Temperature Trends in North American Mountains: A Global Context
Nick Pepin1 & Jessica Lundquist2
1.Department of Geography, University of Portsmouth, U.K., nicholas.pepin@port.ac.uk
2 Department of Civil and Environmental Engineering,
2.Department
Engineering University of Washington,
Washington Seattle,
Seattle jdlund@u.washington.edu
jdlund@u washington edu
Surface Temperature Data
Introduction
1084 GHCN/CRU Surface Sites
It is difficult to predict future temperature changes due
to global warming in mountainous areas due to the
effects of complex topography, often not represented
i global
in
l b l climate
li t models.
d l Some
S
observational
b
ti
l studies
t di
suggest that warming rates are amplified at high
elevations (Diaz & Bradley 1997, Chen et al. 2003,
Diaz et al. 2003, Bradley et al. 2006), partly a result of
the positive feedback associated with melting
mountain glaciers and decreases in snow cover
(Barry 2006). However on a global scale, the
observational evidence for stronger and more
significant temperature trends at high elevations than
in lowland areas is not clear cut (Beniston et al. 1997,
Pepin & Seidel 2005, You et al. 2008) and trends also
depend on mean annual temperatures, local
topographical exposure and land-use (Pepin &
Lundquist 2008).
Our global surface data consists of monthly mean temperatures
from 1084 stations from the GHCN (Global Historical Climate
Network) and CRU (Climatic Research Unit, University of East
Anglia) datasets, ranging from 500 m to over 4700 m in elevation.
The datasets have been homogeneity adjusted for changes in
instrumentation, site, observing practices and site characteristics
(Peterson & Vose 1997, Jones 1994, Jones et al 1999, Jones &
Moberg 2003).
Global Distribution
Asia
280
N.America
552
Europe
162
S. America
33
Africa
41
Australia
14
Antarctica
2
Frequency Distribution of N American Station Elevations in GHCN/CRU Dataset
Frequency
40
60
60
76% of the sites show positive trends (warming) although only
74% of these (57% of all sites) are statistically significant (see
Santer 2000). Only 29% of the apparent negative cooling
trends (concentrated in the central and eastern U.S. and
represented as dark blue) are statistically significant.
20
1000
1500
2000
El
Elevation:
ti
metres
t
2500
3000
Forest/tundra transition at around 3500 m on Niwot Ridge, Colorado, U.S.A.
50
60
40
40
-0.6 to 0
0 to 0.25
0.25 to 0.5
0.5 to 0.75
0.75 to 1
20
-150
r = -0.178: p < 0.001
-100
501 to 999
1000 to 1499
1500 to 1999
2000 to 2499
2500 to 2969
-100
30
-125 -120 -115 -110 -105 -100
r=-0.520: p<0.001
.1
Surface Temperature Trend - deg C/yr
0
.05
-.05
5
Surface Temperature Trend: deg C/yr
--.05
0
.05
.1
.1
Surface Temperature Trend: deg C/yr
0
.05
-.0
05
33
0.127+/0.051
0.057+/-0.095
0.149+/-0.072
0.174+/-0.087
Europe
162
0.041+/0.040
0.061+/-0.079
-0.008+/0.072
0.070+/-0.051
41
0.140+/0.040
0.168+/-0.074
0.110+/-0.068
0.140+/-0.072
280
0.151+/0.027
0.108+/-0.045
0.173+/-0.050
0.172+/-0.043
Australia
14
0.134+/0.066
0.130+/-0.109
0.193+/-0.138
0.091+/-0.109
Antarctica
2
-0.063+/0.176
NA
NA
NA
Asia
HI
MT
MV
Conclusions
Mean warming in North American mountains (+0.123°C/decade) exceeds the global average,
and over 80% of mountain sites have positive slopes.
Mountain summits and freely draining hillslopes are more consistent in their temperature
response than low elevation sites (less variation in trend magnitudes) although we need more
summit sites to be more confident. Such locations are more exposed to the free atmosphere and
less dependent on local surface effects such as land-use change and urbanisation. Thus
mountain peaks/exposed slopes are better indicators of regional scale forcing.
There is an enhancement of warming rates at sites where mean annual temperatures are at
present near 0°C and as temperatures rise well above this critical threshold warming rates
reduce. This is clear evidence that cryospheric changes (earlier snowmelt in spring, reduced
snowpack, melting glaciers) are magnifying temperature response in this critical zone.
-10.00
FL
Mean trend magnitudes and uncertainty are given for high, middle and low elevation
stations in each continent. Elevation bands were defined for each continent by
dividing the stations into three equal categories based on elevation (exact
boundaries will be different for each continent). For North America the boundaries
are 745 and 1295 m.
There is no systematic increase in warming rates at higher elevations for our dataset, at least in
North America. However we have relatively few sites above treeline.
N America: Surface Temperature
Temperature Trend
Trend Magnitude
Magnitude vs
vs Mean
Mean Annual
Annual Temperature
Temperature
B
Boxplots
l t off N A
American
i
T
Temperature
t
Trend
T d Magnitudes
M
it d for
f Topographic
T
hi Classes
Cl
3000
0.161+/-0.030
S. America
35
Distribution of stations over a) North America, and b) Western U.S.
Colour coding shows station elevation.
FL=flat: HI=hillslope: MT=mountain summit: MV=incised valley
2500
Low
Elevation
0.122+/-0.020
In North America the mean temperature trend (+0.123°C/decade) is slightly higher
than the global average (+0.119°C/decade) and is stronger at the lower elevations.
40
North American Trend Magnitudes versus Elevation, Topography and Mean Annual Temperature
1500
2000
Elevation: metres
Middle
Elevation
0.088+/-0.022
45
501 to 999
1000 to 1499
1500 to 1999
2000 to 2499
2500 to 2999
-150
N th America:
North
A
i
Surface
S f
Temperature
T
t
Trend
T d Magnitude
M
it d vs Elevation
El
ti
High
Elevation
0.123+/0.014
Africa
Number of
Sites
Mean surface temperature trends for 1948-2002 are listed for each continent in the
table above. Uncertainty in mean trends is quantified using 95% confidence
intervals. Red shading indicates significant warming for the continent as a whole
(p<0 05) For combined stations this is true on all continents except Antarctica,
(p<0.05).
Antarctica and
in North America there is significant warming at all elevations on average.
0
500
The map to the right illustrates whether sites in North America
show warming or cooling surface trends over the period 19482002. Figures are shown in degC/decade. The strongest trends
are seen in the north of the region, over Canada, and in parts
of the far west of the U.S. (green, orange or red colouring)
often over +0.25°C/decade.
Surface Trend
Deg C/decade
552
N. America
Global and Continental-Scale Mean Trends:
Setting the North American Continent in Context
Spatial Patterns of Temperature Trends
1000
Continent
In North America (552 sites) the elevation distribution is skewed
with many more low and medium elevation sites than higher ones.
The highest lies just below 3000 m. Most mountain sites are
therefore from the forest zone, as the forest/tundra ecotone (as
shown in the photo below) lies above 3000 metres in the southern
part of the continental interior. However all sites are of high
quality specifically evaluated for long-term
quality,
long term climate study.
study
20
We examine patterns of temperature trends observed
over the 20th century in North American mountains
using climate records from two high quality global
surface datasets: The Global Historical Climate
Network (GHCN) version 2 (Peterson & Vose 1997)
and Climate Research Unit (CRUv2) (Jones &
Moberg 2003).
500
Table 2: Mean Temperature Trends by
Continent & Elevation Band: 1948-2002
80
100
•
•
•
•
•
•
•
•
0.00
10.00
Mean Annual Temperature: deg C
20.00
The first scatter plot shows temperature trend magnitude (as measured by the gradient of an ordinary least squares regression line: degC/year) versus station elevation, for
North America. There is a significant negative relationship between trend magnitude and elevation in a statistical sense. However we must be careful in our interpretation. This
relationship is caused by the concentration of the steepest warming trends at lower elevations (where there are more stations anyway). There is also more variability in trend
magnitudes at lower elevations. Topography is also important (second figure). Mountain summit temperature trends are less variable than at more incised sites (particularly
mountain valleys) (standard deviation test: p=0.0689, taking the smaller number of summit sites into account), presumably because the former sites are more exposed to the
free-atmospheric influence, which will eradicate local variability. Other reasons for the increased trend magnitudes at low elevations may include urbanisation (concentrated at
lower elevations). The decreased variability of trends at higher elevations means both that future mountain trends should be easier to forecast, and that they are good indicators
of global trends.
trends
The final scatter plot shows the influence of mean annual temperature on the rate of warming. There is dramatic enhancement of warming rates when present mean annual
temperatures are near 0°C. This is strong influence of the importance of the snow and ice feedback mechanism, in enhancing warming rates where long-term snow pack decline
is more likely. Between 15 and 0°C (red lines on the scatterplot) the correlation between temperature trend magnitude and mean annual temperature is -0.520. For the few
stations with mean annual temperatures below freezing, the warming rate is reduced.
Further work is investigating the effects of aspect and continentality on warming rates. Preliminary analyses show that these two variables do not show simplistic relationships
with trend magnitudes. Continentality is a difficult concept to define, and depends on latitude and prevailing wind patterns as well as distance to the coast, and aspect effects
combine both the influence of differential radiative loading and variable exposure to the free atmospheric flow, which again depends on the local synoptic climatology.
30.00
Above this zone there appears to be decreased sensitivity, but there are few stations (only 17
stations with mean annual temperatures below freezing).
freezing) Explanations could include increased
snowfall and surface cooling at highest elevations but this requires further investigation.
Although small rises in temperature mean melting glaciers and shrinking snow-fields in mountain
regions, these phenomena on their own are not evidence of increased warming rates universally
in mountains relative to lowlands. We must not confuse signs of warming (e.g. consequences)
with the size of the warming itself, and there may be opposite sensitivities in the critical melt
zone and immediately above.
References
Barry, R.G. (2006), Progress in Physical Geography 30, 285-306.
Beniston,, M. et al. ((1997)) Climatic Change
g 36,, 233-252.
Bradley, R.S, Vuille, M, Diaz, H.F, Vergara, W. (2006), Science 312, 1755-1756.
Chen, B, Chao, W.C. & Liu, X. (2003), Climate Dynamics 20, 401-413.
Diaz, H.F. & Bradley, R.S. (1997), Climatic Change 36, 253-279.
Diaz, H. F, Grosjean, M. Graumlich, L. (2003), Climatic Change 59, 1–4.
Jones, P.D. (1994) J. Climate 7, 1794-1802
Jones, P.D., New, M., Parker, D.E., Martin, S. & Rigor, I.G. (1999) Rev. Geophys. 37, 173-199.
Jones, P.D. & Moberg, A. (2003), J. Climate 16, 206-223.
Pepin, N.C. & Seidel, D.J. (2005) Journal of Geophysical Research 110, D03104: doi:10.1029/2004JD005047.
Pepin, N.C. & Lundquist, J. (2008), Geophys. Res. Lett. In press.
Peterson, T. & Vose, R.S. (1997) Bull. Amer. Met. Soc. 78, 2837-2848
Santer, B.D. et al. (2000), Journal of Geophysical Research, 105, 7337-7356.
You, et al. (2008), Geophys. Res. Lett. 35, L04704: doi:10.1029/2007GL032669.