It is said frequently that the climatic data for Antarctica for the past 30 years – the period for which records are far superior to previous times – shows clear signs of mankind’s influence. However, it is important to look at such changes in a historical context. This is important else we attribute any and every change seen in the past few decades to mankind alone. When it is done today’s changes are seen in a new light.
Antarctica’s climate has never been stable, and there have many significant changes in the past few thousand years, indeed the past few hundred as well. Proxy indicators from ice cores show abrupt alterations in atmospheric circulation and temperature. One of the most dramatic changes was the intensification of the circumpolar westerly winds between 6000 and 5000 years ago and between 1200 and 1000 years ago.
Going back even further there is strong evidence for dramatic changes obtained the synthesis of ice core isotope proxy records for temperature showing that there was a warm period, much warmer than Antarctic is today, between 11,500 and 9000 years ago, a period sometimes referred to as the early Holocene climatic optimum. The ice sheets responded to this warmth. Analysis of grounding lines in the marine-based parts of the West Antarctic ice sheet, at the head of the Ross Ice Shelf, show that the sheets started to retreat to their current position from a position close to the edge of the current Ross Ice Shelf 7000 – 9000 years ago.
It is also clear that the climate of the Earth’s polar regions are linked. Intensification of atmospheric circulation in the Northern Hemisphere (indicated by a stronger Siberian High and northern circumpolar circumpolar westerlies and a deeper Icelandic Low) and to a lesser degree in the Southern Hemisphere (stronger circumpolar westerlies and deeper Amundsen Sea Low) occurs 8200 – 8400 years ago. Date from Siple Dome (West Antarctica) and the Greenland Ice Sheet Project ice core climate proxies for northern and southern circumpolar westerlies show remarkably concurrent major intensification periods between 6000 and 5000 years ago and starting 1200 – 600 years ago. It appears that Antarctic events start earlier and less abruptly than those in Greenland.
The most dramatic changes in atmospheric circulation during the Holocene noted in the Antarctic are the abrupt weakening of the southern circumpolar westerlies 5200 years ago and intensification of the westerlies and the deepening of the Amundsen Sea Low starting 1200–1000 years ago.
Following the early Holocene warming Siple Dome data indicate significant cooling between 6400 and 6200 years ago, followed by relatively milder temperatures over East Antarctica 6000 – 3000 years ago that lasted until about 1200 years ago in the Siple Dome area. There then seems to have been a flattening and a slight decline in temperature starting 1200 – 1000 years ago, followed by warming in the last few decades.
The timing of these climatic changes can also be deduced from observations of abandoned Adelie penguin colonies along the coast of Victoria Land. This is because penguins depend on sea ice extent. Research into penguin rookeries suggest a ‘‘penguin optimum’’ associated with a warmer climate and less sea ice between 4000 and 3000 years ago. This ‘optimum’’ ended abruptly 3000 years ago, as the inland lakes began to fill and the coastal lakes began to decrease in size. It seems the abandoned rookeries were reoccupied between 1200 and 600 years ago, also suggesting warming along the southern Victoria Land coast.
The abrupt climate change commencing 1200 – 1000 years ago is the most significant Antarctic climate event of the past 5000 years. Its onset is characterised by strengthening of the Amundsen Sea Low and the southern circumpolar westerlies with cooling both at Siple Dome. It is this event that provides the underpinning for centennial and perhaps shorter-scale natural variability upon which future climate change over Antarctica might operate.
In the past 300 years there has been major changes in what appears to have been a significant shift in atmospheric circulation above the entire trans-Antarctic region between about 1700 and 1850. Concurrently there has been abrupt climate change in the North Atlantic as well as a significant change in atmospheric circulation in the North Pacific.
The close of the cooling event is coincident with a major transition from zonal to mixed flow in the North Pacific, indicating once again a global-scale association between Antarctic and North Pacific climate. It was also coincident with the onset of the modern rise in CO2, which has been followed by the warmest temperatures of the last 700 years in West Antarctica, indeed possibly the warmest of the past 10,000 years.
Understanding of such events, and the associations between the abrupt climate change recorded in both hemispheres, is important in predicting the impact and timing of future abrupt climate change possibly forced by anthropogenic changes in greenhouse gases and aerosols.
The Past 50 Years
It is only in the past 50 years that the continent has a first person instrumental record. Looking back a few hundred years further much important information about the climate of Antarctica has come from a network of recently available shallow firn cores. This data reveals that significant regional climate changes has taken place.Atmospheric temperatures have increased markedly over the Antarctic peninsula, as we shall see this is probably linked to nearby ocean warming and intensification of the circumpolar westerlies.
Glaciers are retreating on the Antarctic peninsula, as well as in Patagonia, and on the sub- Antarctic islands, and in West Antarctica adjacent to the peninsula.
The penetration of marine air masses has become more pronounced over parts of West Antarctica.
The Antarctic troposphere has warmed during winter while the stratosphere has cooled all year-round.
The upper kilometre of the circumpolar Southern Ocean has warmed.
Antarctic Bottom Water across a wide sector off East Antarctica has freshened, and the densest bottom water in the Weddell Sea has warmed.
Given these observed changes it is important to consider what has not changed in the past 50 years:
Over most of Antarctica, near-surface temperature and snowfall have not increased significantly.
Atmospheric circulation over the interior has remained in a similar state for at least the past 200 years.
The total sea ice cover around Antarctica shows no significant change since reliable satellite monitoring began in the late 1970s, despite large but counteracting regional changes.
Natural or Not?
How can natural variations, of which there are plenty, be distinguished from purported man-made ones?
Many factors must be influencing Antarctic climate, and there are bound to be many unknown influences. Known ones include: Orbital insolation, Greenhouse gases, Solar variability, Ice dynamics and aerosols. Also, the relation between the ice sheets and climate is not simple.
It has been argued that the inhomogeneity of Antarctic climate change in space and time implies that recent Antarctic climate changes are due to a combination of strong multidecadal variability and anthropogenic effects, although when this opinion is tracked back to its origin it is more of a feeling than something based on unequivocal evidence.
A point often made is that until the warming of the last few decades, major changes in temperature were preceded by or coincident with changes in atmospheric circulation. For example, consider the climate change event commencing 1700 and ending by about 1850, during which circulation and temperature acted synchronously in both polar regions. The cooling is coincident with an increase in the frequency of El Nino events, and with an increase in solar energy output.
Modern warming is not preceded by or coincident with change in atmospheric circulation, suggesting that recent warming is not like the natural variability of the last few thousand years and that therefore is a consequence of anthropogenic forcing. This is an interesting argument though it must be realised that given our limited understanding of Antarctic climate and what has happened in the past, the logic of this case is suggestive, but by no means proof, of man’s influence.
The Antarctic Peninsula
In any map of global temperature changes in recent decades the Antarctic peninsula stands out as one of the fastest warming places on Earth. The largest annual warming trends are found on the western and northern parts of the Antarctic Peninsula, with Faraday/ Vernadsky having the largest statistically significant (<5% level) trend at +0.56 deg C/decade from 1951 to 2000. Rothera station, some 300 km to the south of Faraday, has a larger annual warming trend, but the shortness of the record and the large interannual variability of the temperatures render it statistically insignificant.
It is a highly localised effect. The region of significant warming extends from the southern part of the western Antarctic Peninsula north to the South Shetland Islands. The rate of warming decreases north away from Faraday/Vernadsky (50 year long record), with the long record from Orcadas (100 year long record) in the South Orkney Islands showing a warming trend of only +0.20 deg C/ decade.
The large winter season warming of 5°C over the past 50 years at Faraday is believed to be associated with a significant decrease in winter sea ice over the Amundsen-Bellingshausen Sea. It should be stressed that the reason why there was more sea ice in the 1950s and 1960s is not known but it may have been linked to weaker or fewer storms to the west of the peninsula and greater atmospheric blocking. Indeed, a greater frequency of blocking anticyclones would have meant weaker northerly winds to the west of the Antarctic Peninsula, allowing the sea ice to advance farther north during the winter resulting in colder temperatures on the western side of the Peninsula.
The other side of the peninsula seems to be warming because of a different reason. On the eastern side the greatest warming is during the summer months and appears to be associated with the strengthening of the circumpolar westerlies since the mid-1970s. These stronger winds have resulted in more relatively warm, maritime air masses crossing the peninsula and reaching the low-lying ice shelves, as well as the adiabatic descent and warming of these winds crossing the Antarctic Peninsula’s topography.
Around the rest of the coastal region of the continent there have been few statistically significant changes in surface temperature over the last 50 years. Inland the Amundsen-Scott Station at the South Pole has shown a statistically significant cooling in recent decades that is thought to be a result of fewer maritime air masses penetrating into the interior of the continent. Satellite-deduced surface temperature and ice core proxies for temperature in the interior over the past 200 years, although based on only eight records distributed over the ice sheet, suggests no discernible trend over recent decades, but a 0.2°C warming for the past century.
Taking these observations into account, I maintain it is a brave scientist who looks at the data, especially the warming of the Antarctic Peninsula, as sees the unequivocal influence of mankind because “the climate models predict the greatest warming in the polar regions.”