The three contributing factors need to be combined into a single aggregated measure—the impact of human caused climate change (impact). Impact must still be translated into societal consequences, which we will do in step 5.
Combining the contributing factors requires considering how they might interact. Three straightforward approaches capture a very broad range of possible interactions. For each approach the values for the contributing factors (Low, Moderate, or High) map directly onto impact (i.e., impact can be Low, Moderate, or High).
Impact could take the lowest, highest, or average value among the three contributing factors.
Using the lowest value of the contributing factors, assumes a single point safety system in which any of the three factors confers protection from the other two. For example, if there were no climate change at all, then it would not matter how sensitive physical systems, biological resources, and social institutions are to climate changes. Using the highest value of the contributing factors assumes a single point failure system in which any of the three factors determines impact no matter how lucky we get with the other factors. For example, if human society were highly dependent on existing physical systems, biological resources, and social institutions, then even small climate changes could result in high levels of impact. The equal weight average approach assumes that each factor contributes equally to impact.
 The counterfactual will become an important issue in step five. For now, by the impact of human-caused climate change we consider impact relative to pre-industrial climate conditions. When we consider societal consequences, we must include the possibility of different counterfactuals, including the potential for other influences that might contribute additional warming or cooling influences. This is because the consequences of human caused warming could be offset by cooling influences (e.g., volcanic eruptions, meteor impacts, nuclear war) or exacerbated by warming influences.
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