The Fourth Assessment report of the Intergovernmental Panel on Climate Change (IPCC) noted that the global average surface air temperature had increased by 0.65° C ± 0.2° C for the period 1901-2005 (IPCC, 2007). There is now strong evidence that anthropogenic activity in the form of increasing greenhouse gas emissions is responsible for this near-surface warming that has been observed in the global data. However, the increase in temperature is not uniform across the Earth. Land areas have warmed more than the oceans, and the polar regions have experienced some of the largest increases in temperature, especially over recent decades. But even here the pattern of change is complex. In this paper we examine how temperatures have changed across the Arctic and Antarctic over approximately the last 50 years—the period for which we have reliable observations from manned stations in the Antarctic and for which the temperatures in both polar regions can be compared. We also examine change in other quantities, such as sea ice extent and permafrost, particularly focussing on the mechanisms that are responsible for the changes.
Near-surface air temperature
Figure 1 shows that based on annual mean surface air temperature data for 1950-2010 the three areas that have experienced the greatest warming are all at high latitude and consist of (a) Alaska/northern Canada/the Arctic Ocean, (b) Siberia and (c) the Antarctic Peninsula.
Alaska has a number of stations with long climate records and it is possible to construct Alaska-wide mean temperature estimates. Figure 2 indicates that the state as a whole has experienced a warming of about 2°C since 1950, although the temperature increase has not been uniform, but has been characterised by a jump in the mid-1970s. The temperature across Alaska is strongly influenced by the depth of the Aleutian Low, which is the climatological low pressure centre located in the vicinity of the Aleutian Sea. When the low is deep there is extensive advection of warm air from the south across the state giving relatively high temperatures. The reverse situation of a weak Aleutian Low results in more incursions of cold, northerly flow across the state and lower temperatures. The depth of the Aleutian Low is rather variable, but is strongly influenced by the phase of the Pacific Decadal Oscillation (PDO). The PDO is a high-low latitude climatic link or teleconnection that operates on relatively long time scales (Zhang et al., 1997). The shift in the temperature regime across Alaska in the mid-1970s can be linked to a change in the PDO, with a consequent deepening of the Aleutian Low and greater warm, southerly flow across Alaska.
The warming across Siberia in recent decades has been associated with stronger westerly flow across the North Atlantic and into central Eurasia. Climatologically the mid-latitude westerly are a result of low surface pressure over the Arctic and higher pressure in the sub-tropics. The primary mode of variability of the high and mid-latitude areas is an oscillation in this high-low latitude pressure differences resulting in changes in the strength of the westerlies. This mode of variability is known by a number of names, including the Arctic Oscillation, the Northern Annular Mode (NAM) and the North Atlantic Oscillation (NAO) (Quadrelli and Wallace, 2004). We have a long record of the variations in the NAO/NAM and Figure 3 shows its winter fluctuations since 1950. Between 1950 and about 1990 the index was more negative giving weaker westerlies and generally colder winters across Europe and Siberia. However, from the late 1980s the NAO/NAM switched to a positive state resulting in the advection of warm air from the North Atlantic into the Arctic Ocean and northern Russia. These warmer conditions have resulted in the disintegration of extensive areas of permafrost, causing damage to buildings and the fracture of oil-carrying pipelines. The stronger westerlies have also brought more snow to northern Russia resulting in greater river discharge during the spring snow melt season. Records show that the discharge from the six largest Eurasian rivers into the Arctic Ocean has increased by 7 per cent between 1936 and 1999, equivalent to an increase from 0.058 to 0.062 Sv.
While the NAO/NAM can vary naturally, model experiments have suggested that higher concentrations of greenhouse gases can shift the NAO/NAM into its positive phase. However, since the late-1990s, at a time of record levels of greenhouse gases, the NAO/NAM has become more neutral, indicating the difficulty of separating natural climate variability from anthropogenic influences.
The third area that has experienced significant warming over the last 50 years is the Antarctic Peninsula. We have a relatively large number of meteorological reporting stations in this region and they show the complexity of change. Broadly, the greatest warming on the maritime western side of the Peninsula has been during the winter, with the largest trend on the eastern side during the summer and autumn. This can be seen in the monthly surface air temperature trends for Faraday/Vernadsky on the western side and Esperanza on the east, which are shown in Figure 4.
Faraday/Vernadsky Station has experienced a large statistically significant (<1 per cent level) trend in annual mean temperature of +0.55°C per decade for the period 1951-2010. The warming in winter (June-August) for the same period has been +1.06°C per decade, with the largest monthly trend of +1.21°C per decade being during July (Figure 4). On the western side of the Peninsula there is a high anti-correlation during the winter between the sea ice extent over the western Bellingshausen Sea and the surface temperatures. Recent research based on passive microwave satellite imagery (Turner et al., in press) suggests that monthly ice concentrations here have decreased by up to 25 per cent since the late 1970s creating a polynya-like feature along the west coast of the Peninsula. This has been a very localised loss of sea ice, hence the region with very large warming is quite limited. Warming has been experienced from the southern part of the western Antarctic Peninsula north to the South Shetland Islands, but the rate of temperature increase decreases away from Faraday/Vernadsky, with the long record from Orcadas on Signy Island only having experienced a warming of +0.2oC per decade. However, it should be noted that this record covers a 100-year period rather than the 60 years for Faraday/Vernadsky.
The very cold temperatures during the 1950s and 1960s suggest much more winter sea ice during that time and a progressive reduction since then. Data are very limited from that time, but King and Harangozo (1998) found a number of ship reports from the Bellingshausen Sea covering that period when sea ice was well north of the locations found in the more recent satellite era, suggesting some periods of greater sea ice extent than found in recent decades. However, there is very little sea ice extent data before the late 1970s so we have largely circumstantial evidence of a mid-century sea ice maximum at this time (de la Mare, 1997). At the moment it is not known whether the warming on the western side of the Peninsula has occurred because of natural climate variability or as a result of anthropogenic factors.
As can be seen from Figure 4, temperatures at Esperanza station on the eastern side of the Peninsula have risen most during the summer and autumn months. Here the temperatures have increased during the summer and autumn by +0.37°C per decade and +0.40°C per decade, respectively over 1953-2010. The temperature increases during these seasons have been linked to a strengthening of the westerly winds that have resulted in more relatively warm, maritime air masses crossing the high orographic barrier of the Peninsula and reaching the low-lying ice shelves on the eastern side. A rise in air temperature at this time of year can be most damaging for the ice shelves as temperatures are close to freezing point and a small warming can push temperatures above 0°C. Much of the Larsen-B ice shelf disintegrated in March 2002 releasing many icebergs into the Southern Ocean and this event has been linked to the stronger westerly winds and the higher temperatures experienced on the eastern side of the Peninsula.
The stronger westerly winds in recent decades have been attributed to the shift of the Southern Annular Mode (SAM) into a more positive phase (Marshall et al., 2006). The SAM is the primary mode of variability in the atmospheric circulation of the extratropical areas of the Southern Hemisphere and consists of an oscillation of mass (as measured by surface barometric pressure) between high and mid-latitudes (Thompson and Wallace, 2000). Changes in the SAM can be observed as anomalies of opposite sign in mean sea-level pressure (MSLP), with periods of positive (negative) anomalies at high latitudes (mid-latitudes) developing before the anomalies reverse. As MSLP is normally lower over the Antarctic continent compared to the sub-tropics the climatological winds over the Southern Ocean are from the west. When the SAM become more positive the westerly winds become stronger. Figure 5 shows that over 1979-2009 the westerly winds increased by about 15-20 per cent over the latitude range 60-70° S as the SAM became more positive. This not only affected the advection of maritime air onto the eastern side of the Peninsula but limited the drawdown of CO2 into the Southern Ocean (Le Quéré et al., 2007), which is a primary sink of atmospheric CO2.
The SAM has a large natural variability and increasing concentrations of greenhouse gases can move the SAM into its positive phase. However, model experiments have suggested that the increase of greenhouse gases make a smaller contribution to the change than stratospheric ozone depletion (Arblaster and Meehl, 2006). The loss of stratospheric ozone (the ‘ozone hole’) is a phenomenon of the Austral spring. However, its effects propagate down through the atmosphere over the subsequent weeks so that the greatest decrease in MSLP and increase in westerly winds over the Southern Ocean have been during the summer and autumn. There is therefore strong evidence that the ozone hole has played a significant part in the summer and autumn warming on the eastern side of the Antarctic Peninsula.
While the Peninsula has warmed over recent decades the rest of the continent has experienced relatively little change. Across the interior there are only two staffed stations with long records, Amundsen-Scott at the Pole and Vostok on the high plateau of East Antarctic. Over 1957-2009 Amundsen-Scott Station has only small trends in all seasons, none of which are statistically significant. Vostok, has also not experienced any statistically significant change in temperatures, either in the annual or seasonal data, since the station was established in 1958.
Thompson and Solomon (2002) considered the Antarctic surface temperature trends over 1969-2000 and showed that the contribution of the SAM was a warming over the Antarctic Peninsula and a small cooling along the coast of East Antarctic. They only considered the months of December to May, which is when the largest change in the SAM has taken place. Thompson and Solomon (2002) attributed the trends primarily to changes in the polar vortex as a result of the development of the Antarctic ozone hole.
Sea ice extent
There has been a marked decrease of Arctic sea ice extent in recent years that has been reported well beyond the scientific literature. In the north the loss of ice has been greatest in September (Figure 6), with a decrease of over 10 per cent per decade. In September 2007 the extent of Arctic sea ice reached a new minimum of 4.1 million km2, which was 39 per cent below climatology, and some 23 per cent below the previous minimum in 2005. The ice attained a record minimum in 2007 because of a complex chain of events stretching back to the 1990s. The age, area and thickness of the sea ice had decreased in recent years as a result of the ‘flushing’ of much of the ice out of the Arctic Basin in the early 1990s, which pre-conditioned the decline (Nghiem et al., 2007). The NAO/NAM was particularly positive from 1989 to 1995, which advected a considerable volume of multi-year sea ice out of the Arctic into the Atlantic, so that the area of multi-year sea ice over the Arctic Ocean decreased from over 5.6 to 2.7 million km2, as sea ice drifted away at twice its usual speed. The record minimum of ice in 2007 was also influenced by the high atmospheric pressure over the region that summer, which gave less cloud and allowed more solar radiation to reach the surface. Although the September Arctic sea ice extent has not dropped to such a low value in subsequent years the overall trend is still downward and there is increasing evidence that the decline is a result of anthropogenic activity in the form of higher greenhouse gas concentrations.
Across the Southern Ocean the extent of sea ice over 1979-2010 has actually grown in every month of the year (Figure 6), with the largest increase of almost 4 per cent per decade being in March. For the year as a whole the Antarctic sea ice extent has increased at a rate of 153,700 km2 per decade (1.28 per cent per decade), which is significant at <1 per cent level.
Spatially the pattern of autumn ice concentration change (Figure 7) has consisted of a small increase around the coast of East Antarctic (which is consistent with the small decrease of temperature there) and a larger increase across the eastern Weddell Sea. However, the most marked change has been a dipole of change between the Antarctic Peninsula and the Ross Sea, with a decrease over the Bellingshausen Sea and an increase over the Ross Sea. This latter pattern of change is consistent with a deepening of the Amundsen Sea Low (ASL), which is the climatological low pressure cell that is located in the South Pacific sector of the Antarctic coastal region. The deeper ASL has given stronger northerly flow to the west of the Peninsula resulting in less sea ice and warmer air temperatures down the western side of the Peninsula. In contrast the deeper low has given stronger cold southerly flow off the Ross Ice Shelf giving more sea ice in this area. It has been proposed that the ozone hole has given this pattern of atmospheric change and played a significant part in the increase of Antarctic sea ice extent (Turner et al., 2009). However, it has also been suggested that oceanic change may have played a part (Zhang, 2007).
Predicted changes over the Twenty First Century
Over the next century it seems likely that atmospheric greenhouse gas concentrations will increase, so producing further warming across the surface of the Earth, especially over the land areas. Bracegirdle et al. (2008) used the IPCC predictions to examine how a number of aspects of the Antarctic climate system may change by 2100. They found that if greenhouse gas concentrations doubled, the temperatures across the Antarctic were expected to increase by about 0.33 ± 0.1°C per decade on land and 0.26 ± 0.1°C per decade in the ocean/sea-ice zone. This is approximately the same change as the mean change for the land areas of the Earth. It is expected that with such an increase in greenhouse gas concentrations the amount of sea ice across the Southern Ocean would decrease by about a third (Bracegirdle et al., 2008).
Larger increases in annual mean temperature are expected across the Arctic, with values of 0.6-0.8°C per decade over the northern parts of the continents, and the largest values of 1°C per decade over the Arctic Ocean (IPCC, 2007). There is extensive debate about when the Arctic Ocean will become ice free in the late summer. The more conservative estimates suggest that this will happen sometime during the second half of this century, although some predictions place this date before 2030 (Stroeve et al., 2008).
The polar regions have experienced very different changes over recent decades with a loss of sea ice across the Arctic Ocean and an increase around the Antarctic continent. These changes are consistent with the large surface air temperature increases in the north and the general pattern of little change or even a small cooling in the south. There is now strong evidence that the changes in the Arctic are a result of the increasing concentrations of greenhouse gases. Over the Antarctic the loss of stratospheric ozone has had a major impact on the climate system of the continent and in many ways shielded the Antarctic from the impact of increasing greenhouse gas concentrations. However, over the next century we expect the ozone hole to ‘heal’ and for there to be a warming at the surface, of 3-4°C, which is broadly the same as expected across the land surfaces of the Earth.
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