Saturday, January 22, 2011

USFS report on past and future climate trends for Tahoe National Forest and Eldorado National Forest

A recent internal U.S. Forest Service report on climate change effects in Tahoe National Forest and neighboring Eldorado National Forest paints a picture of climate change already under way locally, projecting an extremely uncertain but likely more hazard-prone future for those living nearby and downstream - with a possible 9 degree F mean temperature rise which would equate to the Tahoe Basin's elevation dropping by 2500 feet, and with likely increased fire activity, shrublands spreading up into coniferous areas, and a possibly doubled maximum flow rate of Sierra rivers yielding "a substantial increase in flood risk in flood-prone areas of the Central Valley".

Written by Forest Service ecologist Hugh Safford and UC Davis botanist Chris Mallek, the December 2010 report - titled "A summary of current trends and probable future trends in climate and climate-driven processes in the Eldorado and Tahoe National Forests and the neighboring Sierra Nevada"- looks at local and regional trends in the last century or so, and then reviews the existing literature to project how climate change is likely to affect the region during the rest of this century.

1. Nevada City

The Nevada City weather station on Banner Mountain, with records extending back to 1893, is one of four stations used in the report. And here climate change is evident - Mallek and Safford note that our station's mean annual temperature has increased by 2.4 degrees F, primarily from increased nighttime temperatures (a fingerprint of human-caused global warming); atypically, the station's maximum daytime temperatures have actually decreased. (Report coauthor Safford declined to speculate on why; your author suggests it's likely the reforestation of Nevada City and environs, which were pretty much denuded during the Gold Rush.)

The Nevada City station has also seen an increase in average annual precipitation, now up to almost 64 inches from around 46 inches in the 19th century readings.

2. Existing local and regional trends linked to climate change

The report notes that numerous changes have already been observed:
  • A decrease in early spring snowpack over the period 1950-1997;
  • An earlier spring thaw;
  • "Strong increases" in fire "frequency, size, total area burned, and severity" over the past several decades, where climatic variables account for up to half the increase;
  • Increased tree mortality in the West, including the Sierras, attributed to regional warming and drought stress;
  • Changes in vegetation patterns - more oaks and fewer alpine and subalpine habitats, though subalpine plants have also been moving into areas that used to be permanent snowfields;
  • Movement of wildlife - the species that have shifted their range, have generally moved upslope.
3. Future predictions

What lies in store for Tahoe National Forest and environs is inherently uncertain, since projecting its future entails climate modeling, and the resolution of the Global Climate Models (GCMs) now in use isn't fine-grained enough to allow confidence in regional predictions, even on a state level. Some climate model runs predict substantially increased precipitation for California, others predict a substantial decrease. But the change from current conditions could be major: one study simulating future climate for the Tahoe Basin found the temperature soaring by 9 degrees F, equivalent to dropping the basin's elevation by 2500 feet - with moisture increasingly coming as rain, from 35% snow now to just 10-18% snow by 2100.

The Tahoe Basin simulation effort also projected "a continuing trend toward earlier snowmelt and runoff during the water year; increased in drought severity, especially toward the end of the century; and dramatic increases in flood magnitude in the middle third of the century"; plus the potential reduction by a quarter, of the snowpack duration in summer.

An increased flood risk is considered "a high probability outcome", since peak runoff is more dependent on temperature than changes in average precipitation. The projected more intense rainfalls in a warmer climate should make the flooding worse; and under the wettest scenarios, maximum streamflow could more than double, an unpleasant prospect for those downstream.

And the increase we've seen in wildfire activity that's already linked to climate change is expected to "persist and possibly accelerate":
"If fire becomes more active under future climates, there may be significant repercussions for old growth forest...If fuels grow more rapidly and dry more rapidly - as is predicted under many future climate scenarios - then both [fire] severity and frequency may increase. In this scenario, profound vegetation type conversion [e.g. changing from pines & cedars to shrubs] is all but inevitable."

Commentary and updates
  • It would be interesting to hear an explanation from organizations promoting adaptation (preparation) over mitigation (prevention), as to how we in the Sierras can go about preparing for a future in which all that we're fairly sure of is that things will be substantially different. If an ounce of prevention were ever worth a pound of cure...
    (The answer, to the extent that there is one, involves ruggedizing.)
  • KQED Climate Watch yesterday posted on an intriguing study that found many California plant species had shifted their elevation lower since the 1930s, not upward as you'd expect if temperature were driving the change. The authors suggest this "downslope" shift is likely due the wetter climate we've experienced so far, "allowing plants to exist in warmer locations than they were previously capable of".
  • In its discussion of increased tree mortality, the USFS report didn't mention climate-linked changes in bark beetle infestations, which are currently devastating forests in the Rocky Mountains. Coauthor Hugh Safford explained:
    "Thus far we are not seeing big climate driven outbreaks of beetles in the Sierra Nevada, although in 2003 and 2004 millions of trees died after a 4-5 year drought in SoCal, many because of a huge pine beetle outbreak. That said, there are relatively large areas of pine beetle mortality in whitebark and lodgepole pine in subalpine forests in the Warner Mountains (NE CA) and the S. Sierra (east side, Inyo National Forest), and scientists and managers are tracking these outbreaks closely to see whether they will "erupt". One of the things that may be keeping beetle mortality down at higher elevations in the Sierra Nevada is the fact that precipitation has been steady or even increasing in the Sierra, whereas precipitation (or at least water balance) has been dropping in the Rockies, i.e. trees may be more stressed in the Rockies."
  • A minor issue with the report: other listings for the Nevada City weather station with records from the 1890s put it at 2781 ft. elevation (*).
  • An edited and shortened version of this post will likely run in the Nevada City Advocate's February March ? issue (it's posted online here).

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