The choice of stabilisation level

The last few sections have addressed the main greenhouse gases and how their concentrations might be stabilised. To decide how appropriate stabilisation levels should be chosen as targets for the future we look to the guidance provided by the Climate Convention Objective (see box on pages 291-2), which states that the levels and timescales for their achievement should be such that dangerous interference with the climate system is avoided, that ecosystems are able to adapt naturally, that food production is not threatened and that economic development can proceed in a sustainable manner.

The balancing of these scientific, economic, social and political criteria presents a large challenge. In Chapter 9 (see box on page 280) the concept of Integrated Assessment and Evaluation was introduced which involves employment of the whole range of disciplines in the natural and social sciences. Taking all factors into consideration will involve different kinds of analysis, cost-benefit analysis (which was considered briefly in Chapter 9), multicriteria analysis (which takes into account factors that cannot be expressed in monetary terms) and sustainability analysis (which considers avoidance of particular thresholds of stress or of damage). Further, because uncertainty is associated both with many of the factors that have to be included and with the methods of analysis, the process of choice is bound to be an evolving one subject to continuous review - a process often described as sequential decision-making.

In Figure 10.2 were presented options for the stabilisation of carbon dioxide and other greenhouse gases in terms both of the concentration of CO2e (a) and of the global average temperature increase since pre-industrial times (b). It is the latter measure that is closer to what actual climate change is all about. The information in Figure 10.2 and Table 10.3 covers a wide range of possible temperature increases up to over 6 °C. However, attention has recently focused on the lower end of the range, between 2 and 3 °C.

In Chapter 7 we found that many of the studies of impacts of climate change had been made under the assumption that the atmospheric CO2e concentration had doubled from its pre-industrial value of280 ppm to about 560 ppm and that the global average temperature had increased from its pre-industrial value by a best estimate of 3 °C - an estimate that was raised in the 2007 IPCC Report from its earlier value of 2.5 °C (see Chapter 6, page 143). We enumerated in Chapter 7 the substantial impacts that apply to this situation together with estimates of their associated costs.25 In Chapter 9 it was pointed out, even considering only those costs that can be estimated in terms of money, that estimates of the cost of the likely damage of impacts at that level of climate change were substantially larger than the mitigation costs of stabilising CO2e concentration at doubled pre-industrial CO2. We also noted that damage due to anthropogenic climate change is likely to rise more rapidly as the amount of carbon dioxide in the atmosphere increases. These considerations suggest that limits set at 560 ppm CO2e or 3 °C would be too high.

A widely publicised target for the maximum allowable rise in global average temperature is 2 °C. It was proposed by the European Union over 10 years ago26 and has been recently reiterated by the EU,27 by governments (e.g. by Chancellor Merkel of Germany before the 2007 G8 conference) and by many other organisations.28

How important is it to aim at 2 °C rather than say 3 °C? Further perspective on this can be obtained from Table 7.1 which indicates that the impacts at 2 °C above pre-industrial are substantially less severe compared with those at 3 °C, for instance in terms of water stress, extinction of species, coral mortality, decreased crop productivity, ocean acidity, increased floods, droughts and storms and risk of more rapid sea level rise. As an example, we can turn to Figure 6.12 to compare more quantitatively the risk of drought for different increases in global average temperature. Noting from Figure 6.4a, that for the A2 SRES scenario, as followed by Figure 6.12, global average temperature increases by 2 °C and 3 °C above the pre-industrial value by about 2050 and 2075 respectively. As mentioned in Chapter 6 (page 158) and shown in Figure 6.12 the proportion of land area under extreme drought increased from around 1% in 1980 to 2% or 3% now and is projected to increase to about 10% in 2050 and 20% in 2075, indicating a likelihood of a rise from today in the risk of extreme droughts of a factor of up to 8 for a rise of 3 °C in global average temperature compared with a factor of up to 4 for a 2 °C rise.

Stabilisation below 2 °C might also avoid some of the worst impacts; for instance, some of the large-scale dieback of forests and the transition of the biosphere from a sink to a source for carbon dioxide (see box in Chapter 3 on page 48-9) that would otherwise occur around the middle of the twenty-first century. It would also reduce the risk mentioned earlier in the chapter of large-scale release of methane as the Arctic ocean or tundra warm.

How achievable is 2 °C? From Figure 10.1b and Table 10.3, it can be seen that 2 °C implies an equilibrium radiative forcing of 2.5 W m-2 and about 450 ppm stabilisation for CO2e. Since these are best estimates, what we can say is that 450 ppm CO2e and 2.5 W m-2 should provide a 50% chance of achieving 2 °C.

We now need to know what 450 ppm CO2 e implies for CO2 itself. As was explained in Chapter 6 page 149 (see also Figure 3.11), the negative radiative forcing of anthropogenic aerosols at the present time approximately balances the positive contributions from greenhouse gases other than carbon dioxide. Earlier in the chapter, it was pointed out that options are available for preventing further increases and reducing the contributions from methane, nitrous oxide and halocarbons. Also aerosol scenarios show little reduction in the magnitude of the aerosol contribution during the next few decades.29 These considerations provide a rational basis for the time being for coupling the 2 °C temperature target with a stabilisation target of 450 ppm for CO2 alone (Figure 10.3). A similar argument couples together a 3 °C temperature target with 550 ppm for carbon dioxide alone. It is in fact these assumptions that are made in Chapter 11 when the challenging implications of such targets are presented for the future of the energy and transport sectors.

In accepting the targets of 2 °C and 450 ppm as a basis for action, I must however inject two notes of caution. The first is that the calculations I have presented are based on best estimates only. They have not allowed for uncertainties; 450 ppm stabilisation for carbon dioxide only provides a 50% chance of achieving 2 °C. An 80% chance of achieving 2 °C would require stabilisation at about 380 ppm, the current atmospheric carbon dioxide level.30

The second point of caution pertains to the future of sulphate aerosols that provide the largest component towards aerosol cooling mentioned in the last paragraph. Because they lead to serious low level pollution and also to 'acid rain', there is large pressure to control and reduce the sulphur dioxide emissions that are the precursors of sulphate aerosols. Reductions can also be expected as the consumption of coal and oil is phased out. Many future aerosol scenarios therefore show large reductions of sulphur emissions especially during the second half of the twenty-first century. Now, because the life time of aerosols in the atmosphere is very short (a few days) reductions in aerosol emissions lead almost immediately to large changes in aerosol concentrations and hence also in radiative forcing. Compensating changes in forcing through changes in CO2 emissions can only occur much more slowly because of the long life time of carbon dioxide in the atmosphere (Chapter 3 page 37). This means that, in order to maintain a 2 °C target for global average temperature rise, reductions in global sulphur dioxide emissions need to be anticipated well in advance by matching reductions in CO2 emissions. In fact, that anticipation should begin now; it will likely mean that global CO2 emissions should be reduced close to zero by 2050 and total greenhouse gas (CO2e) emissions to zero before the end of the century. I return to these issues in the section entitled A Zero Carbon Future in Chapter 11, page 378.

Is it possible to consider targets lower than 2 °C and 450 ppm carbon dioxide? Considering the most important greenhouse gas, carbon dioxide, as we have already noted, its long life in the atmosphere provides severe constraints on the future emission profiles that lead to stabilisation at any level. The concentration of carbon dioxide in the atmosphere is already above 380 ppm which means (Figure 10.2) that stabilisation of carbon dioxide alone below 400 ppm would require an immediate drastic reduction in emissions. Such reduction could only be achieved at a large cost and with some curtailment of energy availability and would almost certainly breach the criterion that requires 'that economic development can proceed in a sustainable manner'.

Many are, however, asking the question whether a 2 °C target will be adequate to stabilise the climate against very damaging and irreversible change.

Prominent among these is Professor James Hansen at the NASA Institute for Space Studies at New York. In a recent paper,31 Hansen argues largely from palaeoclimate evidence that a 350 ppm target is necessary to avoid the danger of rapid collapse of the Greenland and West Antarctic ice-sheets and other serious non-linear processes (see Table 7.5). Such a target could only be realised after substantial overshoot in the early years and would probably also require a large programme over many decades of sequestration of carbon dioxide already in the atmosphere. The possibility of such a programme has also recently been proposed by Professor Wally Broecker of Columbia University in the USA.32 Targets such as that aimed at 2 °C that may be set now are bound to be reviewed and revised during the next few years and decades as more information becomes available regarding how 'dangerous' climate change can be defined and avoided.

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