date: Wed, 24 Nov 2004 11:07:17 +0000 from: Sarah Raper subject: my zero draft for CH10 to: Tom Wigley Dear Tom, At the end of this email please find my zero draft for ch10. I would much appreciate it if you would look through it. I have reviewed literature since the TAR and how that reflects on what we did in the TAR, and outlined what might be done in the AR4. It's only zero draft and likely most of it won't stay but it does set the tone for where we are going so its quite important for that. It is already about 3/4 the length of what I am allowed! cheers, Sarah PS will have some space for forcing in Collin's section. 10.5.2 Range of responses from different scenarios. The TAR projections with a SCM presented a range of warming over the 21st Century for all the SRES scenarios. The construction of the TAR Figure 9.x was pragmatic. It used a simple model tuned to AOGCMs that had a climate sensitivity within the long-standing range of 1.5 - 4.5 advocated by the IPCC. Models with CS outside that range were discussed in the text and allowed the statement that the presented range was not the extreme range indicated by AOGCMs. The figure was based on a single anthropogenic forcing estimate for 1750 to 2000, which is well within the range of values recmommended by TAR ch 6, and is also consistent with that deduced from model simulations and the observed temperature record (TAR ch 12.). To be consistent with TAR Ch 3. climate feedbacks on the carbon cycle were included. The resulting range of global mean temperature change from 1990 to 2100 given by the full set of SRES scenarios is 1.4 to 5.8 degC. Since the TAR several studies have examined the TAR projections and attempted probabilistic assessments. Allen et al, 2001 show that the forcing and SCM tunings used in the TAR give projections that are in agreement with the observationally constrained probabilistic forecast (Allen et al. 2000), reported in TAR ch x, stating that under the IS92a scenario anthropogenic warming is likely to lie in the range 0.1 deg to 0.2 degC over the next few decades. As noted by Schneider (2001), Jones (2000) and Moss and Schneider (2000), giving only a range of warming results is potentially misleading unless some guidance is given as to what the range means in probabilistic terms. Wigley and Raper (2001) interpret the warming range in probabilistic terms, accounting for uncertainties in emissions, the climate sensitivitiy, the carbon cycle, ocean mixing, and aerosol forcing. They give a 90% probability interval for 1990 to 2100 warming of 1.7 deg to 4.9 degC. As pointed out by Wigley and Raper (2001), such results are only as realistic as the assumptions upon which they are based. Key assumptions in this study were; that each SRES scenario was equally likely, that 1.5 to 4.5 corresponds to the 90% confidence interval for the CS, that carbon cycle feedback uncertainties can be characterised by the full uncertainty range of abundance in 2100 of 490 to 1260 ppm given in the TAR. The aerosol pdf was based on the uncertainty estimates given in the TAR together with constraints based on fitting the SCM to observed global- and hemispheric-mean temperatures. Several studies have used observational constraints to determine the range of likely future climates under specific emissions scenarios. Because CS is only weakly constrained by the observations Knutti et al (2002) admit higher warming for the specific scenarios studied compared to the TAR SCM projections. However, when they also constrain CS to be in the range 1.5 to 4.5 deg C they get results consistent with the those of the TAR. Stott and Kettleborough 2002 bypass the need to specify the CS and scale scenarios on the assumption that a model that over- or under-estimates the response by a ceratin fraction now will continue to do so by a similar fraction in the future. They give probabilistic results for specific emissions scenarios which admit higher warming than given in the TAR. Stott and Kettleborough 2002 mention that the reduction of SO2 emissions in the SRES scenarios in the latter half of the century increases the uncertainty range consistent with past observations. However, Wigley and Raper 2001 with their method, report that the 21st C decline in SO2 emmissions leads to a reduction in the effect of the very large present-day uncertainty range in aerosol forcing on future projections. Webster et al. (2003) use the probabilistic emissions projections of Webster et al.(2002) which consider present uncertainty in SO2 emissions, and allow the possibility of continuing increases in SO2 emissions over the 21st C, as well as the declining emissions consistent with SRES. Their main results give a CS pdf not unlike that used by Wigley and Raper (2001) but for aerosol forcing their pdf gives substantially smaller forcings compared to both Wigley and Raper (2001) and Knutti et al. (2002). This is likely to be a compensatory effect because they did not explicitly consider natural forcings. Since their climate model pdfs were constrained by observations and are mutually dependant the effect of the lower present day aerosol forcing on the projections is not easy to determine but there is no doubt that their projections tend to be lower where they admit higher SO2 emissions. Only the first of these studies (Wigley and Raper, 2001) considers the effect of carbon cycle feedbacks. None of the studies consider abrupt changes which are examined in another section. The aim of this section is to bring together information on emissions scenarios from WGIII, on forcings from Chapter 2, on the carbon cycle from Chapter 7, on attribution from chapter 8, and on model assessment in Chapter 9, together with the AOGCM responses examined in this chapter and to make projections with simplified models that are consistent with that information. There will be a figure 10.x1 comparable to TAR Figure 9.14, so that the new projections can be compared to the old. Observational constraints will be considered at least for the near term using information from Chapter 8 (attribution). As well as the SRES scenarios there may be new information from WGIII, with the possiblility of probabilistic and longer-term scenarios. It is likely that Chapter 2 will provide probabilistic forcings. In TAR Figure 9.14 a climate feedback on the carbon-cycle was included. We will seek the latest information on the strength of this feedback and its uncertainties from Chapter 7. These uncertainties will be concatenated where possible in probabilistic form to produce a new presentation of results in Figure 10.x2 (SCM) and Figure 10.x3 (EMIC, possible contribution from Knutti). It may be necessary to have an additional figure showing longer-terms scenarios (Figure 10.x4). The SCM will be tuned to emulate the AOGCMs using the PDMCI AR4 AOGCM modelling exercise data. For Figure 10.x1 the climate sensitivities will be the AOGCM effective climate sensitivities at the time of CO2 doubling and the ocean heat uptakes in the SCM will match those of the AOGCMs in the 1% CO2 increase experiment. Figures 10.x2 and 10.x3 may present results based on pdfs of the climate sensitivity and other inputs. Probabilistic temperature projections do not give true probabilities of occurrance but are conditional on the assumptions made in their construction. To convey this it may be wise to present more than one set of probabilistic projections, using for example different pdfs for the climate sensitivity (cf Box on pdfs of climate sensitivity).