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. 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