Weather Window | Climate change and snowpack depletion at Lake Tahoe
Special to the Tribune
TAHOE/TRUCKEE – Warnings about regional climate change was kicked up a notch with the recently released report by Robert Shibatani, a Sacramento-based hydrologist who is also CEO of The Shibatani Group Inc. Presented at the California Water Law & Policy Conference on April 19-20, 2012, 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 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 needs both now and into the future.” 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. 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 two degree Celsius (3.6° F) warming over the temperature averages from the 1961-1990 time frame. Air temperature is expected to steadily increase in the 21st century, but this report assumes 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 during the next 10 years or so, Shibatani anticipates a greater than 50 percent reduction in the Sierra snowpack’s April 1 snow water equivalent (SWE) by the early 2020s. As a hydrologist, it’s an event he considers of “Katrina-esque proportions.” He states 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.”
Climatologists are not predicting a significant shift in the average amount of precipitation California receives each year, but Shibatani argues that 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 2050s, the report projects a 76 percent reduction in April 1 SWE for the Sacramento watershed fed by the Sierra and northern mountains. If Shibatani’s projections verify, by the 2070s the April 1 snow survey will reflect an 89 percent SWE loss compared to the 1990s. 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 predictions come true it could be disastrous for California and create tough challenges for Tahoe resorts, but he may be getting ahead of himself with the rapid extinction of the Sierra snowpack. During the past century daily air temperatures at Tahoe City have trended warmer, with overnight lows up more than 4° 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. 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 the National Oceanic and Atmospheric Administration 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. There were some interesting adjustments in this most recent climate update which, at least temporarily, contradict Shibatani’s conclusion the Golden State is on an accelerated pace for warmer temperatures. The NWS took a look at six key climate stations in California, ranging from Redding Airport south to Modesto Airport. Daily temperatures went down at four of the six sites, averaging about half a degree. Only Modesto and Redding were warmer.
– Tahoe historian Mark McLaughlin is a nationally published author and professional speaker. His award-winning books are available at local stores or at http://www.thestormking.com. You can reach him at email@example.com.
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