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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L18707, doi:10.1029/2007GL031253, 2007
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Disappearing ‘‘alpine tundra’’ Köppen climatic type in the western
United States
Henry F. Diaz1 and Jon K. Eischeid1
Received 6 July 2007; accepted 27 August 2007; published 27 September 2007.
[1] We examine changes in the areal extent of the Köppen
‘‘alpine tundra’’ climate classification type for the
mountainous western United States following the work of
Kottek et al. (2006). We find a significant decline in the area
occupied by this climate category. In the early decades of
the 20th century, the mean temperature of the warmest
month in the areas of the western U.S. with nominal alpine
tundra climates ranged largely between 8.5°C and 9.5°C.
In the last 20 years (1987– 2006), rising temperatures have
caused a significant fraction of these areas to exceed the
10°C threshold for alpine tundra classification. The result
has been a 73% reduction in coverage of this climatic type.
The remaining classified alpine tundra in the last 20 years
now averages between 9°C– 10°C during the warmest
month, so that continued warming past the classification
threshold, would imply that areas where this climate type is
found today in the West will no longer be present.
Citation: Diaz, H. F., and J. K. Eischeid (2007), Disappearing
‘‘alpine tundra’’ Köppen 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 past
decade that indicate that the world’s mountain regions
comprise an area where the effects of global warming is
likely to be amplified. In particular, many of these studies
have documented an amplification of temperature trends
with height [Diaz and Graham, 1996; Liu and Chen, 2000;
Diaz et al., 2003; Bradley et al., 2004]. To examine changes
in climatic types that may have occurred during the past
century, we make use of the Köppen climate classification
system, which has been used to evaluate ongoing and future
climate change impacts on characteristic regional climate
types [Lohmann et al., 1993; Fraedrich et al., 2001; Kottek
et al., 2006; see also Dang et al., 2007]. Recent strong
warming 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 streamflow [Stewart et al., 2005; Regonda et al., 2005] suggest
that climatic types in the region may also be changing.
[3] We have used the Köppen climate classification
system to evaluate changes in climatic types across the
United States. The classification algorithm was applied to
the PRISM (Parameter-elevation Regressions on Independent Slopes Model) gridded data set of surface temperature
1
at 4-km resolution [Daly et al., 2002]. We have also used
the PRISM temperature data to evaluate maximum and
minimum temperature trends with elevation in the West; a
comparison of means and trends in the PRISM data set with
other data sets is given in the auxiliary material.1 Here we
report on changes in one particular phenotype—the alpine
tundra, which is equivalent to Köppen type-E (polar climates). Briefly, polar climates are defined as occurring if the
mean 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 of
the warmest month lies in the interval (0°, 10°C]. We note
that the percent area coverage of this climate type is rather
small, occupying only about 0.2% of the area of the western
United States, or about 15,000 km2. There were 1226 4-km
pixels 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 the
period 1979– 2006 for mountainous areas in Colorado, the
Pacific Northwest and the Sierra Nevada, which contain
most of the areas classified as ‘‘alpine tundra’’. This period
was chosen for two reasons. First, it corresponds to a period
of rapidly warming surface temperatures in the West (see
below). Second, it enabled us to compare elevational differences in temperature changes in PRISM with changes
derived from two independent data sets: the MSU lower
troposphere satellite-derived temperatures [Spencer et al.,
2006] for the domain of the western US that starts around
1979, and a homogenized version of the SNOTEL temperature record provided by the Natural Resources Conservation Service, USDA, which also starts around this time.
2. Analysis
[5] To illustrate the recent warming trends across the
entire West, Figure 1 shows mean annual temperature
changes for the U.S. region west of about 102°W longitude.
The last 20 years of record (1987 –2006) is a period of
consistently higher temperatures—an increase of 1°F
(0.6°C) averaged over the entire region compared to the
long-term (period-of-record) mean. This in turn has resulted
in a dramatic reduction—as much as 73%—in the area
classified as alpine tundra in the West, compared to the first
decades of the 20th century (Figure 2). This relatively large
change in the coverage of alpine tundra climate in the
western U.S. is quite remarkable. We should note that the
mean monthly temperature of the warmest month in most of
the areas we classified as alpine tundra (the key parameter in
NOAA Earth System Research Laboratory, Boulder, Colorado, USA.
This paper is not subject to U.S. copyright.
Published in 2007 by the American Geophysical Union.
1
Auxiliary materials are available in the HTML. doi:10.1029/
2007GL031253.
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elevation, while the trends in the mean maximum temperature increases with height only in the Colorado Rockies.
The trend in the other two mountain ranges is positive, but
rather more uniform with height. We also note that precipitation trends in the West over the period 1979 –2006 (not
shown), using a data set of snow water equivalent values
from snow-depth measurements together with collocated
PRISM data show declines during this time. The drier
conditions prevalent over the last few decades in the West
would also have contributed to the decline in the areal
extent of the alpine tundra in this region.
[7 ] We also examined spring season (March – May,
MAM) temperature trends for the same three regions
because generally the largest climate trends and impacts
signals in the West reported in the literature are associated
with this transition season, and because a connection can be
made between amplification of the warming trends in the
West with elevation and surface changes related to diminished western snow packs [Mote et al., 2005]. The results
Figure 1. Time series of annual mean temperature
anomalies (°C) for the western United States, 1901 –2006.
(top) Values computed from divisional data (blue curve)
available from the National Climatic Data Center, NOAA in
Asheville, NC, and from PRISM. (bottom) Difference
between the two curves above. The mean difference
between the two curves in the period 1901– 1930 (1987 –
2006) is 0.2°C (0.05°C).
Köppen Type E) in the West is close to the critical threshold
of 10°C. This is illustrated in Figure 3 (left), which shows
the temperature changes in pixels that exceeded the classification threshold in the 1987 – 2006 period compared to the
earlier (1901– 30) period. Figure 3 (right) also indicates that
below elevations of about 3000 m, the mean temperature of
the warmest month today is quite near the critical 10°C
threshold, and so one would expect that a continuation of
the recent warming trends will cause further reductions in
the areal coverage of this climate type.
[6] We further illustrate the warming at high elevations in
the West in Figure 4, which show the distribution of linear
trends for annual mean minimum and maximum temperatures as a function of ground elevation for areas where the
highest mountain regions in the West are located. The
results are presented as distributions of linear trends computed for successive 250 m elevation bins. In all three cases,
the trend in minimum average temperature increases with
Figure 2. Distribution of Köppen classification ‘‘E’’
(tundra climates, E-T) corresponding to the ‘‘alpine tundra’’
climate in the western United States considered in this
study. (top) Areas classified as K-E for the period 1901–
1930. (bottom) Same as for Figure 2 (top), except for the
period 1987 – 2006. These areas are colored red and
comprise all the 4-km pixels in the PRISM data that are
classified as E-T.
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Figure 3. (left) Distribution of mean annual temperature of the warmest month for pixels that were classified as ‘‘alpine
tundra’’ in 1901 – 1930 (blue curve) and for the subset of pixels that exceeded the 10°C threshold, and hence were no longer
classified as such in 1987– 2006. (right) Temperature distribution of pixels that are still classified as alpine tundra in both
periods. Note the elimination of this category at the lowest elevation bin. Solid dots represent the median value in each
250-meter bin.
(not shown because of space limitations) indicate that for
the period 1979– 2006, significant increases in MAM maximum and minimum temperatures have taken place, with
mean annual minimums displaying substantial amplification
with altitude. Annual average maximums, however, do not
generally exhibit this vertical amplification of the warming.
3. Summary and discussion
[8] Average temperature in the western United States has
risen considerably in the last 20 years—about 0.6°C. This
period is the warmest such interval in the instrumental
record. Based on the PRISM data set, the warming has
generally been accentuated at the higher elevations (above
about 2000 m) where trends in excess of 1°C are calculated.
Using the Köppen climate classification system, an area of
the western US of around 20,000 km2 was typed as alpine
tundra for the period 1901 – 30. In contrast, for the period
1987 – 2006, the same climate type covered only 336 4-km
pixels—a decline in area coverage of about 73%.
[9] The U.S. Climate Change Science Program recently
took up the question of whether there was agreement
between observations and climate model simulations on
the nature of temperature changes at the surface and in
the troposphere [Karl et al., 2006]. In particular, for global
average temperature, 1979 through 2004, the report concluded that they could not state definitively whether trends
at the surface were different from those in the troposphere.
The results presented here are for the mountainous regions
of the western United States since 1979, where studies have
documented significant changes to snow packs and the
timing of the seasonal melting [e.g., Stewart et al., 2005;
Mote et al., 2005], and where these hydroclimatic changes
have been associated with significant vegetation-related
impacts [cf. Breshears et al., 2005].
[10] We have focused here on one particular western U.S.
ecotone—the alpine tundra—as defined by a simple metric,
namely the mean temperature of the warmest month. A
range of different observations indicate that environmental
aspects of global warming are beginning to show, with
significant impacts already documented in a number of
phenological, biological, and hydrological indicators in
the West. These include earlier blossoming of shrubs in
spring, massive infestations of western forests by insect
pests, intensified wildfire seasons, and earlier spring runoff.
Our results suggest that the prevailing climate associated
with alpine tundra landscapes in the West may soon be out
of equilibrium with present climate and may largely
disappear.
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Figure 4. Distribution of annual mean minimum and maximum temperature trends expressed as total trend for the period
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H. F. Diaz and J. K. Eischeid, NOAA Earth System Research Laboratory,
325 Broadway, Boulder, CO 80305, USA. ([email protected])
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