SURVEY OF GREENLAND INSTRUMENTAL TEMPERATURE RECORDS…polarmet.osu.edu/jbox/pubs/Box_2002_Greenland_Temperature_Analy… · SURVEY OF GREENLAND INSTRUMENTAL TEMPERATURE RECORDS:

INTERNATIONAL JOURNAL OF CLIMATOLOGY

Int. J. Climatol. 22: 1829–1847 (2002)

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.852

SURVEY OF GREENLAND INSTRUMENTAL TEMPERATURE RECORDS:1873–2001

JASON E. BOX*Cooperative Institute for Research in Environmental Sciences (CIRES), CB 216, University of Colorado, Boulder, CO 80309, USA

Received 8 February 2002Revised 11 July 2002

Accepted 22 July 2002

ABSTRACT

Temporal and spatial variability are analysed in Greenland instrumental temperature records from 24 coastal and threeice sheet locations. Trends over the longest period available, 1873–2001, at Ilulissat/Jakobshavn indicate statisticallysignificant warming in all seasons: 5 °C in winter. Trends over the 1901–2000 century in southern Greenland indicatestatistically significant spring and summer cooling. General periods of warming occurred from 1885 to 1947 and 1984to 2001, and cooling occurred from 1955 to 1984. The standard period 1961–90 was marked by 1–2 °C statisticallysignificant cooling. In contrast to Northern Hemisphere mean temperatures, the 1990s do not contain the warmest yearson record in Greenland. The warmest years in Greenland were 1932, 1947, 1960, and 1941. The coldest years were 1918,1984, 1993, and 1972, several of which coincide with major volcanic eruptions. Over 1991–2000, statistically significant2–4 °C warming was observed in western Greenland, 1.1 °C warming at the ice sheet summit (3200 m), although this isstatistically insignificant. Annual temperature trends are dominated by winter variability. Much of the observed variabilityis shown to be linked with the North Atlantic oscillation (NAO), sea ice extent, and volcanism. The correlation of coastaltemperature anomalies with the NAO is statistically significant, in autumn and winter at western and southern sites.Warming from 1873 to 1930 and subsequent cooling persists after the removal of the NAO signal. Temperature trends areoften opposite between west and east Greenland. This apparent teleconnection is spurious, however, given insignificanteast–west correlation values. Frequency peaks correspond with periods of 3.7, 14.3, 9.1, 5.5–6.0, 11.1, and 7.1 years inboth temperature and NAO. Copyright 2002 Royal Meteorological Society.

KEY WORDS: Greenland; climatology; temperature; North Atlantic oscillation; volcano

1. INTRODUCTION

Understanding Greenland climate is crucial, given that major changes in Arctic climate have been recentlyobserved and climate models predict high-latitude amplification of the enhanced greenhouse effect (Houghtonet al., 2001). There is mounting evidence that climate changes in the most recent decade are unprecedentedin historical times. Nine of the ten warmest years globally have occurred between 1990 and 2001 (WMO,2001). It is very likely that the decade of the 1990s is the warmest in the past 1000 years (Mann et al.,1999; Crowley, 2000). Climate fluctuations influence global ice masses and, in turn, contribute to sea leveland ocean circulation changes, occasionally in the form of rapid ice discharges (Heinrich events) associatedwith abrupt climate change (Bond et al., 1993). The Greenland ice sheet is of particular interest becauseit appears to have contributed more to sea-level change than Antarctica during the climatic optimum ∼135thousand years ago (Cuffey and Marshall, 2000). Greenland ice-sheet melt between 1865 and 1990 contributedabout 3.0 ± 1.6 cm to global sea-level rise (Zuo and Oerlemans, 1997). Under specified greenhouse-gas andsulphate-aerosol forcings, a coupled atmosphere–ocean general circulation model predicts that glacier melt

* Correspondence to: Jason E. Box, Byrd Polar Research Center, The Ohio State University, 1090 Carmack Road, Scott Hall Room108, Columbus, Ohio, 43210-1002, USA; e-mail: [email protected]

Copyright 2002 Royal Meteorological Society

1830 J. E. BOX

equivalent to 13.2 cm of sea-level rise will occur between 1990 and 2100, with 58% from melting of theGreenland ice sheet (Gregory and Oerlemans, 1998).

Greenland is the largest island on Earth, 82% covered by a single ice sheet (1.6 × 106 km2) with roughly1500 m average elevation. Greenland is over 2500 km in latitudinal extent, extending 770 km below the ArcticCircle. Consequently, there are large north–south contrasts in solar irradiance and temperature at any time ofyear. Greenland’s topography opposes atmospheric circulation, enhancing Northern Hemisphere meridionalheat exchanges (Barry and Kiladis, 1982; Kristjansson and McInnes, 1999). Temperatures around Greenlandhave been analysed between 1880 and 1955 by Putnins (1970) and for 1961–90 by Cappelen et al. (2001).Monthly mean temperatures show a large degree of interannual variability, increasing northward, particularlyin winter. Putnins (1970) identified five temperature trend periods: 1880–95, 1895–1909, 1909–29, 1929–43,and 1943–55. The most consistent period was 1909–29, when steady warming occurred. Significantcooling is said to have occurred at all stations during 1929–43. The 1943–55 period was characterizedby warming, except at Upernavik. Trends for these periods are investigated in this paper and with a largerdata set.

Other previous work relevant to Greenland temperature changes includes analysis of upper air temperaturetrends (Kahl et al., 1993) and changes in extreme air temperatures in the Arctic (Przybylak, 1997). Theformer work determined that, between 1959 and 1986, trends were predominantly negative in Greenlandbetween 850 and 500 hPa, with a small springtime positive tendency in north and eastern Greenland at 400to 300 hPa. Extreme temperatures trends between 1951 and 1990 are characterized by reductions of theannual maximum and minimum temperatures around Greenland by −0.25 °C and −0.75 °C respectively, with+0.25 °C warming in the northeast. The work of Jones et al. (1999) to determine the global distributionof surface temperature changes over 150 years indicates that temperature trends are typically oppositebetween western and eastern Greenland. Across Greenland, 1998 was between +0.2 and +1 °C warmerthan the 1961–90 period, 3 °C above normal in the adjacent North Atlantic, and −1 °C colder at thesouthern tip. Steffen and Box (2001) derived a 2 °C warming over the ice sheet plateau over 1995–99compared with 1951–60 maps by Ohmura (1987). Abdalati and Steffen (1997a) found a 4.4% per yearincrease in ice sheet melt area between 1979 and 1991, and a corollary trend in coastal temperatureanomalies was noted. Tuomenvirta et al. (2000) show that mean maximum and minimum temperatures hadstatistically significant negative trends in western coastal Greenland during the period 1950–95, while overthe Nordic Seas and Fenno-Scandia the trends were generally positive. The opposite temperature trendsare induced by the North Atlantic oscillation (NAO), an oscillation in the strength of both the IcelandicLow and the Azores High. A weaker than normal Icelandic low is associated with colder than normalnorthwestern European winters, due to a weakening of the North Atlantic westerlies which normally bringmoderating oceanic heat to northwestern Europe. West Greenland temperatures are above normal in thenegative phase of the NAO due to weaker than normal Baffin Bay northerlies. In the opposite, positiveNAO phase, cold Greenlandic winters correlate with mild conditions in northwestern Europe caused by astrengthening of the North Atlantic westerlies and Baffin Bay northerlies. The oscillation in temperaturesbetween Greenland and northwestern Europe is commonly referred to as the temperature see-saw (Van Loonand Rogers, 1978).

Dahl-Jensen et al. (1998) offer a longer-term perspective on temperature changes over Greenland. Coolingover Greenland began 4000 years BP and reversed at 2000 years BP. More recent cold periods were centredat AD 1550 and AD 1850, with warm intervals during the Medieval Warming Period (MWP) near AD 1000and the 1930s temperature maximum. The Little Ice Age (LIA) in Greenland appears to span the periodof approximately AD 1400 to AD 1850. The coupling of ice sheet and coastal temperature variations is not,however, well established.

The availability of a 30 year extension of long-term instrumental temperature data (Cappelen et al., 2001)has prompted the objectives of this study, which, besides an objective survey, seek to answer the followingquestions. Are recent Greenland temperatures unprecedented, as they are for the Northern Hemisphereaverage? What level of control on Greenland temperature anomalies can be attributed to known climateforcings, such as the NAO and volcanism? Which locations exhibit the highest correlation with the NAO?What are the temperature trend values over various periods?

Copyright 2002 Royal Meteorological Society Int. J. Climatol. 22: 1829–1847 (2002)

GREENLAND TEMPERATURE RECORDS 1831

2. DATA

2.1. Temperature data

Two instrumental temperature data sets were merged to produce a time series covering the Greenlandregion. Temperature data between 1958 and 2000 from 27 Greenland locations were obtained from the DanishMeteorological Institute (DMI) (Cappelen et al., 2001). The National Aeronautics and Space Administration(NASA), Goddard Institute for Space Studies (GISS), and Global Historical Climatology Network (GHCN)data (Peterson and Vose, 1997) included 13 Greenland station records with temperature data before 1958. Thecombined data sets are summarized in Table I and Figure 1. In the case of Greenland’s capital Nuuk/Godthab,and Ilulissat/Jakobshavn, the data available from the GHCN begin in 1866. For these two sites, however,the official records begin in 1875 and 1873 respectively (DMI, personal communication; Clayton, 1944a,b).The earliest 7–9 years of data at these two sites are not used in the present study due to the lack ofmetadata. Only a handful of long-term instrumental temperature records from ice cap locations exist. Twoof the longest (15 years) records come from DYE-2 and DYE-3, located above 2000 m on the southwesternand southeastern slopes respectively. Problems exist in the DYE-2 record (and DYE-3), owing to extendeddata gaps and the placement of the temperature sensor too near the building and at a non-standard height of∼25 m (Ohmura, 1987). Temperature measurements much above the standard 2 m height in the Arctic exhibita positive bias and are affected differently by the surface temperature inversion strength. A 14 year record hasbeen produced from automatic weather station (AWS) data collected at the ice cap summit. This ‘Summit’compilation represents the merging of 1987–96 University Wisconsin AWS data (Stearns and Weidner, 1991)with 1996–2001 Greenland Climate Network (GC-Net) AWS data (Steffen and Box, 2001). Gaps of up to2 months in the Summit compilation were supplemented with a passive microwave-based temperature proxy(Shuman et al., 2001).

Other than site relocation, changes in instrument type and measurement frequency are important factors thataffect data homogeneity. Detection of such error has been discussed by Peterson and Vose (1997) and Joneset al. (1999). Information on Greenland measurement practices and station notes is available from SmithsonianInstitute World Weather Records (WWR) (Clayton, 1944a,b; Smithsonian Institution 1959, 1968). Before thewidespread standardization of global weather observations in the International Geophysical Year (IGY) 1957,temperature observations at Greenland stations were made three times daily, at 8 h, 14 h, and 21 h, 45th westmeridian time, i.e. UTC − 3 h. Monthly averages were constructed from the observations using the followingformulae for the mean temperature (mT t). mT t = M + C, where M = 1/9[2(8 h + 14 h) + 5 × 21 h]S andcorrections C represent the diurnal temperature fluctuation S = (T14T08T21)/2, which are applied June throughto September. Slightly differing formula are employed to adjust other site data with differing observationalfrequency. In the IGY, the observation frequency increased at many new ‘synoptic observation’ sites to3 h UTC intervals, i.e. 0 h, 3 h, etc. (Smithsonian Institution, 1959). Continuous measurements were madeby thermographs and automated devices. To measure ambient air temperature accurately, thermometersare situated in the shade, typically provided by a white screen, situated at a height between 1 and 2 mabove the surface. Modern instruments have elaborate thermal radiation shielding, including low-emissivityaluminium cavities with motorized ventilation. Other than visual inspection of the higher-resolution records,one homogeneity test applied to GHCN and DMI data is intra-correlation of nearby stations. Outliers areeliminated by this approach when the difference between nearby stations exceeds a reasonable value. Intra-station correlation is measured in this study to illustrate that the data homogeneity problems are secondaryto major climate signals. Temporal changes in variability are used to investigate potential data homogeneityshifts.

2.2. NAO data

The NAO index is often defined as the difference of sea-level pressure between two stations situated closeto the Icelandic Low and the Azores High. Station-based NAO data from two sources are analysed. One,spanning 1874–2000, is based on the pressure difference of Stykkisholmur, Iceland, minus Ponta Delgada,Azores (Rogers, 1997). Another, spanning 1864–2001, is based on the pressure difference between Lisbon,


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Figure 1. Map showing locations of coastal and ice sheet temperature records used in this study. Crosses indicate the position of theWMO stations (see Table I for station names). Elevation contours are in metres above sea level

Portugal, and Stykkisholmur (Hurrell, 1995). It is possible to combine multiple time series to construct NAOindices that are less sensitive to displacements in the centres of action. These time series, however, do notextend far enough back in time to cover the entire temperature instrumental record. The Rogers (1997) andHurrell (1995) data are certainly sufficient for this study.



Table I. Summary of coastal temperature data sets used in this study

WMOID

Site Name Lat. N(deg)

Lat. N(min)

Lon. W(deg)

Lon. W(min)

Elevation(m)

Earliestyear

Latestyear

Completion(%)

4202 Pituffik/Thule Air Base 76 32 68 45 77 1946 1999 974207 Hall Land AWS 81 41 59 57 105 1982 1999 75b

4210 Upernavik 72 47 56 10 120 1873 1999 914216 Ilulissat/Jakobshavn (Town) 69 13 51 3 39 1961 1978 994220 Aasiaat/Egedesminde 68 42 52 45 43 1949 2001 964221 Ilulissat/Jakobshavn (Airport) 69 14 51 4 29 1873 1999 87c

4230 Sisimiut/Holsteinborg 66 55 53 40 12 1961 1999 974231 Kangerlussuaq/Søndre Strømfjord 67 1 50 42 50 1949 1999 914240 Maniitsoq/Sukkertoppen 65 24 52 52 25 1961 1979 1004242 Sioralik AWS 65 1 52 33 14 1987 1999 944250 Nuuk/Godthab 64 10 51 45 80 1875 2001 954260 Paamiut/Frederikshab 62 0 49 40 13 1958 1999 1004261 Kangilinnguit/Ivigtut 61 12 48 12 10 1875 1966 964270 Narsarsuaq 61 10 45 25 27 1961 1999 994272 Qaqortoq/Julianehab 60 43 46 3 32 1961 1999 1004301 Kap M. Jesup AWS 83 39 33 22 4 1980 1999 65b

4312 Nord 81 36 16 39 34 1952 1999 724320 Danmarkshavn 76 46 18 40 11 1951 2001 984330 Daneborg 74 18 20 13 44 1958 1999 774339 Illoqqortoormiut/Scoresby sund 70 29 21 57 65 1931 1999 784351 Aputiteeq 67 47 32 18 13 1958 1999 894360 Tasiilaq/Ammassalik 65 36 37 38 50 1895 2001 984382 Ikermiuarsuk 61 56 42 4 39 1958 1999 934390 Pr. Christian S. 60 3 43 10 88 1950 1999 92

– Summita 72 35 37 38 3208 1987 2001 924465 DYE-2 66 29 46 17 2332 1974 1988 834475 DYE-3 65 11 43 50 2652 1974 1989 86

a Combination of Shuman et al. (2001) and data from GC-Net Steffen and Box (2001).b The automatic weather station AWS data have gaps throughout the monthly records.c Extended data gap in merged Ilulissat data exists from 1981 to 1991 (inclusive).

3. METHODS

Several statistical techniques are applied to explore Greenland temperature records. These methods includeleast-squares regression, spatial correlation, frequency analysis, autocorrelation, and running variance statistics.Calculations are made over a set of time periods, including periods defined by the World MeteorologicalOrganization as time ‘standards’, i.e. the most recent standard decade (1991–2000), standard 30-year ‘normals’(1901–30, 1931–60, 1961–90), and the standard century (1901–2000). In general, only records with fewerthan 10% missing data are considered in this analysis. One exception is the Ilulissat/Jakobshavn recordspanning 1873–1999, with 13% missing data. This analysis focuses on seasonal and annual periods withoccasional emphasis on monthly statistics. Trends are calculated from linear least-squares regression. Thestatistical significance of trend slopes is measured as 1 − p to assess the reliability of conclusions regardingclimate changes. It should be noted that this statistic fails to measure the significance of trends with no slope.In other words, one cannot state that a nearly constant temperature period is statistically significant. Thecoefficient of the regression is multiplied by the number of years to indicate the magnitude of change overthe period. Analysis showed that the raw data have sufficiently symmetric distributions that transformationwas not necessary. Temperature anomalies are derived by linearly detrending long-term trends in the dataderived by linear regression. Spatial correlation matrices of temperature anomalies over different time periods


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are presented in Section 4 to verify the extent of spatial decay in correlation and cross-island teleconnections.A 15-year interval of running statistics is used to measure changes in variability through time. Time seriesare compared with indices of climate forcing, including volcanic eruptions and the NAO.

Trends in the instrumental temperature record depend somewhat on the temporal scale chosen. Various trendperiods have been identified in monthly data for the 1880 to 1955 interval by Putnins (1970). Slightly differenttrend periods emerge on the seasonal time scale. Given these facts, trends are investigated for various periods.Long-term temperature fluctuations are first described using running annual and select monthly averages, asdone by Putnins (1970). Temperature trends for various time periods are presented, followed by possibleexplanations of interannual features in the time series.

Periodicity in station records exceeding 85 years is investigated by Fourier analysis of monthly temperaturedata. Four sites with sufficient records are considered in the frequency analysis (Upernavik, Ilulis-sat/Jakobshavn, Nuuk/Godthab, Tasiilaq/ Ammassalik; Table I). Periods longer than about 20 years arearguably inconclusive, given the 100 year time series analysed by this technique. Interpretation of periods ofless than 2 years in frequency is not attempted here.

Temporal autocorrelation in temperature records is determined for seasonal and annual data over the1901–2000 century. Lags up to 30 years are considered. The temporal significance scale is measured asthe time at which the Pearson’s correlation coefficient drops below 1/e.

Many of the locations referred to in this study have both Greenlandic and Danish names. Greenlandicnames are used as the principal names in this study, but occasionally both names are given; see Table I.

4. RESULTS AND DISCUSSION

4.1. Long term means and variability

Long-term seasonal average temperatures are presented for the standard period 1961–90 (Table II).Cappelen et al. (2001) provide monthly normals over this period. All summer averages for coastal sitesare above the ice melting point. The variance statistics suggest that summer at a coastal site with an averagetemperature below freezing is highly unlikely. Continuous summer melt conditions are promoted by unlimitedday length. Variance values are consistently largest in winter and smallest in summer. Summer temperaturevariability is dampened by the influence of melting snow and ice, in which temperatures are locked nearthe melting point, given the sink of sensible heat into melting and evaporation (Steffen, 1995). Largestvariances are found at the interior Kangerlussuaq site and other western sites (i.e. Sisimiut and Aasiaat).Smallest variance values are found for east Greenland sites. Consistent with the variance statistics in Table II,Cappelen et al. (2001) discuss the importance of sea ice to coastal temperature variability, given that east andsoutheast sea ice concentrations are consistently greater than in the west and southwest.

The ranking of the five extreme warmest and coldest seasons and annual averages over the standardcentury 1901–2000 is given in Table III. The majority of coldest seasons and years occur before 1920,with the exception of cold years that appear to be the result of large volcanic eruptions, such as the 1992cold associated with the 1991 Mt Pinatubo eruption (Abdalati and Steffen, 1997b) or the 1983 El Chichoneruption. It can be said that coastal sea ice anomalies affect air temperatures and temperature anomalies affectsea ice thickness and concentration in a positive feedback loop. Compared with the 1978–87 period, −3 °Ctemperature anomalies in 1983 and 1984 are associated with 30% more sea ice in Baffin Bay, Davis Strait,and the Labrador Sea (Parkinson et al., 1999). January–March 1983 and 1972 were identified by Chapmanand Walsh (1993) as the largest Arctic ice extent years from 1961 to 1990. 1972 was 2 °C below normal, butdoes not coincide with a major volcanic eruption. The warmest years were 1929 and 1947, generally from1929 to 1941. Over the 1901–2000 standard century, European station temperatures (Oslo and Potsdam) fromthe GHCN indicate that 1941 was among the coldest years, while it ranks among the warmest in Greenland,illustrating the temperature see-saw between Greenland and northwestern Europe (e.g. Van Loon and Rogers,1978). The climate conditions that lead to extremely warm years are uncertain, unless explained by extremenegative phases of the NAO or the absence of volcanic eruptions, as proposed by Robock (2000).



Table II. Temperature (°C) statistics (with two standard deviations in parentheses) for Greenland sites over the 1961–90standard period

Site Winter mean Spring mean Summer mean Autumn mean Annual mean

Pituffik/Thule Air Base −23.0 (5.9 ) −15.3 (3.0 ) 3.3 (2.3 ) −9.2 (3.9 ) −11.1 (2.3 )Aasiaat/Egedesminde −13.0 (7.9 ) −9.2 (4.5 ) 4.6 (1.5 ) −1.9 (1.6 ) −4.9 (3.3 )Sisimiut/Holsteinborg −12.3 (7.7) −7.1 (3.2 ) 5.3 (1.3 ) −1.5 (1.7 ) −3.9 (2.8 )Kangerlussuaq/Søndre Strømfjord −18.0 (8.5 ) −7.6 (5.3 ) 9.3 (1.8 ) −5.0 (3.1 ) −5.3 (3.4 )Nuuk/Godthab −7.1 (5.7 ) −3.7 (3.0 ) 5.5 (1.7 ) −0.3 (1.8 ) −1.4 (2.4 )Paamiut/Frederikshab −6.1 (4.7 ) −2.3 (2.9 ) 4.9 (1.7 ) 0.3 (2.0 ) −0.8 (2.2 )Narsarsuaq −6.3 (6.7 ) −0.1 (4.6 ) 9.3 (1.5 ) 0.9 (2.5 ) 0.9 (2.8 )Qaqortoq/Julianehab −5.0 (5.2 ) −0.6 (3.6 ) 6.5 (1.6 ) 1.4 (0.2 ) 0.6 (2.5 )Danmarkshavn −23.1 (3.7 ) −15.7 (2.5 ) 2.3 (1.0 ) −12.6 (2.8 ) −12.3 (1.6 )Illoqqortoormiut/Scoresby sund −16.0 (4.3 ) −10.5 (3.3 ) 2.6 (1.8 ) −6.3 (2.8 ) −7.6 (2.4 )Tasiilaq/Ammassalik −7.5 (2.9 ) −3.8 (2.7 ) 5.5 (1.2 ) −0.9 (1.6 ) −1.7 (1.5 )Ikermiuarsuk −6.1 (2.4 ) −3.2 (1.9 ) 4.3 (1.8 ) −0.4 (1.6 ) −1.4 (1.3 )Summita −40.9 (5.2 ) −31.7 (4.6 ) −14.9 (3.1 ) −31.5 (2.9 ) −29.7 (2.2 )

a Statistics are for 1991–2000 standard decade.

Table III. Ranking of coldest and warmest years over the 1901–2000 standard century

Site/season Coldest 5 years (coldest on left) Warmest 5 years (warmest on right)

UpernavikWinter 1918 1910 1914 1909 1911 1941 1963 1940 1929 1947Spring 1907 1964 1992 1960 1993 1905 1930 1962 1937 1932Summer 1970 1972 1996 1992 1922 1939 1928 1960 1957 1931Autumn 1917 1958 1996 1918 1904 1995 1960 1946 1998 1928Annual 1918 1914 1992 1907 1910 1936 1928 1939 1940 1947

Ilulissat/JakobshavnWinter 1910 1914 1918 1911 1906 1964 1980 1929 1947 1963Spring 1992 1907 1914 1972 1961 1947 1929 1928 1962 1932Summer 1992 1972 1919 1914 1918 1928 1949 1957 1931 1960Autumn 1918 1917 1920 1944 1904 1995 1928 1915 1939 1960Annual 1918 1910 1914 1992 1972 1941 1936 1929 1928 1947

Nuuk/GodthabWinter 1984 1993 1983 1911 1992 1978 1917 1985 1963 1947Spring 1993 1989 1907 1904 1983 1928 1935 1941 1929 1932Summer 1972 1992 1914 1989 1983 1941 1929 1928 1931 1948Autumn 1982 1920 1983 1913 1904 1915 1955 1966 1952 1960Annual 1984 1983 1993 1992 1972 1928 1936 1929 1947 1941

Tasiilaq/AmmassalikWinter 1918 1920 1971 1905 1981 1947 1926 1932 1946 1929Spring 1990 1914 1969 1906 1983 1945 1941 1939 1923 1929Summer 1983 1992 1970 1987 1989 1950 1933 1936 1947 1932Autumn 1917 1923 1904 1971 1907 1945 1960 1915 1939 1941Annual 1910 1983 1971 1907 1981 1932 1941 1928 1939 1929

4.2. Temperature trends

Presentation of temperature trend results begins with a 30-year update of two figures of running means inPutnins (1970) (Figure 2). Annual mean temperatures from five sites indicate a 1 to 3.5 °C warming from1885 to 1935 for sites north of Ivigtut (Figure 3). This warming is composed of a gradual warming from 1885


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Figure 2. The 21-year running mean January temperatures at Greenland sites with measurements spanning at least 70 years; this figureis a 30-year extension of a graph in Putnins (1970)

Figure 3. The 21-year running mean annual temperatures at Greenland sites with measurements spanning at least 70 years; this figureis a 30-year extension of a graph in Putnins (1970)

to 1915, followed by a greater rate of warming until 1935. The Ivigtut annual trend is positive over the entireseries from 1885 to 1957, corresponding to a 90-year warming of 2 °C. Such warming is large comparedwith the Northern Hemisphere averages, e.g. Mann et al. (1999). The 3–5 °C warming at western Greenlandsites, peaking in the 1930s, was followed by a 1–2 °C decline in temperature until the mid 1980s. Thelargest changes in average temperatures occur in winter and increase with latitude. A 7 °C warming occurredin January at Upernavik from 1915–40, while at the southernmost site, a warming of 3.5 °C is observed.



January temperatures decline from about 1875 until 1915, as opposed to annual temperatures, which increasefrom 1885. The only period specified by Putnins (1970) with a large fraction of significant trends is a warmingbetween 1909 and 1929. Interpretation of other time periods is complicated by an abundance of data gaps. Thetrends over standard periods yield a greater abundance of significant trends, primarily because the standardperiods are longer than those specified by Putnins (1970).

Temperatures anomalies for the 1873–1900 period are characterized by relative cold in 1883. The 1883cold period is preceded by the huge Krakatau volcanic eruption. It should also be noted that a negativetemperature trend precedes this eruption. Annual trends over this period are therefore mixed, between coolingfrom 1873 to 1883 and warming from 1883 to 1900.

Trend statistics for various time periods, including standard periods, are given in Table IV. The 1901–30period is characterized by the largest statistically significant annual warming, between 1.9 and 2.3 °C. Warmingis pronounced in winter and spring, between 2.0 and 5.4 °C. January warming exceeds 8 °C at Upernavikand Ilulissat/Jakobshavn. This warming period has also been identified in ice sheet borehole temperatures(Dahl-Jensen et al., 1998) and Northern Hemisphere mean temperatures (Mann et al., 1999).

The only statistically significant trends over the 1931–60 period were negative. The magnitudes are strongestin summer in the south, and in spring and winter at Ilulissat in the west. Ilulissat appeared to warm from Mayto August over this period. A warming signal is found among the stations during October and November,except at Upernavik. Data gaps complicated analysis of this period.

The 1961–90 period is characteristic of widespread cooling of between 1.7 and 2.1 °C; this is pronounced incentral western locations, such as Sisimiut and Nuuk. Summer warming is observed at Pituffik in the extremenorthwest of Greenland. In contrast to western Greenland, warming trends are observed for north and eaststations Danmarkshavn and Illoqqortoormiut/Scoresby sund in all months over this period, except October andNovember at Danmarkshavn. At Illoqqortoormiut there is a statistically significant annual warming (1.7 °C)over this period. Negative southeastern trends are consistent with western stations over this period. Coolingover this period has also been identified by Chapman and Walsh (1993), where there is a dramatic coolingin the Baffin Bay area from 1962 to 1980. Consistent with the results of Przybylak (1997) for extremetemperatures, eastern Greenland temperature variations are smaller than in the west.

Statistically significant warming, between 1.0 and 4.1 °C, is observed for the 1991–2000 standard decade.Monthly trends exceed +10 °C at numerous sites in winter months. Summer warming trends are moderate.Again, in contrast to western Greenland, an opposite sign of temperature change is observed for eastGreenland. Statistically insignificant 1.1 °C warming at the ice sheet summit is observed over the 1991–2000standard decade.

Trends over the period 1873–2001 at Ilulissat/Jakobshavn indicate statistically significant warming in allseasons: 5.0 °C in winter and 2.1 °C annually. The summer warming at Ilulissat should have enhanced nearbyglacier melt rates. As many of these southern coastal sites have winter temperatures near freezing point, thetemperature increases over this period changed some continuous permafrost into discontinuous permafrost(Humlum, 1999). At Nuuk, a statistically significant winter warming of 2.3 °C is observed over this period.The annual trend over this period at Nuuk is 0.7 °C. Summer cooling is observed, but this is not statisticallysignificant.

Two coastal station records yielded monthly statistics for trend analysis spanning the standard 1901–2000century: Nuuk in western Greenland and Tasiilaq in south east Greenland. Collectively, these indicate aseasonal shift in temperatures, with winter warming and spring and summer cooling. This result appears tobe in contrast to the longer 1873–2001 period, but serves to illustrate the fact that trend results subjectivelydepend on the time interval. In this case, the longer period is strongly influenced by winter warming inthe 1873–1900 period, whereas the standard century is dominated by summer temperature variability andwhere positive and negative winter trends cancel each other out. Summer cooling trends of −1.0 °C atNuuk and −0.7 °C at Tasiilaq are statistically significant at the 99% confidence level. On a monthly basis,a 4.6 °C warming trend is observed in January at Upernavik. Note, however, that the Upernavik record isonly 82% complete. Greatest cooling trends are observed in spring for Upernavik and Nuuk, −2.5 °C and−2.3 °C respectively. Annual trends are small and statistically insignificant. Large trends do not emergeover this period due to the cancelling of warming and cooling phases. Thus, it is better to keep in mind


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Table IV. Greenland temperature trends over various periods, including standard periodsa

Site and time period Region Winter Sig. Spring Sig. Summer Sig. Autumn Sig. Annual Sig.

1873–2001Ilulissat/Jakobshavn W 5.0 99 1.4 86 0.7 95 0.9 95 2.1 99

1875–2001Nuuk/Godthab SW 2.3 99 −0.3 45 −0.3 59 0.6 96 0.7 92

1901–2000Nuuk/Godthab SW 0.7 63 −1.4 97 −1.0 99 0.0 10 −0.37 64Tasiilaq/Ammassalik SE 0.5 60 −0.3 46 −0.7 99 0.5 78 0.10 23

1901–30Upernavik W 5.4 99 2.0 86 0.4 49 1.4 85 2.34 99Ilulissat/Jakobshavn W 4.5 96 4.4 98 −0.3 37 0.6 51 2.33 98Nuuk/Godthab SW 2.1 89 3.5 99 1.2 95 1.3 97 1.89 99Tasiilaq/Ammassalik SE 3.8 99 3.4 99 0.9 99 0.6 51 2.24 99

1931–60Upernavik W −2.8 89Ilulissat/Jakobshavn W −2.4 91 −3.6 99 0.4 47Ivigtut SW −0.8 53 0.5 40 −0.4 51 0.4 50 −0.30 43Nuuk/Godthab SW −0.2 11 −0.8 61 −0.8 90Tasiilaq/Ammassalik SE −0.5 42 −0.9 70 −0.9 99 0.5 53 −0.47 73

1961–90Pituffik/Thule Air Base NW −3.7 96 −0.2 16 2.1 99 −2.0 90 −0.48 47Aasiaat/Egedesminde W −4.6 94 −2.0 84 −0.3 46 −0.3 40 −1.78 91Sisimiut/Holsteinborg W −5.0 96 −1.3 80 −0.1 25 −0.5 61 −1.75 96Kangerlussuaq/Søndre Strømfjord W −5.1 94 −1.7 68 −0.4 54Nuuk/Godthab SW −3.2 92 −2.0 97 −0.6 75 −0.6 74 −1.62 97Paamiut/Frederikshab S −2.8 94 −2.0 98 −0.6 77 −0.5 58 −1.53 98Narsarsuaq S −3.2 86 −4.2 99 −0.2 24 0.1 14 −2.07 98Qaqortoq/Julianehab S −2.9 92 −3.1 99 −0.7 83 −0.3 28 −1.72 97Danmarkshavn NE 1.9 90 0.6 53 0.2 51 −0.4 31 0.64 78Illoqqortoormiut/Scoresby sund E 2.2 89 1.8 91 2.3 99 0.8 59 1.74 98Tasiilaq/Ammassalik SE 0.0 3 −0.9 71 −0.8 98 0.7 84 −0.28 45

1991–2000Aasiaat/Egedesminde W 8.0 99 5.5 99 1.7 92 1.1 81 4.07 99Kangerlussuaq/Søndre Strømfjord W 7.8 84 7.0 91 1.2 68 0.5 20 4.14 94Sioralik AWS SW 5.6 92 4.0 92 1.2 77 1.3 83 3.08 97Nuuk/Godthab SW 4.8 92 3.9 90 1.2 74 0.9 85 2.56 99Paamiut/Frederikshab S 4.2 90 3.9 88 0.7 48 −0.1 9 2.12 93Narsarsuaq S 4.1 78 4.5 89 −0.4 33 1.1 62 2.38 89Qaqortoq/Julianehab S 3.8 86 4.0 92 −0.1 7 0.8 62 2.17 91Nord NE 1.5 49 3.9 92 −0.1 5 1.8 74 1.73 84Danmarkshavn NE 1.4 76 2.5 88 −0.8 92 0.2 14 0.85 69Daneborg NE 0.3 11 5.3 98 −0.1 6 −0.3 12 1.27 70Illoqqortoormiut/Scoresby sund E −2.1 73 1.3 44 −1.5 92 −0.5 22 −0.75 55Tasiilaq/Ammassalik SE 0.8 40 1.3 80 0.5 49 1.2 94 1.00 88Pr. Christian S. S 1.4 76 0.5 34 −0.1 12 −0.2 21 0.52 54Summit C 3.8 76 2.6 62 −1.2 47 −0.7 28 1.13 58

a Trends with statistical significance at or above 90% are in bold type. Italic bold values indicate significance at or above 99%.



the distinct periods of warming from 1885–1935, cooling from 1940 to 1985, and warming from 1985 to2001.

4.3. Periodicities

In order of decreasing rank, Fast Fourier transforms (FFTs) indicate frequency peaks corresponding withperiods of 3.7, 14.3, 9.1, 5.5–6.0, 11.1, and 7.1 years over the same standard century (1901–2000, Figure 4).These peaks correspond closely with peaks in the NAO index FFT, with the exception of a shift in the3.7 years temperature peak to 3.85–4.0 years peak for the NAO. The NAO spectral peaks are more uniformin power, while the temperature spectra are dominated by the 3.7 and 14.3 years peaks. Spectral powers forperiods less than 14.3 years are greater at the three western Greenland sites, compared with Tasiilaq, theonly eastern site. The eastern site does not exhibit the strong 3.7 year NAO periodicity. The spectral peakat 11.1 years is suggestive of the cycle of solar activity indicated by the number of sunspots, which itself iswell known to have an 11 year cycle. The other periodicities in temperature appear to be related to the NAO.These periodicities were not always found to exceed peaks in the frequency spectra of Gaussian randomnumbers. The random numbers, however, produce a widely varying spectrum for each new sample.

4.4. Persistence of autocorrelation

Temporal autocorrelation analysis of the station data spanning 1901–2000 indicates that there is lesspersistence in winter temperatures than in summer. Winter autocorrelation values are typically 1 year or less;summer values are 3–4 years. There is a rebound of correlation between a lag of 12 and 14 years, probablycorresponding to the periodicity of 11.1 and 14.3 years identified by the Fourier analysis. There is also a

Figure 4. Fourier spectra of monthly station temperature data (solid line) and NAO data (dotted line) spanning 100 years. Verticaldashed lines indicate spectral peaks in temperature of 3.7, 5.5, 6.0, 9.1, 11.1, and 14.3 years


1840 J. E. BOX

repeating 4 to 5 year oscillation in temporal autocorrelation; this may be related to the NAO periodicity,although it lies below the 1/e significance level.

4.5. Spatial correlation of coastal temperature anomalies

There are spatially meaningful patterns in the correlation of temperature anomalies among the sites. Theperiods 1961–90 and 1991–2000 are chosen to illustrate this (Table V). A steady decay of correlation withdistance is evident in the station records. The correlation length scale, defined here as the distance whenthe correlation drops below 1/e, is roughly 1200 km for the period 1961–90, with values of 1100 km inspring and 1300 km in winter. There is a general lack of correlation between east- and west-coast stations.For example, the annual correlation between Station Nord (NOR) and Pituffik (PIT) is 0.32, despite the factthat they are closer to each other than Pituffik and Kangerlussuaq (KAN), which have an annual correlationcoefficient of 0.52. Correlation coefficient values are typically below 0.3 for all east versus west comparisons.There is a notable correlation between the DYE-2 ice cap station with nearby western and southern sites. Asimilar tendency is evident for DYE-3 with nearby eastern stations, although to a lesser extent (not shown).The coherent patterns of spatial correlation in the data set suggest that homogeneity problems associatedwith changing measurement practices are minimal. Differences exist between the eastern and western stationsin terms of extreme yearly temperatures (Table III) and trends (Table IV), as there is a lack of east–westcorrelation.

4.6. Temperature anomalies and the NAO

There is a significant correlation of western Greenland coastal temperature anomalies with the NAO,particularly in autumn and winter (Table VI, Figure 5). The literature points to the importance of the NAOfor influencing winter climate (Van Loon and Rogers, 1978; Hurrell, 1995). Here, the correlation betweentemperature anomalies and the NAO is greater in autumn. The Hurrell (1995) station-based NAO index,composed of Portugal and Iceland pressure differences, has a correlation with temperature anomalies that is0.04 lower than the Rogers (1997) NAO index, composed of Azores and Iceland data. The results presentedhere are based on the Rogers (1997) data. The statistical significance of the correlations with temperatureanomalies are above 99% in most cases. To illustrate the temperature see-saw between Greenland andnorthwestern Europe, the correlation is typically made between Ilulissat/Jakobshavn and Oslo, Norway, orCopenhagen, Denmark. The highest correlation with the NAO is, however, with Qaqortoq/Prins ChristianSund, at the southeast tip of Greenland. A high annual correlation exists between the NAO and temperatureanomalies at the interior land site of Kangerlussuaq (1961–90). This high correlation is dominated by thewinter signal (r = −0.63). NAO variations are linked with sea ice flux changes in northeast Greenland (Kwokand Rothrock, 1999) and with sea ice anomalies in the Davis Strait (Parkinson et al., 1999).

4.7. Greenland temperature fluctuations and volcanism

The largest negative temperature anomalies in Greenland records are preceded by large volcanic eruptions.Results from Crowley (2000) indicate that volcanism explains roughly 15–30% of the variability in globaltemperatures. Large volcanic eruptions inject sulphate aerosols into the stratosphere, which reflect incomingsolar radiation, cooling the climate. The atmospheric residence time of the aerosols is between 1 and 3 years(Robock, 2000). The June–July 1991 eruption of Mt Pinatubo, for example, is associated with significantreductions of ice sheet melt extent (Abdalati and Steffen, 1997b). Positive sea ice extent anomalies in BaffinBay and the Labrador Sea in 1983–84 and 1993 (Parkinson et al., 1999) are preceded by the 1982 El Chichonvolcanic eruption. The largest eruptions between 1873 and 2001 were Askja, Iceland, in March 1875; Krakatau,Indonesia, in August 1883; El Chichon, Mexico, in April 1982; and Mt Pinatubo, Philippines, in June 1991(Robock, 2000). The cooling associated with major volcanic eruptions is indicated in Figure 6 for the largesteruptions, i.e. Krakatau, El Chichon, and Mt Pinatubo. Monthly temperature anomalies are between −6.5 and−10 °C for west Greenland stations at a lag of between 5 and 10 months. El Chichon is the closest volcano inlatitude separation and produces the largest temperature negative anomaly, yet does not do so for 2–5 months



Tabl

eV

.In

terc

orre

latio

nof

sele

cted

coas

tal

tem

pera

ture

anom

alie

sov

ertw

ope

riod

s

1991

–20

00Si

teR

egio

nPI

TU

PVA

AS

ILU

SIS

KA

NN

UK

PAM

NA

RN

OR

DA

ND

NB

ILL

APU

TAS

IKE

PRC

1961

–90

PIT

NW

0.80

0.72

0.50

0.56

0.66

0.54

0.49

0.48

0.32

0.17

0.18

0.09

0.17

0.25

0.28

0.26

UPV

CW

0.72

0.87

0.81

0.84

0.79

0.70

0.61

0.52

0.30

0.14

0.23

0.15

0.12

0.35

0.40

0.31

AA

SW

0.56

0.78

0.99

0.86

0.88

0.81

0.69

0.60

0.26

0.10

0.19

0.15

0.11

0.36

0.45

0.38

ILU

W0.

560.

840.

910.

940.

970.

920.

840.

810.

09−0

.36

−0.1

7−0

.03

−0.0

20.

580.

800.

66SI

SW

0.50

0.70

0.90

0.84

0.85

0.89

0.79

0.62

0.22

0.00

−0.0

5−0

.09

0.00

0.17

0.43

0.23

KA

NW

0.52

0.67

0.83

0.83

0.83

0.83

0.78

0.74

0.22

0.08

0.19

0.22

0.25

0.38

0.58

0.41

NU

KSW

0.44

0.63

0.87

0.77

0.91

0.78

0.89

0.75

0.30

0.07

0.18

0.16

0.08

0.31

0.52

0.53

PAM

SW0.

350.

520.

760.

640.

800.

690.

920.

790.

290.

060.

130.

160.

050.

340.

500.

57N

AR

S0.

290.

420.

720.

610.

720.

670.

840.

840.

25−0

.01

0.10

0.10

0.03

0.30

0.60

0.51

NO

RN

E0.

250.

280.

090.

150.

080.

130.

01−0

.03

−0.0

30.

500.

350.

200.

130.

060.

080.

21D

AN

NE

0.19

0.14

0.09

0.19

0.03

0.16

−0.0

1−0

.02

0.03

0.61

0.35

0.46

0.21

0.20

0.08

0.19

DN

BN

E0.

150.

050.

060.

220.

000.

13−0

.06

−0.0

80.

000.

520.

860.

350.

370.

170.

050.

26IL

LE

0.08

0.03

0.05

0.09

0.03

0.15

−0.0

6−0

.08

0.00

0.34

0.60

0.70

0.80

0.66

0.48

0.36

APU

SE−0

.04

0.11

0.17

0.27

0.15

0.26

0.09

0.07

0.12

0.16

0.44

0.51

0.66

0.72

0.55

0.29

TAS

SE0.

100.

280.

420.

450.

360.

450.

320.

290.

360.

100.

280.

330.

510.

740.

700.

56IK

ESE

0.12

0.27

0.49

0.47

0.45

0.46

0.44

0.44

0.51

0.04

0.24

0.25

0.32

0.53

0.74

0.68

PRC

S0.

250.

330.

500.

470.

500.

450.

540.

580.

570.

040.

120.

090.

170.

250.

470.

64D

Y2

S0.

370.

390.

570.

530.

480.

600.

640.

67−0

.06

0.05

0.08

0.38

0.41

0.52

Cor

rela

tions

ator

abov

e0.

5ar

em

arke

din

bold

type

.


1842 J. E. BOX

Table VI. Correlation of the NAO index with Greenland coastal temperature anomaliesa

Site and time period Region Winter Sig. Spring Sig. Summer Sig. Autumn Sig. Annual Sig.

1901–30Upernavik W −0.54 99 −0.42 98 −0.55 99 −0.54 99 −0.51 99Ilulissat/Jakobshavn W −0.46 99 −0.47 99 −0.57 99 −0.54 99 −0.51 99Nuuk/Godthab SW −0.60 99 −0.69 99 −0.50 99 −0.51 99 −0.58 99Tasiilaq/Ammassalik SE −0.28 85 −0.22 76 −0.48 99 −0.50 99 −0.37 90

1931–60Upernavik W −0.51 99 −0.31 86 −0.44 98 −0.63 99 −0.47 96Ilulissat/Jakobshavn W −0.54 99 −0.24 79 −0.43 97 −0.59 99 −0.45 94Nuuk/Godthab SW −0.49 99 −0.44 98 −0.43 98 −0.63 99 −0.50 99Tasiilaq/Ammassalik SE −0.06 25 0.02 10 −0.04 17 −0.60 99 −0.17 38

1961–90Pituffik/Thule Air Base NW −0.52 99 −0.14 52 0.13 52 −0.65 99 −0.30 76Aasiaat/Egedesminde W −0.57 99 −0.33 92 −0.22 77 −0.69 99 −0.45 92Sisimiut/Holsteinborg W −0.60 99 −0.44 98 −0.24 79 −0.59 99 −0.47 94Kangerlussuaq/Søndre W −0.64 99 −0.28 84 −0.25 79 −0.66 99 −0.46 90

StrømfjordNuuk/Godthab SW −0.61 99 −0.51 99 −0.27 84 −0.61 99 −0.50 95Paamiut/Frederikshab S −0.62 99 −0.51 99 −0.30 89 −0.51 99 −0.49 97Narsarsuaq S −0.59 99 −0.48 99 −0.37 95 −0.64 99 −0.52 98Qaqortoq/Julianehab S −0.63 99 −0.47 99 −0.36 95 −0.63 99 −0.52 98Danmarkshavn NE 0.20 71 0.25 82 −0.17 62 0.17 63 0.11 70Illoqqortoormiut/Scoresby E 0.27 85 0.19 69 0.18 63 0.31 90 0.24 77

sundTasiilaq/Ammassalik SE −0.17 64 0.06 26 −0.20 71 −0.17 62 −0.12 56Ikermiuarsuk SE −0.26 80 −0.21 72 0.03 14 −0.16 59 −0.15 56

a Trends with statistical significance at or above 90% are in bold type. Italic bold values indicate significance at or above 99%.

longer than the other major eruptions. There is no obvious cooling signal at east Greenland sites, consistentwith results from Robock and Mau (1995). There was also no obvious cooling associated with the 1875Askja eruption in west Greenland. Interpretation of the effect of volcanoes is complicated given the variableproximity of the eruption to Greenland.

The climate has a tendency for warming associated with an adjustment to periods of no volcanic events(Robock, 2000). The longest period of warming (1912 to 1963) is also the longest period within theinstrumental temperature record that is uninterrupted by volcanism. The three warmest annual means atNuuk occur during this period (Table III). Some negative temperature anomalies, such as in 1972, do notcoincide with major volcanic events.

4.8. Temporal changes in variability

The standard deviation of annual mean temperature varies by a factor of two to three over the 1901–2001period, with coherent semi-decadal oscillations that are consistent among sites (Figure 7). As with trendamplitudes, variance increases with latitude. Periods of minimal variance appear to coincide with trendreversals. Each warming or cooling period is characterized by an increase in variance followed by a decrease,before the trend reversal. The cause of the temporal temperature variability itself is unclear. The patterns inFigure 7 bear little resemblance to annual NAO or sunspot data.

Temporal changes in variability are used to investigate potential data homogeneity shifts. There is a reversalof variance from decreasing to increasing trend at the time of the IGY. However, the semi-decadal oscillationsbefore and after the IGY, when measurement practices are consistent, are just as large. Therefore, fluctuations



Figure 5. Seasonal and annual instrumental temperature with 3-year running mean from Nuuk/Godthab in coastal southwest Greenland,1875–2000. Annual NAO index over the same period is illustrated in the lowest graph


1844 J. E. BOX

Figure 6. The 3-month running average temperature anomalies, indicating the cooling effect of major volcanic eruptions

in variance through time do not yield results that suggest a large effect of changing measurement frequencyand instrument type on data homogeneity errors.

4.9. Glacial response to temperature anomalies

Greenland ice sheet and fjord glacier advance and retreat are the ensemble result of factors influencedby both long-term climate and ice dynamics and present climate fluctuations. Thinning of Greenland’s iceat low elevations between 1994 and 1999 has been measured by repeat laser altimeter surveys (Krabillet al., 2000). Surface height reductions, greater than 10 m per year in a few regions, however, cannot beexplained by melting alone, but must also be due to the effect of local ice dynamical variations. Presentglacier retreat may be attributable to warming since the LIA, ending 150 years BP. Weidick (1985) suggeststhat the present observed widespread recession of fjord glaciers is the result of warming since a coolingapproximately 3000 years BP, which has ended some 300 to 100 years BP. The timings of warming andcooling have been verified and more precisely dated based on Greenland ice sheet borehole temperature data(Dahl-Jensen et al., 1998). Putnins (1970) notes a correlation between fjord glacier fluctuations and stationtemperature anomalies. In the short term, climate variations might affect fjord glacier advance and retreat bythe lubrication effects of surface meltwater communication with the glacial bed. Given global warming trendsand model predictions (Houghton et al., 2001), the future Greenland mass balance is expected to be negative



Figure 7. The 15-year running standard deviation of annual temperatures from Greenland coastal records spanning 1901–2000

(Gregory and Oerlemans, 1998). A more detailed comparison of coastal temperature trends and Greenlandice is hampered by the complex interaction of ice dynamics and climate.

In the hypothetical event of a future reversal from the past 30-year positive tendency in the NAO, awarming in western and southern Greenland would be expected. Given that the western slope of Greenlandis more gradual than the eastern slope, a very large area would be exposed to melt. The effect of future NAOnegative tendencies on glacier melt will also be pronounced in south Greenland, where there is a statisticallysignificant summer NAO–temperature correlation. Western sites such as Aasiaat and Kangerlussuaq, with astrong autumn NAO correlation, would exhibit increased glacier and permafrost mass loss.

5. CONCLUSIONS

This study explored a long-term monthly temperature data set from Greenland locations prompted by theavailability of 30 additional years of data to complement an older detailed climatology by Putnins (1970).Based on temporal and spatial statistics, distinct and meaningful patterns of temperature are evident inGreenland instrumental temperature records spanning 1873–2001. These include a steady decay of spatialcorrelation, a lack of correlation between west and east coasts, and the presence of opposite temperature trendsbetween west and east coasts that are themselves not statistically linked. Statistically significant temperaturetrends are observed for various periods, namely western Greenland warming from 1873 to 1930 and 1985 to2001, and of cooling from 1930 to 1990. Statistically insignificant 1.1 °C warming at the ice sheet summit(3200 m) is observed over the 1991–2000 standard decade. A longer record is required for ice sheet sitesto establish the level of coupling with coastal sites. The limited length of ice sheet records prevents a solidstatistical result. There is, however, a suggestive correlation between ice sheet sites and nearby stations. Thelargest changes in temperature have occurred in winter, implying only a minor significance of Greenland


1846 J. E. BOX

coastal temperature variability on ice sheet melt rates. However, the NAO exerts a highly significant controlon western and southern Greenland temperature anomalies, particularly in autumn and winter, and notably insummer at southern coastal sites. Therefore, the melt rates in some regions are significantly influenced by theNAO. Recent Greenland coastal temperatures are not unprecedented, as they are for the Northern Hemisphereaverage. The warmest decades in Greenland over the last century were the 1930s and 1940s. Recent west andsouth Greenland temperature extremes would be more in line with global temperatures if the annual NAOvalues were not mostly positive from 1970 to 2000. Long-term globally observed temperature trends persistafter removing the NAO signal from coastal temperature data, e.g. the warming in the 1930s. Further, thecooling effect of large volcanic eruptions and the proposed warming in the absence or large volcanic events(Robock, 2000) persists after removing the NAO signal. Sea ice extent and concentration increases are foundin association with cold periods apparently caused by volcanic eruptions.

The 1873–2001 western Greenland warming trends observed in this study are meaningful in the contextof observed Greenland ice sheet melt rates. The mass balance of the ice sheet sector south of 73 °N latitudeand west of Kap Farvel was negative for the second half of the last century (Ohmura et al., 1999). Thisappears not to be affected by changes in precipitation, implicating the observed warming and potentialice dynamical changes. Global temperatures are expected to increase further this century (Houghton et al.,2001), contributing 58% of global sea level rise between the years 1990 and 2100 (Gregory and Oerlemans,1998). Given temperatures approaching those of the most recent climatic optimum, ∼2.5 thousand years ago(Dahl-Jensen et al., 1998), the possibility of future rapid melting and ice discharges (Heinrich events) shouldbe investigated further.

ACKNOWLEDGEMENTS

This work was supported by the NASA Program for Arctic Regional Climate Assessment (PARCA)under grant number NAG5-10857. This work was performed at the Cooperative Institute for Researchin Environmental Sciences in Boulder, Colorado, USA. Comments from K. Steffen, J. Heinrichs, and theindependent reviewers were very helpful.

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