Recent warming in Greenland in a long-term instrumental (1881–2012) climatic context: I. Evaluation of surface air temperature records

Mean SAT. Mean SAT for the most recent 2001–11 period ranges from −14.7 °C at 04320 Danmarkshavn, on the icy north-east Greenland coast where seasonal sea-ice cover is near its maximum extent (NSIDC 2012) to 1.5 °C at 04270 Narsarsuaq in the far south (table 2(a)). Mean SAT at the latter location has crept significantly above the freezing point from a low value of −0.4 °C in 1971–2000, which like many other sites was similar to 1881–1910 or 1891–1920 mean SAT (table 2(a)), and station 04260 Paamiut has recently been not far from the melt threshold (−0.8 °C for 2001–11). Mean SAT for the latest decade is also generally slightly, but not significantly, ∼0.1–0.4 °C, above the 1930s/40s warm peak depicted by the 1921–50 and 1931–60 normal periods. Indeed, analysis of CGT3 11 year running means representing 1895–2011 yields the highest means of −3.2 °C for 1926–36 and 1927–37, compared with −3.8 °C for 2000–10. However, a Related Samples Wilcoxon Signed Ranks Test shows no significant difference at the p = 0.10 level. The lowest running mean CGT3 is −6.9 °C for 1983–93, compared with the early record low of −6.1 °C for 1896–1906 and 1897–1907, suggesting particularly pronounced spring cooling—compared as we shall see with the other seasons—during the 1980s and early 1990s prior to the recent warming. CGT3 for the latest decade is 1.8 °C above the 1891–1920 and 1971–2000 values, and CGT2 for 2001–11 is 2.1 °C above 1971–2000 mean CGT2. The greatest overall increase in mean SAT is observed for 04221 Ilulissat, which shows a 3.1 °C positive difference between 1881–1910 and 2001–11.

Turning now to mean SAT trends, these are most significant, showing striking strong warming, during the 1981–2011 and 1991–2011 periods, of typically 3–5 °C for these respective time periods but up to nearly 8 °C at 04220 Aasiaat (table 3(a)). This is generally greater than the earlier 1911–40 and 1901–30 warming trends of 2.3–3.9 °C for those time periods for several sites. Aasiaat is also the hotspot of the more recent 2001–11 spring warming (1.7 °C) for the whole Greenland coast; the reason for this is unknown but merits attention. More modest warmings of 1–2 °C since 1981 are seen along the north-east Greenland coastline; perhaps lingering sea-ice is muting the spring warming signal in this region. It is also noteworthy that most stations show a strong 1–3 °C spring cooling for 1961–90 that was greatest in the south (3.5 °C at 04270 Narsarsuaq), in line with previous reports of Greenland cooling for this time period (Hanna and Cappelen 2003). Daily maximum and minimum SAT. The available data show greater recent (1981–2011 and 1991–2011) warming in mean daily minimum, than daily maximum, air temperatures at 04231 Kangerlussuaq, but an opposite pattern at 04270 Narsarsuaq, with rather similar warmings for minimum and maximum temperatures at 04360 Tasiilaq (tables S2(a) and S4(a) in the supplementary materials available at stacks.iop.org/ERL/7/045404/mmedia). Warmings for the last 30 years are generally between 3 and 5 °C except for 04220 Aasiaat which experienced much stronger 7–8 °C mean daily minimum temperature increases over the last 20 and 30 years (table S4(a)). The Greenland coastal averaged CGT3 early mid 20th century warming is much more pronounced for the daily minimum temperatures but the subsequent (1941–1970) cooling is much greater in the CGT3 daily maximum temperatures. Both the mean daily minimum and daily maximum temperatures in spring had little change or even declined slightly, but not significantly, over the last decade.

Mean SAT. The warmest sites during 2001–11 are 04270 Narsarsuaq and 04231 Kangerlussuaq, with mean SAT both slightly above 10 °C for this time period, and the coldest site is 04351 Aputiteeq at only 2.6 °C, affected by its location on a promontory and proximity to the cold East Greenland Current (table 2(b)). In general, mean SAT was ∼1–2 °C warmer than during the 1971–2000 cold period. The 2001–11 mean SAT values are clearly above 1931–60 mean values at all sites, by an average of 0.8 °C for CGT3 but 1.7 °C warmer for 04211 Upernavik and only 0.2 °C warmer for 04360 Tasiilaq. This is confirmed by analysis of running means of CGT3, which reached or exceeded 7.0 °C for five successive summer periods since 1997–2007, whereas there was only one previous comparable 11-year period, 1926–36, that averaged as high as 7.0 °C. Indeed the last two 11-year periods 2000–10 and 2001–11 averaged 7.4 °C. A related samples Wilcoxon signed ranks test indicates a 91% probability that CGT3 2001–11 (or 2000–10) mean temperature is statistically warmer than the previous 1926–36 warm peak. The lowest 11-year running mean was 5.6 °C for 1912–22 and 1913–23, which was closely approached (5.8 °C) during the later cold peak in 1963–73.

Tasiilaq experienced 1.0 °C warming in its mean SAT between 1891–1920 and 2001–11, but the difference between the same time periods was much greater at 2.7 °C for 04211 Upernavik. Some of the greatest warming of recent times was seen at 04230/34 Sisimiut, which has a 1.5 °C difference in its 2001–11 and 1981–2011 mean SAT (table 2(b)).

Summer warming trends are generally highly significant, although at generally 2–4 °C, less than spring trends, for 1981–2011 and 1991–2011. The hotspot of warming was, once again, 04230/34 Sisimiut at 3.1 °C for 1981–2011 and 3.7 °C for 1991–2011, although 04211 Upernavik also warmed particularly strongly (3.8 °C) during the later period (table 3(b)). The available data for 04339 Ittoqqortoormiit show no significant change for 1991–2011, and even significant summer cooling in its mean SAT for 2001–11; there is also (insignificant) cooling for the same time period at 04320 Danmarkshavn (table 3(b)). CGT2 and CGT3 warming trends of 2.0–2.3 °C for 1981–2011 and 1991–2011 are much in excess of HadCRUT3v northern hemisphere temperature increases of ∼0.6 °C for the same time periods (table 3(b)). These CGT3 trends are also well above the CGT3 warming of 1.2 °C for 1911–40 (table 3(b)). Daily maximum and minimum SAT. Here significant warmings of ∼1.0 °C are evident even for the shorter latest period of 2001–11 (tables S2(b) and S4(b) available at stacks.iop.org/ERL/7/045404/mmedia). The hotspot of warming for the last decade was 04211 Upernavik which has increases of 2.0 (1.7) °C in its mean maximum (minimum) temperatures. The cool spot was 04339 Ittoqqortoormiit which experienced slight, non-significant coolings. Temperature increases in southern Greenland for the 1981–2011 and 1991–2011 period were generally greater for maximum than minimum temperatures (tables S2(b) and S4(b) available at stacks.iop.org/ERL/7/045404/mmedia).

Mean SAT. Mean temperatures for 2001–11 range from −10.7 °C at 04320 Danmarkshavn to +2.9 °C at 04390 Ikerasassuaq (table 2(c)), both locations being on the eastern side of Greenland but the latter in the extreme south (figure 1). Four sites (04230 Sisimiut, 04250 Nuuk, 04360 Tasiilaq and 04382 Ikermiuarsuk) have mean SATs that have risen above the freezing threshold since 1961–90, reflecting the lengthening of the melt season for adjacent zones of the ice sheet and nearby marginal glaciers. The site with the greatest mean SAT increase (2.2 °C) between the 1961–90 and 2001–11 periods is 04339 Ittoqqortoormiit, and the sites with the least change (0.9 °C) are 04221 Ilulissat and 04270 Narsarsuaq. Mean SAT for 2001–11 was generally 0.2–0.7 °C above the earlier, 1921–50 warm peak at most sites, although there is only 0.1 °C difference between 1991–2011 and 1921–50 or 1931–60 in CGT3. Running mean analysis of CGT3 shows the warmest 11 year period of −0.3 °C during 2000–10, compared with the previous warm peak of −0.6 °C for 1927–37; the temperature difference between the two periods is statistically insignificant. Like Spring, autumn running mean CGT3 temperatures dipped almost to early twentieth century values: −1.8 °C for 1977–87, just prior to the recent strong warming, compared with −2.1 °C for 1896–1906, 1897–1907 and 1898–1908.

Long-running stations 04211 Upernavik, 04221 Ilulissat and 04270 Narsarsuaq indicate ∼1.7–1.8 °C of warming since 1881–1920. The trend analysis shows generally significant warming trends of ∼2–3 °C for 1981–2011 focused on, and inland of, Greenland's mid-west and south-west coasts, and slightly lower but still significant warming of 1.7 °C at 04360 Tasiilaq in the east (table 3(c)). However, over the last 12 years 2001–11, autumn temperatures actually declined by typically just over 1 °C, with statistically significant declines for CGT3 (−1.3 °C) and quite a number of stations on Greenland's west and east coasts: most notably −2.1 °C at 04231 Kangerlussuaq and 04339 Ittoqqortoormiit. Unlike for other seasons, none of the (for autumn slight) cooling trends of 1961–90 are statistically significant. Daily maximum and minimum SAT. As noted above, and in contrast to other seasons, autumn temperatures underwent often significant decreases during the latest 2001–11 period: this is true of both the minimum and maximum temperatures, with little to distinguish between them (tables S2(c) and S4(c) available at stacks.iop.org/ERL/7/045404/mmedia). However, overall, there was typically 1–2 °C warming—generally significant—for the overall 1981–2011 period.

Mean SAT. This is the season with the greatest temperature changes and trends. The difference in mean SAT between 1881–1910 and 2001–12 is as much as 7.8 °C for 04211 Upernavik but 'only' 4.4 °C for 04270 Narsarsuaq and also a relatively small 3.5 °C difference at 04250 Nuuk between 1891–1920 and 2001–12 (table 2(d)). Mean SAT was typically 3–5 °C higher for 2001–12 than 1971–2000 (e.g. 5.3 °C higher at 04211 Upernavik), which represents a large increase in mean temperatures for this relatively short time period. Mean 2001–12 temperatures are also evidently 1–3 °C above the warmest normal periods during the early mid 20th Century (table 2(d)). Running mean analysis reveals −7.8 °C for CGT3 for 2001–11 and 2002–12 compared with a whole degree lower at −8.8 °C during two peaks marking the Early Twentieth Century Warming, i.e. 1926–36 and 1938–48. The 2001–11 winter temperature peak is significantly higher than 1938–48 (98% confidence) but not significantly higher than 1926–36, due to relatively high temperature variability during this season.

Mean SAT trends for 1991–2012 are in excess of 10 °C along Greenland's west and north-west coastline at 04211 Upernavik, 04220 Aasiaat and 04230/34 Sisimiut, although trends are much lower on the east side of Greenland: only 2.0 °C at 04339 Ittoqqortoormiit (table 3(d)). A similar pattern is seen with the 1981–2011 warming trends: now 8–9 °C in places along the west coast but only 2–3.5 °C in east Greenland. However, it is important to note that for both time periods, the east coast warmings are still significant. Mean SAT trends for the latest 2001–12 period were greatest (5.3 °C) at 04231 Kangerlussuaq (an inland west Greenland site near the ice-sheet margin at Sondre Stromfjord), and 3–4 °C and significantly high at most other west coast stations, and positive but not significant in the east (table 3(d)). The overall CGT3 trend of 5.5 °C for 1981–2010 is well in excess of trends for all climatological standard normal periods during the early mid 20th century, and exceeds contemporaneous northern hemisphere warming by an order of magnitude. The 1960s–80s cooling was clearly focused along the west and north-west coasts (−4 to −5 °C trends at several stations), whereas at the same time several east coast stations saw either significant warmings or not much change (table 3(d)). Daily maximum and minimum SAT. The recent warming is greater for minimum than maximum temperatures, at 6.0 and 5.0 °C, respectively, for CGT2 for 1991–2012 (tables S2(d) and S4(d) available at stacks.iop.org/ERL/7/045404/mmedia). This is also true of the early 20th Century warming: i.e. 4.5 and 3.7 °C for 1901–30 and 1911–40 for daily minimum temperatures, compared with 3.5 and 2.9 °C for the same periods for daily maximum temperatures. The mid-late 20th century cooling, e.g. for the period 1941–70, is also far more marked and significant in the minimum than maximum temperatures. Minimum temperature trends since 1981 reach 9–10 °C for 04220 Aasiaat (table S4(d) available at stacks.iop.org/ERL/7/045404/mmedia). Mean winter daily maximum temperatures have recently reached within 1 °C of freezing at 04270 Narsarsuaq and 04272 Qaqortoq during 2001–12, although mean winter daily minimum temperatures remain at >6 °C below freezing. Mean winter daily minimum temperature increased only 1.6 °C between 1891–1920 and 2001–12 at 04270 Narsarsuaq, compared with a 7.6 °C increase at 04211 Upernavik (table S3(d)). Compared with mean daily maximum temperatures, mean daily minimum temperatures tended to be greater for 2001–12 in relation to equivalent data from the early mid 20th century.