Førland.indd 113Førland & Hanssen-Bauer 2003: Polar Research 22(2), 113–124 A combination of instrumental records and reconstructions from proxy sources indicate that Arctic air temperatures in the 20th century were the highest in the past 400 years (Serreze et al. 2000). During the 20th century an increase in annual precipitation has been observed at higher northern latitudes (Hulme 1995; Dai et al. 1997). The Norwegian Arctic has experienced a substan- tial increase in precipitation during the 20th cen- tury, but although the temperature has increased during the latest decades there is no signifi cant long-term trend in annual temperatures (Førland et al. 2002; Hanssen-Bauer 2002). Global climate models project signifi cant increase of temperature and precipitation in high northern latitudes as greenhouse gas con- centrations increase (IPCC 2001). The warm- ing in high northern latitudes may be amplifi ed by feedbacks from changes in snow and sea ice extent and thawing of permafrost. The freshwater budget in the Arctic has become an increasingly important consideration in the context of global climate change (Walsh et al. 1998) as it may be linked to the intermittency of North Atlantic deep water formation and the global thermohaline cir- culation, which is a major determinant of global climate (Aagaard & Carmack 1989; Mysak et al. 1990). The Arctic freshwater budget is driven by Past and future climate variations in the Norwegian Arctic: overview and novel analyses Eirik J. Førland & Inger Hanssen-Bauer Sparse stations and serious measuring problems hamper analyses of climatic conditions in the Arctic. This paper presents a discussion of measuring problems in the Arctic and gives an overview of observed past and projected future climate variations in Svalbard and Jan Mayen. Novel analyses of temperature conditions during precipitation and trends in fractions of solid/liquid precipitation at the Arctic weather stations are also outlined. Analyses based on combined and homogenized series from the regular weather stations in the region indicate that the measured annual precipitation has increased by more than 2.5 % per decade since the measurements started in the beginning of the 20th century. The annual temperature has increased in Svalbard and Jan Mayen during the latest decades, but the present level is still lower than in the 1930s. Downscaled scenarios for Svalbard Airport indicate a further increase in temperature and precipitation. Analyses based on observations of precipitation types at the regular weather stations demonstrate that the annual fraction of solid precipitation has decreased at all stations during the latest decades. The reduced fraction of solid precipitation implies that the undercatch of the precipitation gauges is reduced. Consequently, part of the observed increase in the annual precipitation is fi ctitious and is due to a larger part of the “true” precipitation being caught by the gauges. With continued warming in the region, this virtual increase will be measured in addition to an eventual real increase. E. J. Førland & I. Hanssen-Bauer, Norwegian Meteorological Institute, Box 43 Blindern, NO-0313 Oslo, Norway, e.forland@met.no. 114 Past and future climate variations in the Norwegian Arctic river runoff, accumulation/ablation of glaciers and precipitation over the Arctic Ocean. The observed and projected increases in temperature and precipitation in Svalbard and Jan Mayen (Fig. 1) thus have broad implications for Arctic and, perhaps, global climate and the monitoring of climatic trends in these regions is therefore also important in a global context. Analyses of climatic trends in the Arctic are hampered by the sparse station network and seri- ous measuring problems. The large year-to-year variations imply that the climate signals have to be strong to be statistically signifi cant. Although the scenarios indicate substantial climate chang- es in the Arctic, it is not necessarily the case that the fi rst signifi cant “greenhouse signal” will be found in this region. Real climatic trends may be masked or amplifi ed when analyses are based upon inhomogeneous series. Studies have revealed that inhomogeneities in Arctic climate series are often of the same magnitude as typi- cal long-term trends (Hanssen-Bauer & Førland 1994; Nordli et al. 1996). For Arctic stations in Canada, Mekis & Hogg (1999) demonstrated that the precipitation trend was almost doubled after adjusting the series. Accordingly, it is of crucial importance to adjust series for inhomogeneities when they are used in studies of long-term cli- mate variations. The climate and long-term climatic variations at the Norwegian Arctic stations are described in a number of old and new publications, e.g. Bir- keland (1930), Hesselberg & Johannessen (1958), Steffensen (1969, 1982), Hisdal (1976), Vinje (1982), Hanssen-Bauer et al. (1990), Førland et al. (1997a) and Winther et al. (2003). Serreze et al. (2000) review observational evidence of recent changes in the Arctic environment. In this paper a discussion of measuring prob- lems in the Arctic and an overview of past and future climate variations are given in the fi rst sections. In the last sections novel analyses of temperature conditions during precipitation and trends in fractions of solid/liquid precipitation at the Arctic stations are outlined, and it is demon- strated that the observed trends in measured pre- cipitation may deviate from the real precipitation trends. Fig.1. Location of current manual weather stations in the Norwegian Arctic. Bjørnøya: elevation 16 m a.s.l., start of measurements 1920. Hopen: 6 m a.s.l., 1944. Hornsund (Polish): 10 m a.s.l., 1978. Sveagruva 9 m a.s.l., 1978. Barentsburg (Russian): 70 m a.s.l., 1933. Svalbard Airport: 28 m a.s.l., 1975. Ny-Ålesund: 8 m a.s.l., 1974. Jan Mayen: 10 m a.s.l., 1921. 115Førland & Hanssen-Bauer 2003: Polar Research 22(2), 113–124 Meteorological measuring problems in the Arctic General measuring problems The Arctic climate poses several serious chal- lenges for the monitoring of the main weather elements. Icing and wet snow may cause mal- functioning of the various sensors at the weather stations. During the polar night, the combination of darkness and harsh weather occasionally com- plicates manual observations. The combination of dry snow, high wind speed and open tundra increase dramatically the meas- uring errors for precipitation and snow depth at most stations in the Norwegian Arctic. Because of blowing snow, the snow layer on the ground is likely to show large local variations (Winther et al. 1998; Winther et al. 2003), and the regu- lar snow depth measurements at weather stations are therefore seldom representative for the snow accumulation in the station area. Consequent- ly, measurements of snow cover and snow depth have been given little priority at the Norwegian Arctic weather stations. Errors in precipitation measurements Several types of errors are connected to precipita- tion measurements (Goodison et al. 1998). For the Nordic countries, Førland et al. (1996) concluded that the real amount of precipitation (“true pre- cipitation”) might be expressed as: PC = k ⋅(Pm + ∆PW + ∆PE ), (1) where PC is true precipitation, k is the correction factor due to aerodynamic effects, Pm is measured precipitation, ∆PW is precipitation lost by wetting, and ∆PE is precipitation lost by evaporation from the gauge. Other error types, such as splash in/ out, instrumental errors, misreadings etc., are either corrected in routine quality controls or may be neglected under Nordic climate conditions. However, drifting or blowing snow occasionally cause substantial problems in the Arctic. “Precip- itation” caused solely by blowing snow is exclud- ed through the quality control at the Norwegian Meteorological Institute, but there is often a com- bination of precipitation and blowing snow. In such cases it is diffi cult to distinguish the propor- tions of real precipitation and blowing snow. In the Arctic, typical values of the combined effect of wetting and evaporation loss (∆PW + ∆PE) for the Norwegian gauge is 0.10 - 0.15 mm/case for solid, liquid and mixed precipitation (Før- land & Hanssen-Bauer 2000). At most measur- ing sites, wind speed is the most important envi- ronmental factor contributing to the undercatch of the precipitation gauges. Based on results from the World Meteorological Organization Solid Precipitation Measurement Intercompar- ison (Goodison et al. 1998), Nordic precipita- tion gauge studies (Førland et al. 1996) and fi eld measurements in Ny-Ålesund, Førland & Hans- sen-Bauer (2000) deduced correction models for hourly and daily measurements in the Norwegian precipitation gauge under Arctic conditions. The correction factor for solid precipitation was found to increase with increasing wind speeds, and decrease with increasing temperature. By considering typical values of wind speed, temperature and precipitation intensity, Førland & Hanssen-Bauer (2000) suggested the following rough correction factors for unsheltered stations on Spitsbergen: liquid precipitation kl = 1.15, solid ks = 1.85 and mixed km = (0.5 * kl + 0.5 * ks) = 1.5. It should be stressed that using these typical correc- tion factors does not necessarily produce accurate estimates of true precipitation at particular indi- vidual stations. Further, the corrections do not refl ect differences in wind exposure between dif- ferent measuring sites. Adjustment for inhomogeneities Inhomogeneities in climatic series may be caused by relocation of sensors, changed envi- ronment (such as new buildings) and instrumen- tal improvements. To acquire reliable long-term climate series in the Arctic is particularly com- plicated. Because of the harsh weather condi- tions, even small changes at measuring sites may cause substantial changes in measuring condi- tions for precipitation, for example. Identifi ca- tion of inhomogeneities in Arctic climate series is further hampered by the sparse station network. Homogeneity tests based upon comparison with neighbouring stations are diffi cult to perform for Bjørnøya (the southernmost island of Svalbard) and Jan Mayen, for example, as the nearest neigh- bouring stations are more than 300 km away. A detailed survey of the results of the homogeneity analyses for Norwegian Arctic series is given by Nordli et al. (1996). 116 Past and future climate variations in the Norwegian Arctic Temperature and precipitation varia- tions in the 20th century Climate series for the Norwegian Arctic The available climate data from the Norwegian Arctic is rather limited. The present network of synoptical weather stations consists of fi ve sta- tions on Spitsbergen and three stations on other Arctic islands (Fig. 1). The oldest meteorologi- cal observations from the Norwegian Arctic were made during scientifi c expeditions to Svalbard and Jan Mayen. In 1911 a permanent weather sta- tion was established in Green Harbour, western Spitsbergen. Around 1920, weather stations were established at Bjørnøya and Jan Mayen. Long-term climate series for Spitsbergen are established by joining series from different sites, and applying statistical methods (Nordli et al. 1996). For Svalbard Airport the combined series is mainly based upon observations from Green Harbour/Barentsburg and Longyearbyen/ Svalbard Airport. The Ny-Ålesund series is based upon measurements from Isfjord Radio and Ny- Ålesund. The weather stations in Ny-Ålesund and Svalbard Airport were both moved to their present sites in 1975. The long-term temperature and precipitation series are adjusted before 1975 to be valid for the present sites. Temperature There are pronounced fl uctuations in Arctic cli- mate on daily, monthly and annual time scales. The lowest recorded temperature at the Norwe- gian Arctic stations is –46.3 °C, but even during midwinter, temperatures well above zero have been recorded at all stations (e.g. +12.3 °C at Jan Mayen on December 2001). During summer, maximum temperatures above 20 °C have occa- sionally been recorded at Bjørnøya and Svalbard Airport. There are no signifi cant trends in the annual temperature at Svalbard Airport, Bjørnøya and Jan Mayen from the start of the series to the present (Table 1). However, a closer examination of the series reveals three sub-periods with sig- nifi cant trends. From the start in the 1910s there is a positive trend up to the late 1930s, a temper- ature decrease from the 1930s to the 1960s, and from the 1960s to the present the temperature has increased signifi cantly. Figure 2 shows that despite the warming in the recent decades, the warmest two decades on an annual basis are still Fig. 2. Low-pass fi ltered series of annual temperature at Norwegian Arctic stations. The series are smoothed using Gaussian weight- ing coeffi cients and show vari- ations on a decadal time scale. (The last three years of the curves are omitted as fi ltered series are unreliable at the ends.) Table 1. Linear trends (°C per decade) in observed tem- peratures. Statistical signifi cant trends (Mann-Kendall) are in boldface (5 % level) and underlined (1 % level). Station 1912–2001 1910– 1945 1946– 1975 1976– 2001 Bjørnøya (–0.01) (+0.08) –0.29 +0.49 Hopen (+0.05) – –0.53 +0.84 Sveagruva – – – +1.84 b Svalbard Airport a +0.15 +1.20 –0.48 +0.78 Ny-Ålesund a – – –0.40 +0.42 Jan Mayen (–0.08) (–0.20) –0.71 +0.49 Northern Hemisphere (land) +0.07 c +0.14 d –0.04 +0.31 e Global (land) +0.06 c +0.11 d –0.01 +0.22 e a Combined series before 1975 b 1979–2001 c 1901–2000 e 1976–2000 d 1901–1945 117Førland & Hanssen-Bauer 2003: Polar Research 22(2), 113–124 the 1930s and the 1950s. An interesting feature is that the long-term series from the Norwegian Arctic stations show that on a decadal time scale, local temperature minima and maxima largely occur within the same decades for all seasons (Førland et al. 1997a). Because of substantial seasonal differences in standard deviations, the variation in annual mean temperatures is generally more affected by the variation in winter temperature than by summer temperatures. Accordingly, both for the warming up to the 1930s and for the decrease in annual tem- perature from the 1930s to the 1960s, variations in winter temperatures were dominant. However, during the latest three decades, increased spring temperatures gave the largest single contribution to the increase in annual temperature. The temperature level at the Norwegian Arctic stations is currently somewhat lower than in the 1930s. This is contrary to the rest of northern Europe and for the globe as a whole, where the present level is signifi cantly higher than the 1930s level (Parker & Alexander 2002). In Table 1 temperature trends are given for the sub-periods applied in the latest Intergovernmen- tal Panel on Climate Change report (Folland & Karl 2001). During 1946–1975, all stations expe- rienced a negative trend of –0.3 to –0.7 °C/decade. For recent decades (1976–2001), the trend is posi- tive at all stations, with the strongest warming at Hopen, Sveagruva and Svalbard Airport. Despite the rather strong trends since 1976, the Svalbard Airport series is the only one with a statistically signifi cant trend at the 1 % level. Hanssen-Bauer & Førland (1998a) found that though the temperature increase in the Norwe- gian Arctic during the latest decades to a large degree may be explained by changes in atmos- pheric circulation, this is apparently not the case with respect to the early 20th century warming. A model based on regional atmospheric circula- tion indices was able to account for most of the trend from the 1960s to the present, but only about one-third of the observed temperature increase at Svalbard from 1912 to the 1930s and for the subsequent temperature decrease from the 1930s to the 1960s. Fu et al. (1999) suggest that ocean circulation and sea surface temperatures may be important for explaining the warming in the northern North Atlantic region before 1940; they conclude that this warming is not yet fully understood. Precipitation The severe measuring problems discussed above create serious uncertainties in Arctic precipi- tation values. Precipitation in the Arctic is low because air masses are usually stably strati- fi ed and contain only small amounts of water vapour. The normal (1961–1990) annual precipi- tation at Svalbard Airport (190 mm) is the lowest at any Norwegian station. There are large local gradients in precipitation between the Spitsber- gen stations: although the distance is just 35 km, the annual precipitation at Barentsburg is almost three times as high as at Svalbard Airport. By measuring several transects, Sand et al. (2003) revealed both west–east and south–north gradi- ents of snow accumulation on Spitsbergen. They found that the winter accumulation rates along the east coast were about 40 % higher than on the west coast, and that the southern part of the island receives about twice as much winter precipitation as the northern part. Large local precipitation gradients were also found for the Ny-Ålesund area (Førland et al. 1997b). The precipitation dis- tribution was found to be strongly dependant on the large-scale wind direction. With winds from the south and south-west, the precipitation at a glacier (Brøggerbreen) a few kilometres south- west of Ny-Ålesund was about 60 % higher than in Ny-Ålesund, while with winds from the north- west Ny-Ålesund received more precipitation than the glacier. Annual precipitation (Fig. 3) has increased sub- stantially during the 20th century at most of the stations in the Norwegian Arctic. Both at Sval- bard Airport and Bjørnøya the increase is larger than 2.5 % per decade (Table 2). At Jan Mayen most of the increase happened before 1960, while the increase at the other stations is more evenly distributed throughout the 20th century. The course of the precipitation increase at Svalbard parallels the increase in coastal parts of northern Norway (Hanssen-Bauer & Førland 1998b), with a trend that seems to be fairly con- stant throughout the 20th century. However, the relative precipitation increase on Spitsbergen and Bjørnøya is considerably higher than the con- current increases on the Norwegian mainland. It is also higher than the “average high latitude increase” estimated by Hulme (1995). The observed long-term variations in precipi- tation on the west coast of Spitsbergen during the 20th century may be explained largely by varia- 118 Past and future climate variations in the Norwegian Arctic tions in the average atmospheric circulation con- ditions (Hanssen-Bauer & Førland 1998a). Hans- sen-Bauer (2002) concluded that about 70 % of the trend in annual precipitation at Svalbard Air- port during the period 1912–1997 was accounted for by variations in atmospheric circulation. Future climate development Scenarios of temperature and precipitation for Svalbard for the next 50 years have been worked out by Hanssen-Bauer (2002) by empirical down- scaling of an integration with the Max-Planck Institute’s coupled atmosphere–ocean global cli- mate model ECHAM4/OPYC3. The integration (GSDIO) has a physical parameterization which accounts for direct and indirect effects of sulphur aerosols in addition to greenhouse gases, includ- ing tropospheric ozone (Roeckner et al. 1999), and is based upon the emission scenario IS92a (Houghton et al. 1992). The trend in the downscaled temperatures from the GSDIO integration for the period 1960– 2000 is mainly in accordance with what has been observed during that period, while the project- ed annual warming rate up to 2050 is almost fi ve times greater than that observed for the last 90 years (Table 3). A similar warming rate is project- ed in this area by dynamical downscaling based upon the same climate model (Bjørge et al. 2000). The projected warming is statistically signifi cant at the 1 % level in all seasons. Hanssen-Bauer & Førland (2001) conclud- ed that less than 20 % of the warming projected by the GSDIO integration in Svalbard could be accounted for by changes in the atmospheric cir- culation. When compared to the warming during recent decades, this implies that a diminishing part of the projected warming will be attributed to changes in circulation. Some of the warming is probably directly connected to the greenhouse warming. However, the GSDIO integration shows extensive melting of sea ice east of Svalbard, and feedback effects from the melting probably con- tribute signifi cantly to the strong warming in the area (Bjørge et al. 2000). Benestad et al. (2002) compared downscaled temperature scenarios for Svalbard based upon three different global cli- mate models. They concluded that differences between the models concerning sea ice conditions lead to highly variable local temperature projec- tions in Svalbard. The realism of future tempera- ture scenarios is therefore critically dependent on Table 2. Linear trends (% per decade) in observed and projected annual and seasonal precipitation. Statistical signifi cant trends (Mann-Kendall) are in boldface (5 % level) and underlined (1 % level). The magnitude of the trend is relative to the (observed) 1961–1990 normal. Station Period Annual Winter Spring Summer Autumn Bjørnøya 1920–2001 +2.8 +3.9 +4.8 +1.3 +2.1 Jan Mayen 1921–2001 +1.7 +2.5 +4.6 +1.9 –0.6 Svalbard Airport (observed) 1912–2001 +2.7 –0.2 +2.2 +4.9 +3.7 Svalbard Airport (projected) 1961–2050 +1.4 +1.7 +4.6 -0.9 +0.6 Fig. 3. Low-pass fi ltered series of measured annual precipitation at Norwegian Arctic stations. The series are smoothed using Gaus- sian weighting coeffi cients and show variations on a decadal time scale. (The last three years of the curves are omitted as fi ltered series are unreliable at the ends.) 119Førland & Hanssen-Bauer 2003: Polar Research 22(2), 113–124 the reliability of the projected changes in the sea ice concentrations in the region. The downscaled precipitation scenario (Hans- sen-Bauer 2002) also indicates that annual pre- cipitation will increase signifi cantly up to 2050, mainly because of a highly signifi cant project- ed increase in spring precipitation (Table 2, bottom row). Dynamical downscaling (Bjørge et al. 2000) projects an even higher precipitation increase (ca. 2 % per decade) at the west coast of Spitsbergen. Analyses of the Svalbard Airport series indi- cate that precipitation variation to a larger degree than temperature variation may be explained by changes in the atmospheric circulation. This is at least partly due to the local topography, which shelters against precipitation from some sectors while it orographically enhances the precipitation from other sectors. This is also valid for the cli- mate scenario, but the infl uence of atmospheric circulation on the long-term trend in the scenar- io is considerably smaller than it has been during the 20th century. A major part of the projected precipitation trend is accounted for by the tem- perature increase, which is used in the empiri- cal downscaling models as a proxy for increased air humidity. As different climate models seem to show a closer agreement concerning the temper- ature signal than concerning changes in atmos- pheric circulation (Räisänen 2001), this part of the trend is probably also more credible than the part caused by variations in the atmospheric cir- culation. Precipitation characteristics Frequencies of different precipitation types The frequency of precipitation events is substan- tially higher at the island stations Bjørnøya and Jan Mayen than at the Spitsbergen stations Sval- bard Airport and Ny-Ålesund (Table 4). Based on four observations per day (00, 06, 12 and 18 UTC), the average number of events at Jan Mayen implies that there is precipitation about 31 % of the time. The value for Svalbard Airport is 20 %. Snow is the most frequent precipitation type. At Svalbard Airport, for example, around 75 % of precipitation events are reported as snow. At all stations, 15 - 20 % of precipitation events are reported as rain and about 5 % as sleet. Drizzle constitutes 20 % of precipitation events at Jan Mayen but just 4 % at Svalbard Airport. Air temperature during precipitation At Svalbard Airport, 35 % of snow events are reported at air temperatures below –10 °C; at Jan Mayen the value is 13 % (Fig. 4a). At all sta- tions less than 10 % of snow events are observed at temperatures above 0 °C. At Svalbard Airport, 30 % of the cases with sleet are reported for tem- peratures > + 2°C; at Bjørnøya just 5 % (Fig. 4b). For rain (Fig. 4c), around 25 % of the events at Svalbard Airport are reported at temperatures lower than 3 °C; at Bjørnøya the percentage is above 50 %. The geographical differences in tem- perature distribution are smaller for drizzle (Fig. 4d) than for rain. At Svalbard Airport and Ny- Ålesund, ca. 30 % of drizzle events are reported at temperatures below +3 °C; at Jan Mayen and Bjørnøya around 40 %. Wildlife, particularly the reindeer on Spits- bergen, are vulnerable to events with snow crust or icy conditions. Extensive starving and death Table 3. Linear trends (°C per decade) in observed and pro- jected annual and seasonal temperature at Svalbard Airport. Statistical signifi cant trends (Mann-Kendall) are underlined (1 % level). (From Hanssen-Bauer 2002.) Annual Winter Spring Summer Autumn Observed (1912–2000) +0.14 +0.04 +0.37 +0.04 +0.11 Projected (1961–2050) +0.61 +0.99 +0.52 +0.29 +0.62 Table 4. Frequencies of various precipitation types. The fre- quencies are given as percentage of the total number of cases based on 4 observations per day (at 00, 06, 12 and 18 UTC). However, at Ny-Ålesund there are no observations at 00 UTC and the number of cases is adjusted to 4 per day. Bjørnøya Svalbard Airport Ny- Ålesund Jan Mayen No. of cases per year 365.2 289.0 295.6 455.9 Drizzle (%) 16.5 3.9 8.6 20.1 Rain (%) 19.9 16.5 17.0 19.5 Sleet (%) 5.5 4.0 5.8 5.2 Snow (%) 55.3 73.6 65.2 51.8 Freezing rain/ drizzle (%) 0.4 0.3 0.2 1.0 Hail, snow crystals (%) 2.4 1.8 3.2 2.4 120 Past and future climate variations in the Norwegian Arctic of reindeer was reported after severe crust and ground ice formation in November and Decem- ber 1993. Table 5 shows that episodes with rain or drizzle falling at temperatures below 0 °C are rather infrequent at Spitsbergen; it occurs on average just once a year in Longyearbyen, Sval- bard Airport and Ny-Ålesund. At Jan Mayen it is more frequent, with nearly 10 cases per year. The low frequencies of freezing rain/drizzle at Spitsbergen taken into consideration, it seems that it is rather seldom that events with compre- hensive snow crust or ice formation are caused by liquid precipitation at temperatures below zero. Other causes include melting and refreezing at the snow surface, or rain absorbed and subse- quently frozen in the surface snow layer. Fractions of snow and rain as a function of air temperature Figure 5 demonstrates that there are distinct geographical differences in fractions of liquid/ solid precipitation as a function of air tempera- ture. For near zero temperatures on Bjørnøya and Jan Mayen, the fraction of liquid precipitation is generally higher than at the Spitsbergen stations. On Bjørnøya, the precipitation is liquid in 90 % of precipitation events when the temperature is above 1.5 °C, while in Longyearbyen, Svalbard Fig. 4. Cumulative distribution of air temperature during pre- cipitation as (a) snow, (b) sleet, (c, opposite page) rain and (d, opposite page) drizzle. (a) (b) Table 5. Threshold temperature for equal probability of solid and liquid precipitation, and number of events (4 observations/day) per year with liquid precipitation observed for T < 0 °C. Station name Period Threshold temp. (°C ) Events/yr Bjørnøya 1956–1999 0.84 3.0 Svalbard Airport 1975–1999 1.70 1.1 Longyearbyen 1957–1977 1.96 1.0 Ny-Ålesund 1975–1999 1.62 1.0 Jan Mayen 1956–1999 1.03 8.5 121Førland & Hanssen-Bauer 2003: Polar Research 22(2), 113–124 Airport and Ny-Ålesund this threshold is reached at around 3 °C. The “threshold” temperature—where the probability for liquid and solid precipitation is equal— differs at the individual Arctic sta- tions (Table 5). At the stations on Bjørnøya and Jan Mayen, this threshold temperature is below or close to 1.0 °C, while at the Spitsbergen sites it is higher than 1.5 °C. The same pattern is also found for sleet (Fig. 4b), where the median tem- perature is lower on Bjørnøya and Jan Mayen than at the Spits bergen stations. This indicates that, at the same 2 m air temperature, the air mass aloft during precipitation is colder over Spitsber- gen than over Bjørnøya and Jan Mayen. Bear- ing in mind that rain or snow in this region may occur throughout the year, two possible reasons for this apparently more stably stratifi ed air over the island stations during precipitation might be: (1) the contribution from convective precipitation over Spitsbergen caused by solar heating of the ground during summer; (2) during most of the year, Bjørnøya and Jan Mayen are farther from the sea ice border than the Spitsbergen stations, and cold air masses from sea ice covered areas are better mixed because of the longer travel dis- tance over open sea. Trends in annual amounts of solid and liquid precipitation At Hopen and Svea, ca. 60 % of the annual precip- itation amount during the period 1975–2001 was reported as snow, and only about 20 % as rain. At Svalbard Airport and Ny-Ålesund, the fi gures were ca. 45 and 25 %, while ca. 25 % fell as sleet or a mixture of rain and snow. On Bjørnøya and Jan Mayen, the amounts of snow, rain and mixed precipitation were nearly equal. These fractions are based on semi-daily measurements of precipi- tation amounts and the precipitation types report- (c) (d) 122 Past and future climate variations in the Norwegian Arctic ed by the observers. No correction for gauge undercatch has been performed and, according- ly, the fraction of solid precipitation is underes- timated. The fractions of solid precipitation have dimin- ished at all stations during the latest decades, particularly at Svalbard Airport and Jan Mayen. Based on linear regression, the fraction of solid precipitation on Jan Mayen is reduced from 39 % in 1975 to 20 % in 2001. The fraction of annual precipitation reported as mixed precipitation (i.e. sleet, or a combination of rain and snow during the 12h sampling interval) has increased at all stations. Fictitious trends in precipitation amounts Precipitation records from the Arctic are infl u- enced by substantial measuring errors, e.g. caused by undercatch of conventional precipita- tion gauges. As the gauge undercatch is different for snow and rain, and further depends on wind and temperature, changes in climate will result in changes in the undercatch of the gauges. Reduced fractions of annual precipitation falling as snow lead to a reduced annual gauge undercatch, and thus a fi ctitious positive trend for precipitation even if the true precipitation does not change at all (Førland & Hanssen-Bauer 2000). The poten- tial for such artifi cial trends is maximum in areas with strong winds and where a large percentage of the annual precipitation is solid, for example, in the Norwegian Arctic. By applying the rough correction factors pre- sented earlier on the annual amounts of solid, liquid and mixed precipitation it is possible to give crude estimates of “true” precipitation (Pc). In Table 6, the same correction factors for solid, liquid and mixed precipitation have been applied to all the Arctic stations, with no consideration taken to differences in wind and temperature conditions at the stations. The measured values Fig. 5. Fractions of observations classifi ed as liquid precipitation at different temperatures (ra and sn are amounts of precipitation as rain and snow, respectively). Table 6. Changes in precipitation, 1975–2001. Pm is mean measured annual precipitation. Mean correction factor is the ratio between corrected (Pc) and measured (Pm) precipitation. Change is the difference between levels in 1975 and 2001, based on linear regression. Change for measured and corrected precipitation is given both in millimetres and as a percentage per decade of the 1975 level. Precipitation change Pm (mm) Mean correction factor Pc (mm) Measured Corrected Change (mm) Change (%) Change (mm) Change (%) Bjørnøya 396 1.52 602 129 15.0 198 15.2 Hopen 469 1.64 767 –65 –5.0 –119 –5.6 Sveagruva 271 1.66 451 –40 –5.2 –100 –7.6 Svalbard Airport 192 1.56 299 10 2.1 5 0.7 Ny-Ålesund 403 1.56 631 88 9.4 142 9.8 Jan Mayen 680 1.48 1007 –97 –5.2 –203 –7.2 123Førland & Hanssen-Bauer 2003: Polar Research 22(2), 113–124 are not corrected for evaporation and wetting effects. The rough estimates of Pc presented in Table 6 indicate that the true precipitation at all stations in the Svalbard region is more than 50 % higher than the measured values. The estimates in Table 6 are based on a short time period and rather crude approximations, but nonetheless illustrate the importance of correct- ing Arctic precipitation series for undercatch. For most stations there are substantial differences between trends based on measured and corrected values, both in material amounts and also for rel- ative changes. For instance, the increase in meas- ured annual precipitation at Svalbard Airport during the period 1975–2001 is 2.1 % per decade. However, by correcting for gauge undercatch, the increase in the resulting estimates for “true” pre- cipitation is 0.7 % per decade. The scenarios cur- rently available (Table 3) indicate a 3 °C increase in annual temperature from the present to 2050 in Svalbard. For an increase in annual tempera- ture of 4 °C, Førland & Hanssen-Bauer (2000) estimated a fi ctitious precipitation increase of 2 % per decade. This virtual increase, which is caused solely by reduced measuring errors, is thus of the same magnitude as the projected pre- cipitation increase of 1.4 % per decade (Table 2) under global warming. This virtual increase will be measured in addition to an eventual real increase. Conclusions Annual temperatures in the Longyearbyen/Sval- bard Airport area have increased by more than 1 °C since 1910, but because of large interannu- al and decadal variations this trend is not statis- tically signifi cant. Although the temperature has increased signifi cantly since the cold 1960s, the present temperature level is still lower than in the 1930s at all stations in the Norwegian Arctic. Measured annual precipitation in the Svalbard region and at Jan Mayen has increased substan- tially (15 - 25 %) during the last 80 to 90 years. At Bjørnøya, Svalbard Airport and Jan Mayen, the positive precipitation trend during the 20th cen- tury is statistically signifi cant. The fraction of annual precipitation falling as snow has decreased at all stations in the Norwe- gian Arctic during recent decades. This leads to a reduced annual undercatch in the precipita- tion gauges, and consequently a fi ctitious positive trend in measured precipitation. There are substantial differences between measured and “true” precipitation in the Arctic, both with respect to material amounts as well as trends. Accordingly, corrected precipitation values should be used in studies of historical trends as well as for monitoring future trends. Estimates of “true” precipitation are also impor- tant for water balance assessments in the Arctic. Acknowledgements.—Thanks to Jan-Gunnar Winther at the Norwegian Polar Institute for useful suggestions, and to two anonymous reviewers who contributed to the improvement of the manuscript. References Aagaard, K. & Carmack, E. C. 1989: The role of sea ice and other fresh waters in the Arctic circulation. J. Geophys. Res. 94(C10), 1485–1498. Benestad, R., Førland, E. J. & Hanssen-Bauer, I. 2002: Empirically downscaled temperature scenarios for Sval- bard. Atmos. Sci. Lett., doi:10.1006/Asle.2002.0051. Birkeland, B. J. 1930: Temperaturvariationen auf Spitz- bergen. (Temperature variations on Spitsbergen.) Meteor- ol. Z. 47, 234–236. Bjørge, D., Haugen, J. E. & Nordeng, T. E. 2000: Future climate in Norway. Dynamical experiments within the RegClim project. Res. Rep. 103. Oslo: Norwegian Mete- orological Institute. Dai, A., Fung, I. Y. & Del Genio, A. D. 1997: Surface observed land precipitation variations during 1900–88. J. Clim. 10, 2943–2962. Folland, C. & Karl, T. R. 2001: Observed climate variabili- ty and change. In International Panel on Climate Change: Climate change 2001: the scientifi c basis. Contribution of Working Group I to the third assessment report of the Inter- governmental Panel on Climate Change. Pp. 99–181. Cam- bridge: Cambridge University Press. Førland, E. J., Allerup, P., Dahlström, B., Elomaa, E., Jóns- son, T., Madsen, H., Perälä, J., Rissanen, P., Vedin, H. & Vejen, F. 1996: Manual for operational correction of Nordic precipitation data. Klima 24/96. Oslo: Norwegian Meteorological Institute. Førland, E. J. & Hanssen-Bauer, I. 2000: Increased precipita- tion in the Norwegian Arctic: true or false? Clim. Change 46, 485–509. Førland, E. J., Hanssen-Bauer, I. Jónsson, T., Kern-Hansen, C., Nordli, P. Ø., Tveito, O. E. & Vaarby Laursen, E. 2002: Twentieth century variations in temperature and precipita- tion in the Nordic Arctic. Polar Rec. 38, 203–210. Førland, E. J., Hanssen-Bauer, I. & Nordli, P. Ø. 1997a: Cli- mate statistics and long-term series of temperature and precipitation at Svalbard and Jan Mayen. Klima 21/97. Oslo: Norwegian Meteorological Institute. Førland, E. J., Hanssen-Bauer, I. & Nordli, P. Ø. 1997b: Oro- graphic precipitation at the glacier Austre Brøggerbreen, Svalbard. Klima 02/97. Oslo: Norwegian Meteorological 124 Past and future climate variations in the Norwegian Arctic Insti tute. Fu, C., Diaz, H. F., Dong, D. & Fletcher, O. 1999: Changes in atmospheric circulation over the Northern Hemisphere oceans associated with the rapid warming of the 1920s. Int. J. Climatol. 19, 581–606. Goodison, B. E., Louie, P. Y. T. & Yang, D. 1998: WMO solid precipitation measurement intercomparison. WMO/TD- Rep. 872. Geneva: World Meteorological Organization. Hanssen-Bauer, I. 2002: Temperature and precipitation in Svalbard 1912–2050: measurements and scenarios. Polar Rec. 38, 225–232. Hanssen-Bauer, I & Førland, E. J. 1994: Homogenizing long Norwegian precipitation series. J. Clim. 7, 1001–1013. Hanssen-Bauer, I. & Førland, E. J. 1998a: Long-term trends in precipitation and temperature in the Norwegian Arctic: can they be explained by changes in the atmospheric circu- lation patterns? Clim. Res. 10, 143–153. Hanssen-Bauer, I. & Førland, E. J. 1998b: Annual and seasonal precipitation variations in Norway 1896–1997. Klima 27/98. Oslo: Norwegian Meteorological Institute. Hanssen-Bauer, I. & Førland, E. J. 2001: Verifi cation and analysis of a climate simulation of temperature and pres- sure fi elds over Norway and Svalbard. Clim. Res. 16, 225– 235. Hanssen-Bauer, I., Solås, M. K. & Steffensen, E. L. 1990: The climate of Spitsbergen. Klima 39/90. Oslo: Norwegian Meteorological Institute. Hesselberg, T. & Johannessen, T. W. 1958: The recent vari- ations of the climate at the Norwegian Arctic stations. In R. C. Sutcliffe (ed): Polar Atmosphere Symposium, part I, meteorological section, Oslo, Norway, 2–8 July, 1956, Pp. 18–29. London: Pergamon Press. Hisdal, V. 1976: Geography of Svalbard. A short survey. Tromsø: Norwegian Polar Institute. Houghton J. T., Callander, B. A. & Varney, S. K. 1992: Cli- mate change 1992. Cambridge: University Press. Hulme, M. 1995: Estimating global changes in precipitation. Weather 50, 34–42. IPCC (International Panel on Climate Change) 2001: Climate change 2001: the scientifi c basis. Contribution of Working Group I to the third assessment report of the Intergovern- mental Panel on Climate Change. Cambridge: Cambridge University Press. Mekis, E. & Hogg, W. D. 1999: Rehabilitation and analysis of Canadian daily precipitation time series. Atmos.–Ocean 37, 53–85. Mysak, L., Manak D. K. & Marsden, R. F. 1990: Sea ice anomalies observed in the Greenland and Labrador seas during 1901–1984 and their relation to an interdecadal climate cycle. Clim. Dyn. 5, 111–133. Nordli, P. Ø., Hanssen-Bauer, I. & Førland, E. J. 1996: Homo- geneity Analyses of temperature and precipitation series from Svalbard and Jan Mayen. Klima 16/96. Oslo: Norwe- gian Meteorological Institute. Parker, D. E. & Alexander, L. 2002: Global and regional cli- mate in 2001. Weather 57, 328–340. Räisänen, J. 2001: CO2 induced climate change in CMIP2 experiments: quantifi cation of agreement and role of inter- nal variability. J. Clim. 14, 2088–2104. Roeckner, E., Bengtsson, L., Feichter, J., Lelieveld, J. & Rodhe, H. 1999: Transient climate change simulations with a coupled atmosphere–ocean GCM including the tropo- spheric sulphur cycle. J. Clim. 12, 3004–3032. Sand, K., Winther, J.-G., Maréchal, D., Bruland, O. & Mel- vold, K. 2003: Reginal variations of snow accumulation on Spitsbergen, Svalbard, 1997–99. Nord. Hydrol. 34, 17–32. Serreze, M. C., Walsh, J. E., Chapin III, F. S., Osterkamp, T., Dyurgerov, M., Romanovsky, V., Oechel, W. C., Morison, J., Zhang, T. & Barry, R. G. 2000: Observational evidence of recent change in the northern high-latitude environment. Clim. Change 46, 159–207. Steffensen, E. 1969: The climate and its recent variations at the Norwegian Arctic stations. Meteorol. Ann 5, 217–349. Steffensen, E. 1982: The climate at Norwegian Arctic sta- tions. Klima 5, 3–44. Oslo: Norwegian Meteorological Insti tute. Vinje, T. 1982: Frequency distribution of sea ice in the Green- land and Barents seas, 1971–80. Nor. Polarinst. Årb. 1980, 57–61. Walsh, J. E., Kattsov, V., Portis, D. & Meleshko, V. 1998: Arctic precipitation and evaporation: model results and observational estimates. J. Clim. 11, 72–87. Winther, J.-G., Bruland, O., Sand, K., Killingtveit, Å. & Marechal, D. 1998: Snow accumulation distribution on Spitsbergen, Svalbard, in 1997. Polar Res. 17, 155–164. Winther, J-G., Bruland, O., Sand, K., Gerland, S., Mare- chal, D., Ivanov, B., Głowacki, P. & König, M. 2003: Snow research in Svalbard. 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