A natural resource condition assessment for Sequoia and Kings Canyon National Parks: Appendix 22: climatic change

Climate is a master controller of the structure, composition, and function of biotic communities,
affecting them both directly, through physiological effects, and indirectly, by mediating biotic
interactions and by influencing disturbance regimes. Sequoia and Kings Canyon National Park’s
(SEKI’s) dramatic elevational changes in biotic communities -- from warm mediterranean to
cold alpine -- are but one manifestation of climate’s overarching importance in shaping SEKI’s
landscape.

Yet humans are now altering the global climate, with measurable effects on ecosystems (IPCC
2007). Over the last few decades across the western United States, human-induced climatic
changes have likely contributed to observed declines in fraction of precipitation falling as snow
and snowpack water content (Mote et al. 2005, Knowles et al. 2006), advance in spring
snowmelt (Stewart et al. 2005, Barnett et al. 2008), and consequent increase in area burned in
wildfires (Westerling et al. 2006). In the Sierra Nevada, warming temperatures have likely
contributed to observed glacial recession (Basagic 2008), uphill migration of small mammals
(Moritz et al. 2008), and increasing tree mortality rates (van Mantgem and Stephenson 2007, van
Mantgem et al. 2009). More substantial changes can be expected for the future (e.g., IPCC
2007).

Given the central importance of climate and climatic changes, we sought to describe long-term
trends in temperature and precipitation at SEKI. Time and budget constraints limited us to
analyses of mean annual temperature and mean annual precipitation, using readily-available data.
If funds become available in the future, further analyses will be needed to analyze trends by
season, trends in daily minimum and maximum temperatures, and so on.

We chose to analyze data from individual weather stations rather than use interpolated climatic
data from sources such as PRISM (http://www.prism.oregonstate.edu/). In topographically
complex mountainous regions with few weather stations, like SEKI, the addition or subtraction
of even a single weather station through time has the potential to significantly bias trends in
interpolated data. In particular, this analysis was motivated by our questioning of some PRISM
results presented in Appendix 1 (Landscape Context) that compared temperature averages
between two 30-year periods of the 20th Century. Figures 6 and 11 of Appendix 1 indicate that
recent (1971-2000) temperatures in northern Kings Canyon National Park averaged some 2° C
cooler than those of 1911-1940. This would represent a truly profound and persistent cooling,
and seems to be at odds both with the glacial retreats observed in the area over the century
(Basagic 2008), and with the reported PRISM warming of nearly 2° C just to the west of the
cooling (see Figs. 6 and 11 in Appendix 1). We suspect that the extreme localized Kings Canyon
cooling reported by PRISM is an artifact of sparsely-distributed weather stations in the region
being added and discontinued over the span of the 20th Century. For example, data from the
Western Regional Climate Center (http://www.wrcc.dri.edu/coopmap/) suggest that for the
period 1911 through 1924 PRISM must interpolate northern Kings Canyon temperatures based
on a few low-elevation stations -- separated by hundreds of kilometers -- in Nevada and
California’s San Joaquin Valley. In contrast, by 1970 PRISM interpolations will be dominated
by closer, higher-elevation stations (see this report). The single weather station closest to
northern Kings Canyon that has a temperature record at least partly spanning Appendix 1’s two
30-year time periods -- the Independence station, with a relatively continuous temperature record
starting in 1925 -- shows a modest warming, not a cooling, between 1925-1940 and 1971-2000,
further casting doubt on the Kings Canyon cooling shown in Figs. 6 and 11 of Appendix 1. If
funds become available, it will be useful to more formally analyze potential PRISM biases in
long-term SEKI climatic trends. Until then, the analyses of individual weather station records
presented here (effectively an analysis of source data that PRISM uses) are meant to provide a
robust summary of climatic changes in SEKI.