30. Temperature trends in Antarctica - VARIABLE

If there is one region of the planet that is synonymous with climate change, it is probably Antarctica. Climate change, we are told, is melting the ice cap, the glaciers, the ice shelves and the sea ice. As a result penguins may become extinct in 100 years. Or not, because it turns out there are actually a lot more of them than we thought. So what is really happening in Antarctica?

Well, the honest answer is that we don't really know.  Despite being one of the most studied places on the planet, there is virtually no instrumental temperature data from before 1940. The continent has over 260 instrumental temperature records, but most are less than 40 years in length. In fact only about 56 have more than 240 months of data, of a mere 22 have more than 480 months of data. As the following analysis will show, this is insufficient to draw any accurate or definitive conclusions about the current temperature trends for the continent.

Fig. 30.1: A map of Antarctica showing the locations of all the stations with temperature records containing more than 240 months of data.

Part of the problem with analysing the temperature records of Antarctic is the sheer size of the place. It has almost twice the area of Australia, but the weather stations are not evenly distributed. And given its size, it would be inappropriate to simply aggregate trends from opposite sides of the continent, for the same reasons as for Australia; principally, that they are likely to be totally uncorrelated. When looking at the spatial distribution of stations it becomes clear that most are situated on the coast (see Fig. 30.1 above). Those that are inland are usually at altitude, and as I showed in Post 4, the temperatures in the interior of Antarctica are much lower than elsewhere, and have much higher levels of variability. This implies that they should be analysed and aggregated separately.

In addition, the coastal stations appear to exist in three distinct clusters. The most obvious two are the high densities of stations on the peninsula and around the Ross Sea. In contrast, the stations around the Atlantic coast from longitude 45° W to 90° E are more evenly spread. It therefore seems logical to subdivide the stations into four separate groupings: (i) those found on the Antarctic Peninsula; (ii) the interior stations at altitude; (iii) the stations located along the Pacific coast from the Amundsen Sea in the east, to Queen Mary Land in the west via the Ross Sea; (iv) the stations on the Atlantic Coast from 45° W to 90° E. These four groupings of stations are identified in Fig. 30.1 above. 

 

Fig. 30.2: Number of stations active each month that have more than 240 months of data overall.

 

In Post 4 I looked at the three most significant station records for the interior of Antarctica: Amundsen-Scott Base (Berkeley ID - 166900), Vostok (Berkeley ID - 151513) and Byrd Station (Berkeley ID - 166906). The data for Byrd Station was fragmented, while that for both Amundsen-Scott and Vostok indicated negative temperature trends. No other stations in the interior have more than 240 months of data.

Using 240 months as the cutoff, we find that the number of active stations in the other three regions of Antartica that contain this minimum amount of monthly temperature data never exceeds 20, and in the case of the Atlantic coast, it never exceeds 10 (see Fig. 30.2 above). In addition, most of the data is concentrated from 1980 onwards, and only the Antarctic Peninsula has any data before 1950, but even that is miniscule in terms of its total amount.

Fig. 30.3: The mean temperature for the Pacific coast of Antarctica since 1950. The best fit line is fitted to data from 1973-2010 and has an overall trend of 0.55 ± 0.80 °C per century.

If we calculate the mean temperature trend using the data that is available, the results are not great, at least not if you are a firm believer in climate change. The data for the Pacific coast displays a small amount of warming of 0.55 °C per century since 1973 as shown in Fig. 30.3 above (i.e. 0.21 °C in total). The period 1973-2010 was chosen for the best fit calculation because that time-frame is bounded by two peaks in the 5-year moving average. This means that the peaks do not distort the best fit calculation for reasons that I have outlined in the discussion of Fig. 4.7 in Post 4. 

If the best fit in Fig. 30.3 were to be made to all the data, then best fit trend becomes 1.61 °C per century. The dip around 1960 now pulls down the trend line and increases the warming trend, but is this localized dip in the temperature record permanent or just temporary? The answer is that we don't know because there is insufficient data before 1960 to judge.

 

Fig. 30.4: The mean temperature for the Atlantic coast of Antarctica since 1950. The best fit line is fitted to data from 1973-2010 and has an overall negative trend of -0.21 ± 0.60 °C per century.

If we now turn to the Atlantic coast the pattern is the same. The temperature trend is relatively stable from 1970 to 2010 (see Fig. 30.4). If we measure the trend for 1973-2010 in order to compare directly with that for the Pacific coast, we see that the trend is actually slightly negative and equal to -0.21 ± 0.60 °C per century. But again, extending the fitting to all the data changes the trend to a positive one of gradient +0.49 ± 0.31 °C per century. This is, once again a consequence of a dip in temperatures around 1960. This suggests that the temperature fall is real, and not due to measurement errors, but this dip is large enough to completely change the trend from -0.21 °C per century to +0.49 °C per century.

There is one other similarity with the Pacific coast data: the uncertainties in both trends are very large. This is due to the comparatively short time frame for the available data, which illustrates why long temperature records are so valuable. Even 60 years is not long enough.

Fig. 30.5: The mean temperature for the Antarctic Peninsula since 1940. The best fit line is fitted to data from 1973-2010 and has an overall trend of 2.88 ± 0.77 °C per century.

The notable point about the Antarctic Peninsula is that it is the only region of Antarctica where there is clear evidence of a significant warming trend since 1950. But this is no different from what we have seen in Australia and New Zealand, and in this case there is no data before 1940. That means we cannot say whether this warming is new and permanent, or whether, like Australia and New Zealand, it is just a recovery from a temporary cooling phase. In Australia and New Zealand the temperatures in the latter half of the 19th century were just as high as they are now. In the case of Antarctica we just do not know.

Summary

The analysis above allows us to draw the following conclusions.

1. There has been no warming trend in the interior of Antarctica since 1957 (see Post 4).
   
2. The has been no warming trend on the Atlantic coast since 1950, and probably none of any great consequence on the Pacific coast either (see Fig. 30.4 and Fig. 30.3).
3. The only significant recent warming in Antarctic appears to be around the peninsula (as shown in Fig. 30.5). This warming is, however, no greater than that seen in Australia and New Zealand over the same time period (1950-2010), and that warming was preceded by a cooling of almost equal magnitude (see Post 26 and Post 8).
4. We have no idea what the temperature trend anywhere in Antarctica was before 1940.