TAHOE NUGGET #236: CLIMATE CHANGE & SNOWPACK DEPLETION
In response to a provocative prediction for accelerated California climate change that I read in the Sacramento Bee newspaper last week, I felt compelled to contribute the following blog to the dialogue.
Warnings about regional climate change were kicked up a notch earlier this month with the recently released report by Robert Shibatani, a Sacramento-based hydrologist who is also CEO of The Shibatani Group Inc.
This new analysis offers dire predictions for the Sierra snowpack based on projected warming temperatures in California. The report, “Accelerated Climate Change: How a Shifting Flow Regime is Redefining Water Governance in California” focuses on the challenge of managing the Golden State’s water resources as snowmelt and river flow patterns are altered in forced global warming conditions.
It should be understood from the start that according to the Shibatani Group website, the company is “an international leader at assessing, documenting, and explaining the implications of forced climate change, climatic variability, and what that means to water supply and water resource[s]…” Since the group provides professional services and preemptive planning for watershed management based on climate change, the company has a vested interest in the field.
If Sacramento-based hydrologist Robert Shibatani’s projections verify, California is in for some “interesting times.”
Shibatani’s projections are derived from numerous sources that include a 2011 U.S. Bureau of Reclamation-released document on western climate risk assessments. In his base assessment, Shibatani assumes a rapid 2 degree Celsius (3.6F) warming over the temperature averages from the 1961-1990 timeframe. Air temperature is expected to steadily increase in the 21st century, but this report assumes that a dramatic temperature departure “is likely to occur in the next decade,” which explains the “accelerated climate change” reference in the title.
Based on such quick warming over the next 10 years or so, Shibatani anticipates a greater than 50 percent reduction in the Sierra snowpack’s April 1 water content (known as the snow water equivalent (SWE)), by the early 2020s. As a hydrologist, it’s an event he considers of “Katrina-esque proportions.” He states that the only difference between a catastrophic flood event and his prediction of a significantly depleted snowpack within a decade or so “is that it will happen every year and with increasing severity, representing a permanent change.”
The Sacramento River drains Northern California’s main snowpack producing regions. North State provides much of the water that Southern Californians rely upon. Climate change may impact that established system.
Climatologists are not predicting a significant shift in the average amount of precipitation California receives each year, but Shibatani argues that much more of it will be in the form of rain as rapidly warming temperatures drive the freezing level (snow level) much higher. His forecast for future decades is even more ominous. By the 2050’s, the report projects a 76 percent reduction in April 1 SWE for the Sacramento watershed fed by the Sierra and northern mountains.
The amount of water that falls in the Sacramento watershed is expected to remain more or less the same, but the snowpack will cover less terrain and the timing of peak Sierra runoff will be earlier and of shorter duration.
If Shibatani’s expectations are realized, by the 2070s April 1 runoff will drop 90 percent compared to the 1990s.
An accelerated change of this magnitude would be disastrous for California and create tough challenges for Tahoe resorts, but Shibatani may be getting ahead of himself with the rapid extinction of the Sierra snowpack. Yes, over the past 100 years daily air temperatures at Tahoe City have trended warmer, with overnight lows up more than 4 degrees F. and daily maximums up almost 2 degrees since 1910.
However, over the last 10 years temperatures in Tahoe City have actually trended cooler, a change that was reflected in the updated 1981-2010 climate “normals” released in July 2011 by the National Climatic Data Center. The average daily temp in Tahoe City dropped a half degree; not much but not a warming trend either. At the Truckee Ranger Station, the new normal is cooler by one full degree.
Note the parabolic downward trend over the past decade in Truckee’s daily temperatures.
The new precipitation values have also changed. At Tahoe City precipitation has increased during the last decade, with the new normal up 1.6 inches to 32.66 inches, reflecting very wet winters during the early 1980s and the mid-1990s, which offset the worst drought in history during the late 1980s and early 90s. Average precipitation at the Truckee Ranger Station also increased, but only about half an inch. (Precipitation includes rain plus snow melted for liquid content.)
What NOAA calls “normal” is actually based on the most recent 30-year time span, not the whole period of record. Similar to the decadal Census Bureau cycle, every 10 years NOAA drops the oldest decade and updates new station “normals” by adding the most recent decade and adjusting the average.
At the Central Sierra Snow Lab at 6,900 feet near Donner Pass, the duration of the snowpack on the ground has not changed meaningfully since measurements began in 1946.
There were some other noteworthy adjustments in this most recent climate update which, at least temporarily, contradicts Shibatani’s conclusion that the Golden State is on an accelerated pace for warmer temperatures.
The NWS took a look at 6 key climate stations in California, ranging from Redding Airport south to Modesto Airport. Daily temperatures went down at 4 of the 6 sites, averaging about half a degree. Only Modesto and Redding were warmer.
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