Guest Post by Dr Tom M. L. Wigley. Tom is a a senior scientist in the Climate and Global Dynamics Division of the US National Center for Atmospheric Research and former Director of the CRU. He is an adjunct Professor at the University of Adelaide. For his list of papers and citations, click here (his h-index is 70!). Tom is also a good friend of mine and a strong supporter of the IFR.
What would happen to CO2 concentrations, global-mean temperature and sea level if we could reduce total CO2 emissions (both fossil and net land-use change) to zero by 2050? Based on the literature that examines possible policy scenarios, this is a virtually impossible goal. The results presented here are given only as a sensitivity study.
To examine this idealized scenario one must make a number of assumptions. For CO2 emissions I assume that these follow the CCSP MiniCAM Level 1 stabilization scenario to 2020 and then drop linearly to zero by 2050. For the emissions of non-CO2 gases (including aerosols and aerosol precursors) I assume that these follow the extended MiniCAM Level 1 scenario (Wigley et al., 2009). The extended Level 1 scenario provides emissions data out to 2300. Note that the Level 1 scenario is the most stringent of the CCSP stabilization scenarios, one that would almost certainly be very costly to follow using traditional mitigation strategies. Dropping CO2 emissions to zero is a much more stringent assumption than the original Level 1 scenario, in which total CO2 emissions are 5.54GtC/yr in 2050 and 2.40GtC/yr in 2100.
For modeling the effects of this new scenario one must make assumptions about the climate sensitivity and various other model parameters. I assume that the sensitivity (equilibrium warming for 2xCO2) is 3.0C, the central estimate from the IPCC AR4. (Note that the 90% confidence interval for the sensitivity is about 1.5C to 6.0C – Wigley et al., 2009.)
For sea level rise I follow the AR4 and ignore the possible effects of accelerated melt of the Greenland and Antarctic ice sheets, so the projections here are almost certainly optimistic. All calculations have been carried out using version 5.3 of the MAGICC coupled gas-cycle/climate model. Earlier versions of MAGICC have been used in all IPCC reports to date. Version 5.3 is consistent with information on gas cycles and radiative forcing given in the IPCC AR4.
The assumed CO2 emissions are shown in Figure 1.
The corresponding CO2 concentration projection is shown in Figure 2. Note that the MAGICC carbon cycle includes climate feedbacks on the carbon cycle, which lead to somewhat higher CO2 concentrations than would be obtained if these feedbacks were ignored.
Global-mean temperature projections are shown in Figure 3. These assume a central climate sensitivity of 3.0C. Temperatures are, of course, affected by all radiatively active species. The most important of these, other than CO2, are methane (CH4) and aerosols. In the Level 1 scenario used here both CH4 and aerosol precursor (mainly SO2) emissions are assumed to drop substantially in the future. CH4 concentrations are shown in Figure 4. The decline has a noticeable cooling effect. SO2 emissions drop to near zero (not shown), which has a net warming effect.
The peak warming is about 0.9C relative to 2000, which is about 1.7C relative to pre-industrial times. This is below the Copenhagen target of 2.0C – but it clearly requires a massive reduction in CO2 emissions. Furthermore, the warming peak could be significantly higher if the climate sensitivity were higher than 3.0C. For a 3.0C sensitivity, stabilizing temperatures at 2.0C relative to the pre-industrial level could be achieved with much less stringent CO2 emissions reductions than assumed here. The standard Level 1 stabilization scenario, for example, gives a 50% probability of keeping below the 2.0C target.
Figure 5 gives the sea level projection for the assumed scenario. This is a central projection. Future sea level is subject to wide uncertainties arising from uncertainties in the climate sensitivity and in parameters that determine ice melt. As noted above, the projection given here is likely to be an optimistic projection. Note that sea level roughly stabilizes here, at a CO2 concentration of about 320ppm. Less optimistic assumptions regarding the emissions of non-CO2 gases would require a lower CO2 concentration level. Given the unrealistic nature of the assumption of zero CO2 emissions by 2050, this is a graphic illustration of how difficult it would be to stabilize sea level – even at a level more than 20cm above the present level.
T. M. L. Wigley, L. E. Clarke, J. A. Edmonds, H. D. Jacoby, S. Paltsev, H. Pitcher, J. M. Reilly, R. Richels, M. C. Sarofim and S. J. Smith (2009) Uncertainties in climate stabilization. Climatic Change 97, 85-121, DOI: 10.1007/s10584-009-9585-3.