> I have come across a small puzzle, but am unable to solve it myself. I > was looking around a bit to find our present concentration of GHGs > measured in CO2-equivalents.
What you can do is the following. Take the GHG forcing figures from the AR4: http://www.ipcc.ch/SPM2feb07.pdf 1.66 (CO2) 0.48 (Methane) 0.16 (N2O) 0.34 (Halocarbons) Add them up and you get 2.64 W/m2 You then divide that by 3.7 W/m2 (the forcing for doubled CO2) and get 0.71 or 71% ie total GHG forcing is 71% of doubled CO2. Now raise 2 to the power of 0.71 and you get 1.64 (or in other words to get 71% of the forcing we only need 64% of the CO2 concentration increase) and multiply that with 280 ppm, to give you: 459 ppm CO2 equ And instead of your table, you could stick to James Annan's well known theme (climate sensitivity is 3C). Just multiply 71% with 3C and you get 2.14C as the equilibrium response to present GHG forcing. But, total GHG forcing is currently offset by aerosols to a poorly known degree. If you stick in total forcing compared to pre-industrial, namely 1.6 W/ m2, things look considerably better. 1.6 W/m2 is less than half of 3.7 W/m2, and gives you an equilibrium response of 1.3C. Due to thermal lag, only about 0.7C are already realised and another 0.6C are in the pipe-line. One of my pet themes is aerosols. What you'll notice from these figures is the consequences of reducing sulfate emissions towards zero, while say reducing CO2 emissions just enough to keep concentrations constant. As about half the CO2 is sunk at the moment, that's also roughly the reduction required to keep concentration constant. If we do that instantaneously, and simultaneously eliminate aerosols, we've got a slightly better than 50% chance of missing a 2C target. If we keep all forcings constant, however, I get that the climate sensitivity would have to be above 3 times 2/1.3 or more than 4.5 C, which means I think that a 2C target could be met with greater than 95% probability. The critical importance of aerosols is something that is completely lost in the public debate. But it is the loss of aerosol cooling from emissions reductions that means we likely need huge cuts in GHG emissions to avoid 2C with greater than 90% probability. And huge means something like a 50% within 20 years and a 90% within 50. If you combine that with converge and contract for equity reasons, Europe needs 80% cuts by 2020, the US 90% cuts by 2020, and both need 95%plus reductions by 2050. These are fantastical reductions, and I think, mean that either we'll have to accept that 2C isn't so dangerous we couldn't accept a 50% probability of it happening, or we'll have to do some aerosol / tropical cloud albedo geoengineering to compensate for current unintended side effect aerosol geoengineering. We could also continue current sulfate emissions levels, in spite of the fact that the sulfates are emitted right where people live. --~--~---------~--~----~------------~-------~--~----~ 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 -~----------~----~----~----~------~----~------~--~---
