005) suggests a decrease in global anthropogenic emissions from approximately 73 to 54 TgS yr-1 over the period 1980 to 2000." Dividing 73 by 54 I get 1.35, or 35%, which is presumably where the number comes from. What I've just noticed is that this is from 1980 to 2000, rather than from 1990 to 2000. > > 2. If aerosols may have a forcing of similar magnitude as CO2 > Standard estimate is about 1W for aerosols versus 2.4 for GHGs, which is > stretching "similar magnitude" rather I was comparing with CO2 rather than with all GHG's, and the ranges from the AR4 are as follows: 1. 66 [1.49 to 1.83] for CO2 -0.5 [-0.9 to -0.1] aerosols direct effect -0.7 [-1.8 to - 0.3] aerosols indirect effect I don't really understand how they've come up with these uncertainty ranges and what they exactly mean, and in how far it is acceptable to just add them up, but if I just add up, I get -1.2 [-2.7 to -0.4] for aerosols http://www.ipcc.ch/SPM2feb07.pdf >We've already discussed how a > complete cessation of all aerosols could make an observable difference. Yes, you convinced me there that if forcing is 1 W/m2 and climate sensititivity is 3 C for a doubling of CO2 (3.7 W/m2), and it suddenly ceases, then within half a year to a year, you'd expect about 0.1C of warming by comparison with the effect of forcings as shown by seasonal temperature differences. http://en.wikipedia.org/wiki/Image:Climate_Change_Attribution.png As you know, global temperatures dropped between the 1940's and the 1970's, and if you look at the above graphic, the standard explanation seems to be that aerosols rose rapidly up to 1980 and then stayed constant, while greenhouse gases continued rising. > As per the above, I don't think it is plausible that the change in > aerosols had an overwhelming effect on the net radiation balance. Note > that even as much as say 30% change in the overall forcing might not > have seriously contradicted his forecast, which was really a statement > of the fact "there is a substantial net radiative imbalance, sufficient > to dominate natural variability for the next couple of decades assuming > any plausible emissions scenario". With hindsight, not perhaps so > spectacular, but at its time... > > Note that climate forecasts based on the full range of SRES emissions > barely diverge in ~30 years, even though the emissions themselves do. I do understand that GHG emissions accumulate and it's the cumulated effect that matters, rather than the emissions in ~30 years. But aersols don't accumulate, and it's only thermal inertia that buffers the impact. Are there SRES emissions scenarios that are high in GHG's and low in aerosols, or vice versa? The attribution graph I've cited above does not account for thermal inertia, and it's not clear what the underlying forcings are. I'd really want a separate graph for that, including uncertainty ranges. And I didn't quiite find one, but this from the TAR at least gives central estimates: http://www.grida.no/climate/ipcc_tar/wg1/fig6-8.htm Unfortunately it doesn't include the indirect aerosol effect. Coming back to Hansen's prediction. Temperatures weren't montonously climbing in the decades prior to 1987. How did he know there was "substantial net radiative imbalance ..." irrespective of the emissions scenario? Rising aerosols (and natural variability?) were clearly able to stop the rise in the decades before. Maybe I am overlooking something, or see a contradiction where there is none. But, it seems contradictory on the one hand to cite aerosols for the cooling of the decades to the 1970's and then dismiss 30% changes in aerosol forcing subsequent to the 1970's as surely irrelevant compared to the large GHG forcing. --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups Global Change ("globalchange") newsgroup. Global Change is a public, moderated venue for discussion of science, technology, economics and policy dimensions of global environmental change. Posts will be admitted to the list if and only if any moderator finds the submission to be constructive and/or interesting, on topic, and not gratuitously rude. 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/globalchange -~----------~----~----~----~------~----~------~--~---
