Michael Dettinger US Geological Survey, Scripps Institution of Oceanography, La Jolla, CA with figures from Dan Cayan, Noah Knowles, & Iris Stewart Towards a PDF of 21st Century Hydroclimate Changes in California

…in response to accelerations of the greenhouse effect that began 30+ years ago! In the near future, global-warming trends are likely to be superimposed upon the ‘normal’ climate variations that our resource systems accommodate... Extra Heat Trapped by CO2 & other Emissions

Under this particular forcing, climate models yield a narrow range of warming scenarios for the West.

So that by the middle of the 21st Century, even in the coolest of the models, earlier snowmelts & major reductions in snowpacks of western mountain ranges are projected... from Noah Knowles & Cayan, GRL, 2002 No snow left CENTRAL VALLEY Most snow left SIERRA NEVADA

Dettinger et al, 2004 Resulting changes in streamflow timing may mean less “useful” water, i.e., less runoff captured in Western reservoirs for warm-season use. TOTAL RIVER DISCHARGE, APRIL-JULY from ACPI Parallel-Climate model simulations, –30% Sierra-wide, this reduction in spring outflow amounts to about 2/3 of the total maximum flood-control reservation in headwater and lower-basin reservoirs.

With the timing changes will come more severe winter floods… Dettinger et al., 2004;

…and, with runoff leaving basins earlier, summer conditions will be much drier, summer streamflow will decline, & wildfire risks may increase. Dettinger et al., 2004 Simulated Soil Wetness Water stored in soil, inches

But what about precipitation? Overall, climate models provide no consensus regarding the future of precipitation in the West.

Even in the models projecting little change in mean precipitation, extreme precipitation events (at daily time scales) increase markedly… Overall precipitation From the rest of the ppt events From the largest 5% of ppt events Precipitation amount, means shifted PROJECTED TRENDS IN PRECIPITATION BETWEEN 30N-60N From Cayan, in preparation, from ACPI Parallel-Climate model simulations,

…but, in the current models, no consistent trends towards longer multi-year deviations from the (changing) mean. Dettinger and McCabe, in preparation,

Why do some models project a wetter West, and others not? To answer this, you need to know that El Nino’s don’t get involved in global warming trends in all climate models; indeed, in MOST models they don’t! The PCM is a model in which it appears to… El Nino temperatures

Most models act like this… …and only those that don’t, yield wetter Californias! Southern Oscillation Tropical convection … however--although the tropics warm--the atmosphere doesn’t respond as with an El Nino! Measures of atmospheric effects of El Ninos

So…why do some models project a wetter West, and others not? Very preliminarily…it depends on whether global warming looks like El Nino or not. …and the jury is still out as to what El Nino will actually do. Dettinger, in preparation,

…if scenario were warmer… …if warmer… Should we wait for more certainty, or can we already ask “How much wetter must it be to avoid increased flood risks? To retain the current spring snowmelt season?”…

We also don’t know exactly how future greenhouse-gas emissions will evolve.

No time to lay out details now, but… By shuffling the interannual (noisy) fluctuations among the projections, we can objectively develop estimates of the probability distribution functions (PDFs) implied by the 18- member “super- ensemble” of projections…

The procedure yields an estimate of the time- varying joint probability distributions of temperature and precipitation changes during the 21st Century, allowing us to better visualize what the more common projections are doing…rather than being entranced by the outliers.

Here’s what the resulting PDFs at various times in the 21st Century look like… Broadening because of growing model-to-model differences and growing scenario-to-scenario differences. Modest drying is the most common projection result, with a heavy tail towards wetter (due solely to the Canadian model)

Ultimately, such climate PDFs would imply (broadly) corresponding changes in the PDFs of flows… Fig. 16b, Jeton et al., 1996, USGS WRIR Simulated North Fork American flow responses to imposed temperature and precipitation changes

… and the timing of flows. Fig. 17c, Jeton et al., 1996, USGS WRIR Simulated North Fork American timing responses to imposed temperature & precipitation changes Temperature change, ºC Precipitation change, %

NEXT STEPS Add more projections to the ensemble while maintaining a fair and balanced mix Usesingle-model/single-emissions/multiple-ICs,multiple-model/single-emissions/single-IC,single-model/multiple-emissions/single-IC ensembles to characterize contributions to projection scatter Weight the resampling to favor models based on their skill with historical ENSO responses, historical seasonal cycles, and historical climate changes

FINDINGS Northern California temperature projections are broadly in consensus (+3 to +6 or more ºC), enough for earlier flows, more floods, & drier summers Warming already underway and coming fast Northern California precipitation projections are a bit more scattered, with MOST showing small (drier?) changes but with 2 outliers much wetter Whether a scenario yields wetter California depends on how El Nino responds to the warming (which may need to be decided outside the models) How much would precipitation have to change in order to compensate for warming? (± about 25%)

Projected streamflow timings, vs …throughout the West,… Iris Stewart et al., in press

…to be added to historical trends already being observed in the region. Dettinger and Cayan, 1995; Cayan et al., 2001 Stewart et al., in press

Moderate climate changes of mean conditions translate into large changes in the extremes, at both the cold … from ACPI Parallel-Climate model simulations,

…and the warm ends of the thermometer, … from ACPI Parallel-Climate model simulations,

… because of characteristic shapes of the statistical distributions of important climate variables. from ACPI Parallel-Climate model simulations,

At higher resolution, Sierra Nevada climate change may differ from other parts of California… NOTE: See Snyder et al (UCSC), 2002, GRL, or various recent efforts by Miller (UCB) and/or Kim (UCLA), for more discussions of this effect. MM5 Regional-Climate Model output driven by Parallel Climate Model simulations, by Ruby Leung, PNNL. 2001

…because climate changes can vary orographically as well. MM5 Regional-Climate Model output driven by Parallel Climate Model simulations, by Ruby Leung, PNNL. 2001

Wilby & Dettinger, 2000 You probably have heard of climate models that project a wetter California! Hadley Center (HadCM2) Climate model December-February Precipitation Projections P( ) / P( ) +300% with a +3C warming

So, perhaps, we will be able to quantify these uncertainties objectively and thereby accommodate them in our planning.