Disappearing ‘‘alpine tundra’’ Ko¨ppen climatic type in the westernUnited StatesHenry F. Diaz1and Jon K. Eischeid1Received 6 July 2007; accepted 27 August 2007; published 27 September 2007.[1] We examine changes in the areal extent of the Ko¨ppen‘‘alpine tundra’’ climate classification type for themountainous western United States following the work ofKottek et al. (2006). We find a significant decline in the areaoccupied by this climate category. In the early decades ofthe 20th century, the mean temperature of the warmestmonth in the areas of the western U.S. with nominal alpinetundra climates ranged largely between 8.5°C and 9.5°C.In the last 20 years (1987–2006), rising temperatures havecaused a significant fraction of these areas to exceed the10°C threshold for alpine tundra classification. The resulthas been a 73% reduction in coverage of this climatic type.The remaining classified alpine tundra in the last 20 yearsnow averages between 9°C–10°C during the warmestmonth, so that continued warming past the classificationthreshold, would imply that areas where this climate type isfound today in the West will no longer be presen t.Citation: Diaz, H. F., and J. K. Eischeid (2007), Disappearing‘‘alpine tundra’’ Ko¨ppen climatic type in the western United States,Geophys. Res. Lett., 34, L18707, doi:10.1029/2007GL031253.1. Introduction[2] A number of studies have been published in the pastdecade that indicate that the world’s mountain regionscomprise an area where the effects of global warming islikely to be amplified. In particular, many of these studieshave documented an amplification of temperature trendswith height [Diaz and Graham, 1996; Liu and Chen, 2000;Diaz et al., 2003; Bradley et al., 2004]. To examine changesin climatic types that may have occurred during the pastcentury, we make use of the Ko¨ppen climate classificationsystem, which has been used to evaluate ongoing and futureclimate change impacts on characteristic regional climatetypes [Lohmann et al., 1993; Fraedrich et al., 2001; Kotteket al., 2006; see also Dang et al., 2007]. Recent strongwarming trends in the western United States [Mote, 2003]and associated impacts on vegetation [Cayan et al., 2001;Breshears et al., 2005; Westerling et al., 2006] and stream-flow [Stewart et al., 2005; Regonda et al., 2005] suggestthat climatic types in the region may also be changing.[3] WehaveusedtheKo¨ppen climate classificationsystem to evaluate changes in climatic types across theUnited States. The classification algorithm was applied tothe PRISM (Parameter-elevation Regressions on Indepen-dent Slopes Model) gridded data set of surface temperatureat 4-km resolution [Daly et al., 2002]. We have also usedthe PRISM temperature data to evaluate maximum andminimum temperature trends with elevation in the West; acomparison of means and trends in the PRISM data set withother data sets is given in the auxiliary material.1Here wereport on changes in one particular phenotype—the alpinetundra, which is equivalent to Ko¨ppen type-E (polar cli-mates). Briefly, polar climates are defined as occurring if themean temperature of the warmest month is less than 10°C.In the West, only the tundra climate sub-type is present,which is defined as occurring when the mean temperature ofthe warmest month lies in the interval (0°,10°C]. We notethat the percent area coverage of this climate type is rathersmall, occupying only about 0.2% of the area of the westernUnited States, or about 15,000 km2. There were 1226 4-kmpixels classified as ‘‘alpine tundra’’ in the 1901– 30 period,whereas in 1987– 2006, there were only 336 thus categorized—a decline of 73%.[4] We also calculated linear temperature trends for theperiod 1979– 2006 for mountainous areas in Colorado, thePacific Northwest and the Sierra Nevada, which containmost of the areas classified as ‘‘alpine tundra’’. This periodwas chosen for two reasons. First, it corresponds to a periodof rapidly warming surface temperatures in the West (seebelow). Second, it enabled us to compare elevational differ-ences in temperature change s in PRISM with changesderived from two independent data sets: the MSU lowertroposphere satellite-derived temperatures [Spencer et al.,2006] for the domain of the western US that starts around1979, and a homog enized version of the SNOTEL temper-ature record provided by the Natural Resources Conserva-tion Service, USDA, which also starts around this time.2. Analysis[5] To illustrate the recent warming trends across theentire West, Figure 1 shows mean a nnual temperaturechanges for the U.S. region west of about 102°W longitude.The last 20 years of record (1987 –2006) is a period ofconsistently higher temperatures—an increase of 1°F(0.6°C) averaged over the entire region compared to thelong-term (period-of-record) mean. This in turn has resultedin a dramatic reduction—as much as 73%—in the areaclassified as alpine tundra in the West, compared to the firstdecades of the 20th century (Figure 2). This relatively largechange in the coverage of alpine tundra climate in thewestern U.S. is quite remarkable. We should note that themean monthly temperature of the warmest month in most ofthe areas we classified as alpine tundra (the key parameter in1Auxiliary materials are available in the HTML. doi:10. 1029/2007GL031253.GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L18707, doi:10.1029/2007GL031253, 2007ClickHereforFullArticle1NOAA Earth System Research Laboratory, Boulder, Colorado, USA.This paper is not subject to U.S. copyright.Published in 2007 by the American Geophysical Union.L18707 1of4Ko¨ppen Type E) in the West is close to the critical thresholdof 10°C. This is illustrated in Figure 3 (left), which showsthe temperature changes in pixels that exceeded the classi-fication threshold in the 1987 –2006 period compared to theearlier (1901– 30) period. Figure 3 (right) also indicates thatbelow elevations of about 3000 m, the mean temperature ofthe warmest month today is quite near the critical 10°Cthreshold, and so one would expect that a continuation ofthe recent warming trends will cause further reductions inthe areal coverage of this climate type.[6] We further illustrate the warming at high elevations inthe West in Figure 4, which show the distribution of lineartrends for annual mean minimum and maximum temper-atures as a function of ground elevation for areas where thehighest mountain regions