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Climate change - causes and effects

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December 2007

Why should we study Antarctic climate?
The Antarctic region is an important regulator of global climate. The Southern Ocean is a significant sink for both heat and carbon dioxide, acting as a buffer against human-induced climate change. The sea ice that forms around the continent each winter controls the exchange of energy between the Sun and the Earth, and its partition between atmosphere and ocean. As sea ice forms, brine rejected from the ice increases the density of the upper ocean. These waters then sink and form the deep ocean currents that carry heat around the globe.

Changes in global climate can have impacts on the Antarctic environment. The Southern Ocean supports a unique ecosystem that is well adapted to present climate conditions. Changes in ocean temperatures, currents and sea ice will impact on this ecosystem, possibly changing the ocean's capacity to absorb carbon dioxide. Warming of the atmosphere and ocean around Antarctica may lead to increased loss of mass from the Antarctic ice sheets and hence a rise in global sea level. In order to make soundly-based predictions of how the global environment may change over the coming decades and centuries, we need to understand the role played by the Antarctic in the Earth system.


How has Antarctic climate varied over the past 50 years?
Few continuous observations of Antarctic climate are available before the International Geophysical Year of 1957-58. Since this time, surface temperatures have remained fairly stable over much of Antarctica, although individual station records show a high level of year-to-year variability, which could mask any underlying long term-trend. The majority of stations in East Antarctica, including the two long-term records from the high plateau of East Antarctica (South Pole and Vostok) show no statistically-significant warming or cooling trends1. By contrast, large and statistically-significant warming trends are seen at stations in the Antarctic Peninsula. Over the past 50 years, the west coast of the Peninsula has been one of the most rapidly-warming parts of the planet. Here, annual mean temperatures have risen by nearly 3°C, with the largest warming occurring in the winter season1,2,3. This is approximately 10 times the mean rate of global warming, as reported by the Intergovernmental Panel on Climate Change (IPCC). The east coast of the Peninsula has warmed more slowly and here the largest warming has taken place in summer and autumn3.

Significant warming has also been observed in the Southern Ocean. Upper ocean temperatures to the west of the Antarctic Peninsula have increased by over 1°C since 19554. Within the circumpolar Southern Ocean, it is now well-established that the waters of the Antarctic Circumpolar Current (ACC) are warming more rapidly than the global ocean as a whole. A comparison of temperature measurements from the 1990s with data from earlier decades shows a large-scale warming of around 0.2°C in the ACC waters at around 700-1100 m depth21.

Analysis of weather balloon data collected over the past 30 years has shown that the Antarctic atmosphere has warmed below 8 km and cooled above this height. This pattern of warming in the troposphere and cooling in the stratosphere is seen globally and is the expected signature of increases in greenhouse gases, such as carbon dioxide. However, the 30-year warming at 5 km over the Antarctic during winter (0.75°C) is over three times the average rate of warming at this level for the globe as a whole5.

Reliable year-round measurements of Antarctic sea ice extent are only available from the 1970s, when satellite observations first became available. Unlike in the Arctic, where there has been a significant decline in observed sea ice extent over this period, there has been a small but statistically-significant increase in the overall extent of Antarctic sea ice. However, there are strong geographical variations at a regional scale. Sea ice cover has declined substantially in the seas to the west of the Antarctic Peninsula while it has increased in other parts of the Antarctic6.

Subtle but important changes have occurred in the atmospheric circulation around Antarctica. Since the early 1960s, atmospheric pressure has dropped over Antarctica and risen in the mid-latitudes of the Southern Hemisphere, a pattern of variability known as the Southern Hemisphere Annular Mode (SAM)7. These changes have resulted in a strengthening of the westerly winds that blow over the Southern Ocean around Antarctica. Stronger westerlies will impact on ocean currents, upwelling and mixing, but the consequences of such changes have yet to be fully understood.


How has recent climate change impacted on the Antarctic environment?
Recent climate change has driven significant changes in the physical and living environment of the Antarctic. Environmental change is most apparent in the Antarctic Peninsula, where climate change has been largest. Adélie penguins, a species well adapted to sea ice conditions, have declined in numbers and been replaced by open-water species such as chinstrap penguins8. Melting of perennial snow and ice covers has resulted in increased colonisation by plants9. A long-term decline in the abundance of Antarctic krill in the SW Atlantic sector of the southern ocean may be associated with reduced sea ice cover10.

Large changes have occurred in the ice cover of the Peninsula. Many glaciers have retreated11 and around 10 ice shelves that formerly fringed the Peninsula have been observed to retreat in recent years12 and some have collapsed completely. Furthermore, 87% of glaciers along the west coast of the AP have retreated in the last 50 years, and in the last 12 years most have accelerated. The Antarctic Peninsula is contributing to sea-level rise, at about the same rate as Alaska Glaciers.

Analysis of global measurements of atmospheric CO2 indicates that the Southern Ocean carbon sink has weakened significantly since 1981. This reduction in the capacity of the ocean to absorb CO2 has been attributed to increased upwelling of carbon-rich waters associated with strengthening of the westerly winds19. Although future changes in the ability of the Southern Ocean to sequester CO2 are not completely known, this will be a key factor that helps shape global climate.


Has human activity caused the recent changes?
Climate can vary as a result of changes in forcing factors that affect the way energy is exchanged between the sun, the earth and space. These forcings can be of natural origin (e.g. volcanic dust in the atmosphere, variations in solar output and variations in the Earth's orbit about the sun) or a result of human activity (e.g. increases in "greenhouse" gases such as carbon dioxide). Additionally, complex interactions between atmosphere, oceans and sea ice can cause climate variability, particularly on a regional scale, over a timescale of years to decades. Attributing observed changes in climate to particular changes in forcing (or to natural variability) is a difficult process that can only be accomplished by bringing together reliable observations of past and present climate with the results of experiments carried out with sophisticated models of the climate system. Attribution of Antarctic climate change is particularly difficult because of the relatively small number of instrumental climate records available from this region and the short length of the records.

As part of the work undertaken for the Fourth Assessment Report of the IPCC13, about 20 different climate models were run with historical changes to natural and anthropogenic forcing factors to simulate the climate of the 20th century. The simulated changes in Antarctic surface temperatures over the second half of the 20th century vary greatly from model to model with no single model reproducing exactly the observed pattern of change. However, when results from all models are averaged, the resulting pattern of change bears some resemblance to that observed, with greatest warming in the Peninsula region and little change elsewhere20. This result suggests that some of the observed change may have an anthropogenic origin, but the lack of a clear and consistent response to changed forcing between models also suggests that much of the observed change in temperatures may be due to natural variability. The IPCC model experiments fail to reproduce some of the observed features, notably the rapid warming of the lower atmosphere. These differences between modelled and observed changes could be used to argue against attributing change to anthropogenic forcing but some caution is called for as the models used may not adequately represent all of the complex processes that determine temperatures in the polar regions.

Most of the IPCC model experiments do simulate the observed strengthening of the circumpolar westerly winds, suggesting that this phenomenon is a robust response to changed climate forcing. Further experiments have indicated that changes in anthropogenic forcings, particularly stratospheric ozone depletion and increases in greenhouse gases, have made the largest contribution to the strengthening of the westerlies14,15. Recent climate observations show that changes in the strength of the westerlies strongly influence temperature variations on the east coast of the Antarctic Peninsula16. Taken together, these two results suggest that a significant fraction of the recent observed changes in climate in this part of the Antarctic can be attributed to human activity with a reasonable degree of certainty. Further support for this view comes from analysis of marine sediment records which enable us to examine how the extent of Antarctic Peninsula ice shelves has varied over time. While some of the smaller ice shelves in this region have periodically grown and decayed over the past 10000 years17, the Larsen-B ice shelf appears to have been stable throughout this period until it collapsed suddenly in March 200218. This suggests that recent warm temperatures are exceptional within the context of the last 10000 years, making it unlikely that they can be explained by natural variability alone.

Many of the theories that seek to explain the circumpolar warming of the ACC also have the strengthening of the westerly winds as their root cause. Whilst there is not yet a clear consensus on which are the mechanisms that are most important, there is increasing evidence that a significant part of this change is ultimately driven by human activities22.


What further changes can we expect over the next 100 years?
If we make assumptions about how greenhouse gas emissions are likely to change, we can use climate models to predict how Antarctic climate may respond over the coming century. Models predict a warming of a few degrees celsius over much of continental Antarctica. However, as mean temperatures over most of the continent are well below freezing, even this warming will not greatly increase loss of ice from the continent through melting. Indeed, increases in snowfall resulting from a warmer atmosphere (which can hold more water vapour) may actually thicken the Antarctic ice sheets.

Warming is also predicted in and over the oceans surrounding Antarctica. As a result, sea ice cover may decline by around 25% (although there are considerable uncertainties associated with this prediction). Where warmer ocean waters come into contact with the continental ice sheets, loss of ice from the continent will be accelerated.

Although stratospheric ozone levels are predicted to recover as a result of implementation of the Montreal Protocol (and its subsequent revisions), model predictions indicate that the circumpolar westerly winds will continue to strengthen as the effects of increasing greenhouse gases outweigh those of reducing ozone. Further change associated with strengthening winds may be expected in the Southern Ocean environment, including, possibly, further reduction in the strength of the Southern Ocean carbon sink.


References

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