In Colorado River Basin planning, there is a common mistake growing out the the Bureau of Reclamation’s Basin Study. It is made by seizing on the study’s finding regarding the impact of climate change (as exemplified by the results of General Circulation Models, or GCM’s) on the river, quoted here from the executive summary (pdf), and then arguing for a portfolio of policy options to make basin water use sustainable given that new reality:

Under the Downscaled GCM Projected scenario, the median of the mean natural flow at Lees Ferry over the next 50 years is projected to decrease by approximately nine percent, along with a projected increase in both drought frequency and duration as compared to the observed historical and paleo-based scenarios.

This is a helpful prod, but it’s not quite right to interpolate that into “Basin Study predicts 9 percent reduction in flow”, as I frequently see done. That 9 percent number is better thought of as the midpoint in a very uncertain future, as the Basin Study goes on to explain:

The range of this result varies amongst the individual GCM projections that comprise this scenario with some of the GCM projections showing a larger decrease in mean natural flow than nine percent while others showing an increase over the observed historical mean.

The problem is nicely explained in this work from a team at RAND, which worked with the Bureau on the Basin Study:

Reclamation and the water agencies must deal not with a future that is uncertain but well understood; instead, they must plan for a future that is deeply uncertain and one that cannot be described statistically because of a lack of knowledge about how changes will unfold. Under these conditions, developing an optimal management strategy designed to perform well for a single deterministic or probabilistic forecast of future conditions is not very useful; rather, planners need a robust and adaptive strategy—robust in that it performs well over a wide range of possible futures and adaptive in that it can adjust over time in response to evolving conditions.

This recent Ensia piece by Melinda Harm Benson and Robin Craig seems to be heading off in the right conceptual direction:

The concept of resilience holds promise as a new way of addressing the challenges ahead. While not inherently incompatible concepts, resilience and sustainability are not the same. The pursuit of sustainability assumes that we a) know what can be sustained and b) have the capacity to maintain stationarity (i.e., keep the system operating within an unchanging envelope of variability). In contrast, resilience thinking acknowledges disequilibrium and nonlinear, continual change — often as a result of crossing a “tipping point” or threshold — and offers a tool for assessing the dynamic relationships between systems.