TEMPERATURE TRENDS IN ANTARCTICA
Geophysical Institute, University of Alaska, Fairbanks, AK 99775, USA
We analyzed the observational temperature record of meteorological stations in Antarctica proper, the Antarctic Peninsula and some islands located close to Antarctica. We limited our study to these data and did not include temperature values deduced from ice cores (e.g. Mosley-Thompson 1992), which record goes further back in time.
Early this century, a number of expeditions went to Antarctica carrying out meteorological measurements, and Jones (1990) assembled these data. However, these station records are fairly broken and incomplete, absolute calibration is often missing, so that trends are not easy to establish. In general it can be stated that early this century colder temperatures were observed than now. However, there is one station with a good and unbroken record: ORCADAS located close to Antarctica on the South Orkney Island. Its record goes back to early in this century (1904). Hence, close to 100 years of temperature records are available. This station shows a temperature increase of about 1.8°C (Fig.1). The temperature curve shows that the increase was not steady with time; a fact which could hardly be expected and is also not observed on other continents.
Figure 1: Mean annual temperatures of Orcadas for the time period 1904 -1998. Orcadas is located a on the South Orkney Island at 60.7°S, 44.7°W close to the Antarctic Peninsula. The weather station lies at an altitude of 4 m asl. The best linear trend, which was forced through the data points, displays a warming of 1.8°C.
A number of the presently active stations in Antarctica have operated since the* IGY (1958). Most of them are located at the coast and have been operated by Argentina, Australia, Britain, Chile, France, New Zealand, Russia and USA; hence, a fairly substantial record for the last 40 years is available. Schwerdtfeger (1984) lists these and the annual mean temperatures. All these coastal stations show an increase in temperature, the magnitude of which varies (e.g. Streten 1990). It is, of course, more difficult to deduce long term temperature trends from shorter and non identical time periods, as temperature change with time is seldom uniform. Generally it can be stated, that all coastal stations showed a temperature increase. While the amount varied from station to station, a mean value of about 2°C per century can be calculated. The Antarctic Peninsula showed an even higher degree of warming than the coastal stations of Antarctica proper (e.g. King and Turner 1997). Here a value of about twice the amount when compared to the coastal stations was observed (about 4°C per century). This might be caused by positive feedback over the albedo of sea ice and ice shelves, which have decreased for the Peninsula region during in the last 3 decades. Before that time we know little as systematic satellite measurements were not carried out.
The observed increase in temperature for Antarctica is higher than the values found for the Southern Hemisphere. Jones et al. (1986) found an increase of about 0.5°C in the Southern Hemisphere in the 20th Century. This result is in agreement with most model calculations, which predict a higher temperature increase due to increased greenhouse gases in high latitudes. Wendler and Meitner (1993) investigated outgoing long wave radiation (OLR) as seen from satellite (NOAA series) in Mer Dumont d'Urville, Eastern Antarctica. For a 16 year time period they found an increase of 1.8 W/m2. Trends over a 16-year period in any geophysical data is a rather questionable concept in isolation, however, the deduced temperature increase from the radiative data is in agreement with the measured ones of Dumont d'Urville (Periard and Pettré 1991), the only ground station within the study area.
There are only two long term interior stations, namely Vostok (Russia) and Amundsen-Scott Pole Station (USA). The distance between the two stations is 1260 km and both are located on the high Plateau of Eastern Antarctica. While Vostok shows an increase in temperature with time, similar to the one observed for the coastal stations, Pole Station recorded a decrease in temperature (Fig.2). The difference in the absolute values is only an effect of the altitude, Vostok (78.5 S, 106.9 E) being located at a higher elevation (3488m) than Pole Station (2835m). Vostok is also the station at which the coldest temperatures on Earth are being recorded. The absolute minimum was -89.5°C and the mean monthly winter temperatures always dip below -60°C.
It is possible to explain an opposing trends in temperature for coastal and high plateau stations, as due to increased advection of warm air from the north which caused the observed warming at the coastal stations, the temperature gradient with height will have been affected. However, it is more difficult to explain the opposite trend for the two plateau stations located at similar altitudes. Furthermore, the uniform surface of snow (melting never occurs) found at both stations make it difficult to explain it due to aldedo differences or local topographic effects. Hence, we tried to check the quality of the data. For example, errors due to radiation effects can occur, and have been observed for non-ventilated automatic weather stations (AWS). Such errors can become significant during times of calms or very light winds with bright sunshine. Further, if a station is located too close to buildings, heat escaping these buildings might influence the temperature reading.
Figure 2: Mean annual temperatures for Pole Station (2835 m), Vostok (3488 m) and the automatic weather station at Dome C (3280 m). These three station are located on the high plateau of Eastern Antarctica over snow covered flat terrain. For the data sets of Pole Station and Vostok the best linear fit is presented. Pole station showed cooling which can be extrapolated to 2.6°C per century, while Vostok displayed a warming of 2.1°C. The latter one is in agreement with most coastal stations.
The station at Pole is located away from the main camp and professionally maintained by NOAA's Clear Air Laboratory. Looking at the monthly trends, 6 months showed a cooling trend, only 1 month (November) a warming trend, and 5 months did not show any significant temperature change (less than 0.5°C for the 40 year period). Further, there appears no preferred season during which this cooling trend occurred, the months were randomly distributed over the year. This might be taken as an indication that radiation errors, which could occur only in summer, have not affected the measurements of Pole Station. Further, we plotted the mean annual temperatures of Dome C (Wendler and Kodama 1984), where an AWS is located since 1981, also on Fig. 2. Dome C has an elevation of 3280 m and is 1680 km distance from Pole. The agreement with Pole station is better than with Vostok. In addition, we carried out another test comparing Pole station with the AWS, which surrounded it for a certain time period (not shown here). Again, good agreement was found, giving trust in the quality of the measurements.
We know much less about the meteorological setup at Vostok, but the temperature trend of this station is in good agreement with most of the other Antarctic stations. Again, we analyzed monthly time series. Seven months of the year, randomly distributed over the different seasons, showed warming. No monthly trend showed cooling, and 5 months, using the above definition of at least 0.5°C change, were neutral .
The above analyses does not show any systematic errors in either of the two time series of the temperatures and we believe that the trends are real. We correlated the two time series shown in Fig.2 which each other. A high positive correlation coefficient would indicate that both stations would lie in the same climatic zone. We found a r2 value of 0.23. This relatively low value indicates that the two stations do not frequently experience the same deviations from the mean in temperature in identical years. Further it indicates that the climate of the interior of Eastern Antarctica is more complex than one might deduce from the similarity in altitude, surface albedo and geographic setting. Pole Station is, of course, much closer to sea level (Ross Ice Shelf), and cyclones penetrating Antarctica from lower latitudes can reach this station much easier than Vostok. Hence, changes in circulation will affect Pole Station to a greater extend than Vostok. Modeling studies support our findings (Harsmann et al. 2000). While the time scales were different in this modeling study, they showed nevertheless large variations over the high plateau of Antarctica (their figure 8b).
This investigation was supported by NSF/OPP Grant 97-25843. J. Curtis, B. Moore, A. Tivy and G. Weller helped with differnt aspects of this investigation; thanks to all of them.
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Date Last Modified: 4/30/00