SF Bay Watershed Analyst: About the Models

Why are there so many different future climate scenarios and why do scientists insist on using “ensembles” of future climate models?

We have relatively reliable data about past climate and water conditions thanks to weather stations, stream gauges, and other sensors coupled with our Basin Characterization Model that makes physics-based estimates of things that are hard to measure directly, like groundwater recharge and the amount of water used by plants. Regarding the future, we are most certain about LONG TERM TRENDS in temperature and its effects. Meanwhile there remains tremendous uncertainty about future rainfall patterns in our region.

Will we be facing extended droughts, catastrophic floods, or both? The answer is, we don’t know with certainty when or how severe future extreme events will be, but the models do suggest increased variability in our climate. So instead of considering just one physics-based climate-water future scenario for education, research or planning purposes, scientists encourage consideration of MULTIPLE scenarios, or an “ensemble” of computer models that are all physics-based, but may make different, but equally viable assumptions about how our water cycle will respond.

The future projections (often called simply "models") are combinations of GCM (global climate model) and RCP or emissions scenario (the amount of atmospheric carbon human are projected to produce in the future). The models chosen for use in the Watershed Analyst are a sub-set of 14 of the more than 60 models available from the International Panel on Climate Change (IPCC). The plot below displays the models' projections with of two important variables: precipitation and temperature. This helps us to visualize how they differ and see which models are projecting the most extreme cases (wettest and driest, and most increase and less increase in temperature).

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