will comprise some investment, including possible investment in research and development (R&D), in each of the three response options.

# _4.2 Response Options_

# _4.2.1 Dramatic Abatement_

Dramatically and rapidly abating GHG emissions would lower the risks of climate change catastrophes, although determining the relationship between emissions levels and catastrophe probabilities is difficult. The results of the IPCC AR4 suggest a growing level of risk to human well-being as warming goes from 2.5<sup>o</sup> C to 4.5<sup>o</sup> C or higher above preindustrial levels.<sup>13</sup> The IPCC report indicates that limiting temperature increases to around 2.5<sup>o</sup> C would require stabilization of GHG concentrations near 450 parts per million by volume (ppmv) (Fisher et al. 2007). This can be compared to projections of almost 700 ppmv in 2100 under business as usual (Nordhaus 2008) and associated temperature increases in excess of 5<sup>o</sup> C (Yohe et al. 2007). To achieve a target of 450 ppmv, GHG emissions would have to be reduced quickly and dramatically—global emissions would have to start to fall before 2015 and be reduced to less than 50 percent of current emissions by 2050 (Fisher et al. 2007). Even stabilization at 550 ppmv, which could lead to warming between 2.8<sup>o</sup> C and 3.2<sup>o</sup> C, would require global emissions to peak in less than two decades and return to year 2000 emissions by 2040 (Fisher et al. 2007).

A few studies have attempted to estimate temperature and/or emissions levels that would prevent some of the specific catastrophes discussed in Section 2. O’Neil and Oppenheimer (2002) estimate that limiting temperature increases to 2<sup>o</sup> C above 1990 temperatures could conceivably guard against collapse of the West Antarctic ice sheet, while limiting increases to 3<sup>o</sup> C over the next 100 years may prevent collapse of thermohaline circulation in the Atlantic. Thus, both might be prevented if concentrations were stabilized at 450 ppmv. Other studies suggest a higher level of concentrations before substantial weakening of thermohaline circulation in the Atlantic (Keller et al. 2000). The IPCC Fourth Assessment Report estimates several meters of sea level rise for warming between 2<sup>o</sup> C and 5<sup>o</sup> C above 1980–1999 levels as well as weakening of the meridional overturning circulation (Yohe et al. 2007). All estimates are dependent on assumed values of key parameters, particularly climate sensitivity and rate of emissions (Zickfeld and Bruckner 2003) and are thus very uncertain.

More intensive abatement may be needed to prevent a cascading catastrophe. For instance, even with only 2<sup>o</sup> C of warming relative to the late twentieth century, 1–2 billion additional people could face increased water stress in the twenty-first century (Kundzewicz et al. 2007). The comparable range for a 1<sup>o</sup> C warming is 0.4–1.7 billion people. Findings also vary regionally and demonstrate some large jumps in vulnerability with greater warming (Yohe et al. 2007). Conditions could thus be favorable for cascading catastrophes related to water scarcity and conflict for warming less than 2<sup>o</sup> C—though we stress again the lack of knowledge available about sufficient conditions for cascading.

# _4.2.2 Geoengineering_

The term “geoengineering” is defined as intentional large-scale manipulation of the Earth’s environment in order to combat the effects of changes in atmospheric chemistry (National Academy of Sciences 1992; Keith 2000). Current proposals for using geoengineering

> 13 See Yohe et al. (2007, Table 20.8).  Others suggest we may face “dangerous” climate change with as little as a

> 1<sup>o</sup> C warming (Hansen 2005).

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