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Disappearing 'alpine tundra,'Köppen climatic type in the western United States

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Geophysical Research Letters
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Henry F Diaz at University of Hawaiʻi at Mānoa
Henry F Diaz
Jon K. Eischeid
Jon K. Eischeid
Jon K. Eischeid
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Abstract and Figures

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°Cand9.5°C.Inthelast20years(1987–2006),risingtemperatureshavecausedasignificantfractionoftheseareastoexceedthe10°Cthresholdforalpinetundraclassification.Theresulthasbeena738.5°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.
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Disappearing ‘‘alpine tundra’’ Ko¨ppen climatic type in the western
United States
Henry F. Diaz
1
and Jon K. Eischeid
1
Received 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 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 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 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 Ko¨ppen 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 stream-
flow [Stewart et al., 2005; Regonda et al., 2005] suggest
that climatic types in the region may also be changing.
[
3] WehaveusedtheKo¨ppen 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 Indepen-
dent Slopes Model) gridded data set of surface temperature
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 Ko¨ppen type-E (polar cli-
mates). 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 km
2
. 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 differ-
ences in temperature change s 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 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 the
entire West, Figure 1 shows mean a nnual 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
1
Auxiliary materials are available in the HTML. doi:10. 1029/
2007GL031253.
GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L18707, doi:10.1029/2007GL031253, 2007
Click
Here
for
Full
A
rticl
e
1
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.
L18707 1of4
Ko¨ppen 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 classi-
fication 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 temper-
atures 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 com-
puted for successive 250 m elevation bins. In all three cases,
the trend in minimum average temperature increases with
elevation, while the trends in the mean maximum temper-
ature 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 precip-
itation 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–Ma y,
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 dimin-
ished western snow packs [Mote et al., 2005]. The results
Figure 2. Distribution of Ko¨ppen 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. (bott om) 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.
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 di fference
between the two curves in the period 1901 –1930 (1987–
2006) is 0.2°C (0.05° C).
L18707 DIAZ AND EISCHEID: CLIMATIC TYPE SHIFT IN THE WEST L18707
2of4
(not shown because of space limitations) indicate that for
the period 1979–2006, significant increases in MAM max-
imum 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 i n 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 Ko¨ppen climate classification system, an area of
the western US of around 20,000 km
2
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 con-
cluded 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 veget atio n-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 environ mental
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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  • Nov 2022
Endemic plants in high mountains are projected to be at high risk because of climate change. Temporal demographic variation is a major factor affecting population viability because plants often occur in small, isolated populations. Because isolated populations tend to exhibit genetic differentiation, analyzing temporal demographic variation in multiple populations is required for the management of high mountain endemic species. We examined the population dynamics of an endemic plant species, Primula farinosa subsp. modesta, in four subalpine sites over six years. Stage-based transition matrices were constructed, and temporal variation in the projected population growth rate (λ) was analyzed using life table response experiments (LTREs). The variation in λ was primarily explained by the site × year interaction rather than the main effects of the site and year. The testing sites exhibited inconsistent patterns in the LTRE contributions of the vital rates to the temporal deviation of λ. However, within sites, growth or stasis had significant negative correlations with temporal λ deviation. Negative correlations among the contributions of vital rates were also detected within the two testing sites, and the removal of the correlations alleviated temporal fluctuations in λ. The response of vital rates to yearly environmental fluctuations reduced the temporal variation of λ. Such effects manifested especially at two sites where plants exhibited higher plasticity than plants at other sites. Site-specific temporal variation implies that populations of high mountain species likely exhibit asynchronous temporal changes, and multiple sites need to be evaluated for their conservation.
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A Knowledge-Based Approach to the Statistical Mapping of Climate
Article
Full-text available
  • Sep 2002
The demand for spatial climate data in digital form has risen dramatically in recent years. In response to this need, a variety of statistical techniques have been used to facilitate the production of GIS-compatible climate maps. However, observational data are often too sparse and unrepresentative to directly support the creation of high-quality climate maps and data sets that truly represent the current state of knowledge, An effective approach is to use the wealth of expert knowledge on the spatial patterns of climate and their relationships with geographic features, termed 'geospatial climatology', to help enhance, control, and parameterize a statistical technique. Described here is a dynamic knowledge-based framework that allows for the effective accumulation, application, and refinement of climatic knowledge, as expressed in a statistical regression model known as PRISM (parameter-elevation regressions on independent slopes model). The ultimate goal is to develop an expert system capable of reproducing the process a knowledgeable climatologist would use to create high-quality climate maps, with the added benefits of consistency and repeatability. However, knowledge must first be accumulated and evaluated through an ongoing process of model application; development of knowledge prototypes, parameters and parameter settings; testing; evaluation; and modification. This paper describes the current state of a knowledge-based framework for climate mapping and presents specific algorithms from PRISM to demonstrate how this framework is applied and refined to accommodate difficult climate mapping situations. A weighted climate-elevation regression function acknowledges the dominant influence of elevation on climate. Climate stations are assigned weights that account for other climatically important factors besides elevation. Aspect and topographic exposure, which affect climate at a variety of scales, from hill slope to windward and leeward sides of mountain ranges, are simulated by dividing the terrain into topographic facets. A coastal proximity measure is used to account for sharp climatic gradients near coastlines. A 2-layer model structure divides the atmosphere into a lower boundary layer and an upper free atmosphere layer, allowing the simulation of temperature inversions, as well as mid-slope precipitation maxima. The effectiveness of various terrain configurations at producing orographic precipitation enhancement is also estimated. Climate mapping examples are presented.
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Declining Snowpack in Western North America
Article
Full-text available
  • Jan 2005
In western North America, snow provides crucial storage of winter precipitation, effectively transferring water from the relatively wet winter season to the typically dry summers. Manual and telemetered measurements of spring snow-pack, corroborated by a physically based hydrologic model, are examined here for climate-driven fluctuations and trends during the period of 1916-2002. Much of the mountain West has experienced declines in spring snowpack, especially since midcentury, despite increases in winter precipitation in many places. Analysis and modeling show that climatic trends are the dominant factor, not changes in land use, forest canopy, or other factors. The largest decreases have occurred where winter temperatures are mild, especially in the Cascade Mountains and northern California. In most mountain ranges, relative declines grow from minimal at ridgetop to substantial at snow line. Taken together, these results emphasize that the West's snow resources are already declining as earth's climate warms. Joint Institute for the Study of the Atmosphere and the Ocean Contribution Number 1073.
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Seasonal Cycle Shifts in Hydroclimatology over the Western US
Article
Full-text available
  • Jan 2005
Abstract Analyses of streamflow, snowfall temperature, and precipitation in snow - melt dominated,river basins in the western US indicates an advance in the timing of peak spring flows over the past fifty years. Warm temperature spells in spring have occurred much earlier in recent years, which partly explains the trend in the timing of the spring peak flow. In addition, a decrease in snow water equivalent and a general increase in winter precipitation is evident for many,weather stations in the western U.S. It appears that in recent decades more of the precipitation is coming as rain rather than snow. The trends are strongest at lower elevations and in the Pacific Northwest region, where winter temperatures are closer to the freezing-point; it appears that in this region in particular, modest,shifts in temperature are capable of forcing large shifts in basin hydrologic response. We speculate that these trends could be potentially a manifestation of the general global warming,trend in recent decades and also due to enhanced ENSO activity. The observed,trends in hydroclimatol ogy,over the western,US can have significant
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Changes in the Onset of Spring in the Western United States
Article
Full-text available
  • Mar 2001
Fluctuations in spring climate in the western United States over the last 4-5 decades are described by examining changes in the blooming of plants and the timing of snowmelt-runoff pulses. The two measures of spring's onset that are employed are the timing of first bloom of lilac and honeysuckle bushes from a long-term cooperative phenological network, and the timing of the first major pulse of snowmelt recorded from high-elevation streams. Both measures contain year-to-year fluctuations, with typical year-to year fluctuations at a given site of one to three weeks. These fluctuations are spatially coherent, forming regional patterns that cover most of the west. Fluctuations in lilac first bloom dates are highly correlated to those of honeysuckle, and both are significantly correlated with those of the spring snowmelt pulse. Each of these measures, then, probably respond to a common mechanism. Various analyses indicate that anomalous temperature exerts the greatest influence upon both interannual and secular changes in the onset of spring in these networks. Earlier spring onsets since the late 1970s are a remarkable feature of the records, and reflect the unusual spell of warmer-than-normal springs in western North America during this period. The warm episodes are clearly related to larger-scale atmospheric conditions across North America and the North Pacific, but whether this is predominantly an expression of natural variability or also a symptom of global warming is not certain.
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Recent changes in tropical freezing heights and the role of sea temperature
Article
Full-text available
  • Sep 1996
A WIDESPREAD retreat of alpine glaciers1 and melting of tropical ice-cap margins2–7 has been observed in recent decades, over which time a general climate warming at lower altitudes has been documented8. Moreover, some ice-core records provide evidence suggesting that mid-tropospheric temperatures in the tropics have been greater in recent decades than at any time during the past 2,000–3,000 years7. Here we examine the processes controlling mountain glacier retreat by comparing high-altitude air-temperature measurements for the past few decades, to the temperatures predicted by a model atmosphere forced by the observed global pattern of sea surface temperature in a 19-year simulation9. The comparison strongly indicates that the observed changes in freezing-level height (the altitude of the 0°C isotherm) are related to a long-term (over decades) increase in sea surface temperature in the tropics, and the consequent enhancement of the tropical hydrological cycle. Although changes in this cycle are likely to affect high-elevation hydrological and ecological balances worldwide10,11, tropical environments may be particularly sensitive because the changes in tropical sea surface temperature and humidity may be largest and most systematic at low latitudes.
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Changes Toward Earlier Streamflow Timing Across Western North America
Article
Full-text available
The highly variable timing of streamflow in snowmelt-dominated basins across western North America is an important consequence, and indicator, of climate fluctuations. Changes in the timing of snowmelt-derived streamflow from 1948 to 2002 were investigated in a network of 302 western North America gauges by examining the center of mass for flow, spring pulse onset dates, and seasonal fractional flows through trend and principal component analyses. Statistical analysis of the streamflow timing measures with Pacific climate indicators identified local and key large-scale processes that govern the regionally coherent parts of the changes and their relative importance. Widespread and regionally coherent trends toward earlier onsets of springtime snowmelt and streamflow have taken place across most of western North America, affecting an area that is much larger than previously recognized. These timing changes have resulted in increasing fractions of annual flow occurring earlier in the water year by 1–4 weeks. The immediate (or proximal) forcings for the spatially coherent parts of the year-to-year fluctuations and longer-term trends of streamflow timing have been higher winter and spring temperatures. Although these temperature changes are partly controlled by the decadal-scale Pacific climate mode [Pacific decadal oscillation (PDO)], a separate and significant part of the variance is associated with a springtime warming trend that spans the PDO phases.
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Climatic warming in the Tibetan Plateau during recent decades
Article
Adequate knowledge of climatic change over the Tibetan Plateau (TP) with an average elevation of more than 4000 m above sea level (a.s.l.) has been insufficient for a long time owing to the lack of sufficient observational data. In the present study, monthly surface air temperature data were collected from almost every meteorological station on the TP since their establishment. There are 97 stations located above 2000 m a.s.l. on the TP; the longest records at five stations began before the 1930s, but most records date from the mid-1950s. Analyses of the temperature series show that the main portion of the TP has experienced statistically significant warming since the mid-1950s, especially in winter, but the recent warming in the central and eastern TP did not reach the level of the 1940s warm period until the late 1990s. Compared with the Northern Hemisphere and the global average, the warming of the TP occurred early. The linear rates of temperature increase over the TP during the period 1955–1996 are about 0.16°C/decade for the annual mean and 0.32°C/decade for the winter mean, which exceed those for the Northern Hemisphere and the same latitudinal zone in the same period. Furthermore, there is also a tendency for the warming trend to increase with the elevation in the TP and its surrounding areas. This suggests that the TP is one of the most sensitive areas to respond to global climate change. Copyright © 2000 Royal Meteorological Society
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Trends in temperature and precipitation in the Pacific Northwest during the twentieth century
Article
  • Sep 2003
  • NORTHWEST SCI
Documenting long-term trends or persistent shifts in temperature and precipitation is important for understanding present and future changes in flora and fauna. Carefully adjusted datasets for climate records in the USA and Canada are combined and used here to describe the spatial and seasonal variation in trends in the maritime, central, and Rocky Mountain climatic zones of the Pacific Northwest. Trends during the 20th century in annually averaged temperature (0.7degreesC-0.9degreesC) and precipitation (13%-38%) exceed the global averages. Largest warming rates occurred in the maritime zone and in winter and at lower elevations in all zones, and smallest warming rates occurred in autumn and in the Rockies. Largest increases in precipitation (upwards of 60% per century) were observed in the dry areas in northeast Washington and south central British Columbia. Increases in precipitation were largest in spring, but were also large in summer in the central and Rocky Mountain climatic zones. These trends have already had profound impacts on streamflow and on certain plant species in the region (Cayan et al. 2001), and other important impacts remain to be discovered. The warming observed in winter and spring can be attributed partially to climatic variations over the Pacific Ocean, and the buildup of greenhouse gases probably also plays an important role.
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Estimation of Tropospheric Temperature Trends from MSU Channels 2 and 4
Article
  • Mar 2006
The problems inherent in the estimation of global tropospheric temperature trends from a combination of near-nadir Microwave Sounding Unit (MSU) channel-2 and 4 data are described. The authors show that insufficient overlap between those two channels' weighting functions prevents a physical removal of the stratospheric influence on tropospheric channel 2 from the stratospheric channel 4. Instead. correlations between stratospheric and tropospheric temperature fluctuations based upon ancillary (e.g., radiosonde) information call be used to statistically estimate a correction for the stratospheric influence on MSU 2 from MSU 4. Fu et al. developed such a regression relationship from radiosonde data using the 850-300-hPa layer as the target predictand. There are large errors in the resulting fit of the two MSU channels to the tropospheric target layer, so the correlations from the ancillary data Must be relied upon to provide a statistical minimization of the resulting errors. Such relationships depend upon the accuracy of the particular training dataset as well as the dataset time period and its global representativeness (i.e.. temporal and spatial stationarity of the statistics). It is concluded that near-nadir MSU channels 2 and 4 cannot be combined to provide a tropospheric temperature measure without substantial uncertainty resulting from a necessary dependence oil ancillary information regarding the vertical profile of temperature variations. which are, in general, not well known oil a global basis.
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