Schwarzenegger, California and Climate

There’s an op-ed in the 21 January New Scientist from California Gov. Arnold Schwarzenegger laying out his case for action on greenhouse gas reductions:

In California, we are already seeing potentially severe impacts: shrinking of our snow pack in the Sierra Nevada mountains, which reduces our annual water supplies; erosion of our coastline; flooding of farmlands in the Central Valley; and a wide variety of health impacts related to changes in seasonal temperatures.

The snowpack/temperature piece of is being ably explored by Phil Mote and his various coauthors (see here for a bunch of Mote’s other work). But Schwarzenegger’s comments on flooding popped out because of a new paper in GRL by Edwin Maurer of Santa Clara University on the effect of warming on conditions in California during an El Niño.

Maurer and his colleagues did a modelling study of the effect of warming on the hydrologic cycle during El Niño, and found that the whole was greater than the sum of the ENSO/warming parts:

We find that increasing CO2 and rising temperatures cause a greater proportion of winter precipitation to fall as rain causing the typical shift in streamflow timing, as well as increased variability. A wet El Niño increases winter precipitation and snow accumulation with dramatic flow increases. The combination of a wet El Niño and increased CO2 shows all these effects; the striking result, however, is that the combined effects of a wet El Niño and warming are not additive. There is a clear amplification in winter months of mean flows and variability when climate warming is assumed on top of a wet El Niño. This suggests that the risk of winter floods and late-season water shortages may be increased due to the combined effects of El Niño conditions and climate warming.

A couple of things worth pointing out. The authors aren’t saying there will be fewer or more El Niños or La Niñas. That’s a point of controversy, with conflicting literature. They’re saying that, for a given El Niño, this sort of change shows up in their model under warming conditions. And the authors are clear that what they’ve done here is a sensitivity study, not a prediction: “As in the present climate, regional precipitation associated with El Niño will likely vary from one event to another; thus, the results presented here may not be typical of future-climate El Niños.”

Even with the caveats, this is the sort of work that gives folks at the regional level some sense of what they can expect and need to plan to cope with as they adapt to a changing climate.