23 The third RDM step in Figure 2.1, Scenario Discovery helps address these questions (Bryant _et al._ 2010; Hallegatte _et al._ 2012). As described in more detail in Appendix B, we apply scenario discovery algorithms to the database of 1,000 cases to identify the combinations of uncertain conditions that most reliably distinguish those 36 cases in which it satisfies objectives from the 974 cases where it does not.

Scenario Discovery reveals that four conditions together describe a scenario in which the Baseline Strategy satisfies objectives:

1. The increase in rainfall intensity is less than 6%,

2. The Saigon River rises by less than 45 cm, 3. Ho Chi Minh City’s population is les than 18 million, and 4. Ho Chi Minh City’s poverty rate is less then 23%.<sup>18</sup> Assumptions about population distribution and about the vulnerability of the population have much less effect on whether or not the Baseline Strategy satisfies decision makers’ risk management objectives.

Scenario discovery also reveals that future population and poverty rates are statistically less important predictors of strategy performance than are rainfall intensity and Saigon River rise. Moreover, as long as rainfall intensity and Saigon River rise remain below 6% and 45cm, respectively, the Baseline Strategy satisfies objectives in almost the full range of population (up to 19.1 million) and poverty rate (up to 25%) that we consider in this study. Therefore, we focus our attention on rainfall intensity and Saigon River levels in assessing robustness.

Figure 5.4 plots the range of rainfall intensity increase (6%, on the horizontal axis) and Saigon River levels (45 cm, on the vertical axis) under which the Baseline Strategy meets its objectives. We use this visualization to compare these conditions to the best available scientific evidence about future rainfall intensity and Saigon River levels. The IPCC Special Report on Extreme Events (IPCC 2012) suggests that future rainfall intensity could increase by as much as 35%, with a middle estimate of 20% (noted by vertical reference lines on Figure 5.4). These projections significantly exceed the 6% threshold to which the Baseline Strategy is robust. The Vietnamese Ministry of Natural Resources and the Environment (MONRE) projects that eustatic sea level rise will be approximately 30 cm by mid-century, compared to 1980-1999 levels. However, more recent studies suggest that it may higher due to rapid melting of ice sheets and glaciers, which was not taken into account in previous studies. The U.S. National Oceanographic and Atmospheric Administration (2012), for example, suggests an increase of approximately 40 cm by midcentury. When coupled with even modest rates of subsidence, one plausible future is an increase in height of 75 cm (noted by horizontal reference lines in Figure 5.4). Other factors, such as the construction of dikes around the river or faster subsidence, may further increase the height.

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> 18 Appendix B provides a more complete summary of this Section’s scenario discovery results.  In particular, three scenarios, each with relatively low coverage and density, are required to achieve adequate total coverage and density to describe the future conditions where the Baseline Strategy reduces risk for both poor and non-poor populations. For simplicity, we only describe here the one scenario from this group of three with highest coverage and density. However, the best-estimate future conditions shown in Figure 5.2 lie in one of the other two scenarios.