Survey of Greenland Instrumental Temperature Records: 1873–2001
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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 2002 Revised 11 July 2002 Accepted 22 July 2002 ABSTRACT Temporal and spatial variability are analysed in Greenland instrumental temperature records from 24 coastal and three ice sheet locations. Trends over the longest period available, 1873–2001, at Ilulissat/Jakobshavn indicate statistically significant warming in all seasons: 5 °C in winter. Trends over the 1901–2000 century in southern Greenland indicate statistically significant spring and summer cooling. General periods of warming occurred from 1885 to 1947 and 1984 to 2001, and cooling occurred from 1955 to 1984. The standard period 1961–90 was marked by 1–2 °C statistically significant cooling. In contrast to Northern Hemisphere mean temperatures, the 1990s do not contain the warmest years on 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 significant 2–4°C warming was observed in western Greenland, 1.1 °C warming at the ice sheet summit (3200 m), although this is statistically insignificant. Annual temperature trends are dominated by winter variability. Much of the observed variability is shown to be linked with the North Atlantic oscillation (NAO), sea ice extent, and volcanism. The correlation of coastal temperature 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 are often opposite between west and east Greenland. This apparent teleconnection is spurious, however, given insignificant east–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 in both 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 recently observed and climate models predict high-latitude amplification of the enhanced greenhouse effect (Houghton et al., 2001). There is mounting evidence that climate changes in the most recent decade are unprecedented in 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 level and ocean circulation changes, occasionally in the form of rapid ice discharges (Heinrich events) associated with abrupt climate change (Bond et al., 1993). The Greenland ice sheet is of particular interest because it appears to have contributed more to sea-level change than Antarctica during the climatic optimum ∼135 thousand years ago (Cuffey and Marshall, 2000). Greenland ice-sheet melt between 1865 and 1990 contributed about 3.0 ± 1.6 cm to global sea-level rise (Zuo and Oerlemans, 1997). Under specified greenhouse-gas and sulphate-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 Room 108, 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 the Greenland 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 roughly 1500 m average elevation. Greenland is over 2500 km in latitudinal extent, extending 770 km below the Arctic Circle. Consequently, there are large north–south contrasts in solar irradiance and temperature at any time of year. Greenland’s topography opposes atmospheric circulation, enhancing Northern Hemisphere meridional heat exchanges (Barry and Kiladis, 1982; Kristjansson´ and McInnes, 1999). Temperatures around Greenland have 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, particularly in 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. Significant cooling is said to have occurred at all stations during 1929–43. The 1943–55 period was characterized by warming, except at Upernavik. Trends for these periods are investigated in this paper and with a larger data set. Other previous work relevant to Greenland temperature changes includes analysis of upper air temperature trends (Kahl et al., 1993) and changes in extreme air temperatures in the Arctic (Przybylak, 1997). The former work determined that, between 1959 and 1986, trends were predominantly negative in Greenland between 850 and 500 hPa, with a small springtime positive tendency in north and eastern Greenland at 400 to 300 hPa. Extreme temperatures trends between 1951 and 1990 are characterized by reductions of the annual maximum and minimum temperatures around Greenland by −0.25 °Cand−0.75 °C respectively, with +0.25 °C warming in the northeast. The work of Jones et al. (1999) to determine the global distribution of surface temperature changes over 150 years indicates that temperature trends are typically opposite between western and eastern Greenland. Across Greenland, 1998 was between +0.2and+1 °Cwarmer than the 1961–90 period, 3 °C above normal in the adjacent North Atlantic, and −1 °C colder at the southern tip. Steffen and Box (2001) derived a 2 °C warming over the ice sheet plateau over 1995–99 compared with 1951–60 maps by Ohmura (1987). Abdalati and Steffen (1997a) found a 4.4% per year increase in ice sheet melt area between 1979 and 1991, and a corollary trend in coastal temperature anomalies was noted. Tuomenvirta et al. (2000) show that mean maximum and minimum temperatures had statistically significant negative trends in western coastal Greenland during the period 1950–95, while over the Nordic Seas and Fenno-Scandia the trends were generally positive. The opposite temperature trends are induced by the North Atlantic oscillation (NAO), an oscillation in the strength of both the Icelandic Low and the Azores High. A weaker than normal Icelandic low is associated with colder than normal northwestern European winters, due to a weakening of the North Atlantic westerlies which normally bring moderating oceanic heat to northwestern Europe. West Greenland temperatures are above normal in the negative phase of the NAO due to weaker than normal Baffin Bay northerlies. In the opposite, positive NAO phase, cold Greenlandic winters correlate with mild conditions in northwestern Europe caused by a strengthening of the North Atlantic westerlies and Baffin Bay northerlies. The oscillation in temperatures between Greenland and northwestern Europe is commonly referred to as the temperature see-saw (Van Loon and Rogers, 1978). Dahl-Jensen et al. (1998) offer a longer-term perspective on temperature changes over Greenland. Cooling over Greenland began 4000 years BP and reversed at 2000 years BP. More recent cold periods were centred at AD 1550 and AD 1850, with warm intervals during the Medieval Warming Period (MWP) near AD 1000 and the 1930s temperature maximum. The Little Ice Age (LIA) in Greenland appears to span the period of 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 following questions. Are recent Greenland temperatures unprecedented, as they are for the Northern Hemisphere average? What level of control on Greenland temperature anomalies can be attributed to known climate forcings, 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 Greenland region. Temperature data between 1958 and 2000 from 27 Greenland locations were obtained from the Danish Meteorological 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. The combined data sets are summarized in Table I and Figure 1. In the case of Greenland’s capital Nuuk/Godthab,˚ and Ilulissat/Jakobshavn,