Bridging the Gap Between Top-Down and Bottom-Up Climate Vulnerability Assessments: Process Informed Exploratory Scenarios Identify System-Based Water Resource Vulnerabilities

The threat that climate change poses to water resource systems has led to a substantial and growing number of impact studies. These studies follow two approaches: (a) top-down studies are driven by projections of future climate change provided by downscaled general circulation models (GCMs); and (b) bottom-up studies are driven by the systematic evaluation of exploratory scenarios. Top-down approaches produce realistic scenarios rooted in the simulation of thermodynamic and dynamic processes represented in GCMs, but the internal resolution of these processes make it difficult to link vulnerabilities to discrete components of change. Bottom-up approaches link vulnerabilities to discrete components of change through the structured evaluation of exploratory scenarios, but the lack of insight rooted in climate change processes can lead to the development of implausible scenarios. This paper evaluates exploratory scenarios developed through thermodynamic and dynamical guided perturbations motivated by GCM-bound insights. The result is a hybrid approach that bridges a gap between top-down and bottom-up approaches. This yields several advantages. First, emerging vulnerabilities are linked to distinct thermodynamic and dynamic processes that are modeled in GCMs with differential likelihoods and plausible ranges of change. Second, the structured evaluation of process-informed exploratory scenarios link system vulnerabilities to distinct components of climate change. An illustrative case study demonstrates perturbations linked to thermodynamic and dynamical processes have a large impact on stakeholder-defined flood and drought performance, and the structured evaluation of process-informed exploratory scenarios find nuanced infrastructure-specific vulnerabilities that would be difficult to identify using an alternative approach.