7.1.1 Climate commitment introduced
‘Climate commitment’ considers the climate changes that are likely to occur from anthropogenic greenhouse–gas emissions up to the present day. The aim is to convey a measure of what has already been set in train and point out that further increases in global–mean surface temperatures (GSAT) will result even if anthropogenic greenhouse–gas emissions could be stopped immediately due to the sudden cessation of negative aerosol forcing, as discussed later in Section 7.1.2.
Climate commitment can be looked at in a number of different ways, with a range of authors having used various definitions. Wetherald et al. (2001) discussed warming commitment as the difference between realised warming at a specific time and equilibrium warming for a given GHG concentration. Hare and Meinshausen (2006) considered four forms of climate commitment:
1. Constant–emissions commitment. This is the warming that would result from maintaining present emissions. At current levels, this would lead to increasing GHG concentrations and hence further warming.
7.1. CLIMATE COMMITMENT 161 2. A constant–forcing commitment, or constant–composition commitment, with fixed GHG and aerosol concentrations. The warming commitment is due to the slow adjustment of the climate system to changes in radiative forcing. Warming would also continue until a new equilibrium temperature was reached.
3. Abrupt cessation of emissions, or geophysical commitment. The resulting temperatures are a function of the biogeophysical aspects of the climate system adjusting to the GHGs and aerosols in the atmosphere and uptake of GHGs into the biosphere and the ocean.
4. Feasible emissions scenario commitment, where warming is determined from a plausible emissions profile, taking into account current trends and feasible technological, economic and policy changes.
Wigley(2005) examined constant–composition commitments and constant–emissions commit-ments, pointing out that warming estimates are very dependent on climate sensitivity and aerosol forcing. Teng et al. (2006) noted that there is a commitment to additional warming and sea level rise even if atmospheric GHG and aerosol concentrations could be stabilised right now. Stabilis-ing GHG and aerosol concentrations at 2000 levels, their results showed that the globally averaged temperature from an average of 16 AOGCM models will be nearly 0.5◦C ± 0.2◦C warmer by the late 21st–century as compared to the 20th–century. Meehl et al. (2006) included consideration of climate commitment in their study, with GHGs and aerosols fixed at year 2000 levels. A number of model experiments were run with the CCSM3 coupled climate model with results that indicated a 0.4◦C committed warming for the constant–composition case, additional to the 0.6◦C rise over the 20th–century.
However, constant–composition commitment should not be regarded as the unavoidable warm-ing that will eventuate from past greenhouse–gas emissions. Matthews and Weaver (2010) argue that zero–emissions commitment is the more appropriate perspective. Constant–composition com-mitment requires continued GHG emissions otherwise carbon sinks will gradually reduce the at-mospheric concentration of CO2, and hence gradually declining GSAT.
Climate commitment lends itself to investigation with a simplified climate model such as MAGICC, since the different types of commitment can be readily modelled over long time peri-ods. The constant–composition case is effectively one in which radiative forcing is held constant.
GSAT then adjusts over time to a new equilibrium temperature. This was tested, as shown in Fig-ure 7.1, in which the global–mean temperatFig-ure change by 2100 is 1.2◦C, slowly rising to 1.3◦C by 2300 (very similar to that obtained by Matthews and Weaver, 2010).
For their zero–emissions results, Matthews and Weaver (2010) do not appear to have taken into account the loss of aerosols that would accompany zero CO2 emissions since there is no rise in temperature following the changeover point.
The most recent IPCC report discusses many of these issues in Chapter 7 of WG1, includ-ing the work of Brasseur and Roeckner (2005) (IPCC, 2007b, Figure 7.24), which considers the hypothetical removal of anthropogenic sulfate aerosol particles. Their results suggest that this would result in an immediate increase in the global average temperature of about 0.8◦C and pre-cipitation by 3%. This climate commitment case corresponds to constant GHG concentrations but
162 CHAPTER 7. FUTURE TEMPERATURES AND EMISSIONS PATHWAYS
18000 1850 1900 1950 2000 2050 2100 2150 2200 2250 2300 0.5
1 1.5
Anomaly, o C, wrt pre−industrial
Figure 7.1: Constant–composition commitment temperature change, based on a changeover in 2010 to constant radiative forcing.
no aerosols, related to the long atmospheric lifetimes of GHGs (decades to centuries), whereas aerosols have very short lifetimes (a few days only).
A recent paper by Ramanathan and Feng (2008) noted that the 2005 GHG concentration of about 455ppm CO2-equivalent corresponds to an estimated equilibrium temperature of 2.4◦C (1.4–
4.3◦C), which is into the range of ‘dangerous anthropogenic interference with the climate system’.
The planet has not yet experienced this increase because it is masked by the cooling effect of aerosols and the delay in ocean heat transfer, i.e., the equilibrium temperature will not be reached until some time in the future (centuries rather than decades). Ocean expansion, and hence sea–
level rise, will continue for many years to come. Note that the Ramanathan and Feng (2008) temperature change was based on different estimates for direct and indirect aerosol forcing as compared to those of the IPCC AR4.
CO2-equivalent concentration is used as a measure of the combined effect of the long–lived greenhouse–gases, but it has been used in different ways according to what components are in-cluded. For atmospheric concentrations, CO2-equivalent concentration can refer to the concen-tration of carbon dioxide that has the same warming effect as the combined effect of all of the greenhouse–gases, but without the cooling effect of aerosols (for example, Stern, 2007). Alter-natively, CO2-equivalent concentration is the net forcing of all anthropogenic radiative forcing agents including greenhouse–gases, tropospheric ozone, and aerosols but not natural forcings (for example, Meinshausen et al., 2006). In this thesis, all the CO2-equivalent concentration results are for the long–lived greenhouse-gases only (Kyoto protocol), and hence do not include aerosols or tropospheric ozone.1 MAGICC version 6 provides reports for both CO2-equivalent concentrations with all anthropogenic forcings included and for the long–lived greenhouse–gases only.
The zero–emissions commitment and fixed–emissions commitment cases are investigated next using MAGICC.
1The usage here is also not the measurement based on emissions, in which emissions of various greenhouse–gases are converted using their global warming potential (GWP).
7.1. CLIMATE COMMITMENT 163