Climate models and their use in simulating Antarctic climate
The climate of Antarctica is defined here to include the conditions of the atmosphere, ocean, snow and ice across the Antarctic continent and the surrounding Southern Ocean. At a given location a warming climate will be characterised by a long-term background shift (i.e. the signal) combined with shorter-term variability from year to year (i.e. noise). The World Meteorological Organization standard practice is to define the climate of a region based on a 30-year average of parameters of interest (such as temperature). On shorter time scales there are large variations associated with daily weather and major multi-year cycles such as El Niño/La Niña. Changes in climate should therefore be viewed from a multi-decadal perspective, which is the approach here. It is important to note however that there are also components of natural variability that operate over multiple decades and can in some cases affect the effectiveness of even 30-year means for tracking the baseline climate change signal .
The use of climate models in predicting future climate change is the main focus here. However, it is acknowledged that alternative approaches, such as the use of past analogues , can provide alternative views of future Antarctic climate. All major state-of-the-art climate models, which are produced at approximately 30 different modelling centres around the world, are founded on well-established physical laws of geophysical fluid dynamics, such as Newton’s laws of motion. However, limits on computer power mean that model calculations are generally still done at a large scale for global simulations, which involves representing the atmosphere as a series of boxes, which are typically at present 100 km across. This thus presents a major challenge in modelling smaller-scale phenomena (e.g. clouds) and physical characteristics (e.g. complex mountainous terrain) realistically.
Assuming that greenhouse gases will continue to increase in concentration to the year 2100, there is high confidence in a number of changes predicted. Under a medium-intensity assumption of human influence (i.e. an approximate doubling of carbon dioxide concentrations by that point) there is high agreement between the different climate models for the following:
- Antarctic-wide terrestrial annual mean surface warming will occur (two thirds of climate models in the range 1.8°C to 3.3°C) .
- Antarctic-wide terrestrial annual mean snow accumulation rate will increase (by 8% to 18%) .
- Total Southern Hemisphere annual mean sea ice coverage will retreat (by 24% to 42%) .
- The coastal sea-ice production will decrease, along with increased melting of land ice - both have been shown to cause a weakening of the primary global ocean circulation, the thermohaline circulation .
- Southern Ocean water masses, such as the Antarctic Intermediate Water (AAIW) will warm and freshen as the densities at which water masses form become significantly lower. (, ). The AAIW is important for climate change because it is within this water mass that the highest concentration of anthropogenic CO2 is found .
- Increases in snowfall will be accompanied by increased rates of ice discharge [e.g. ,9]. Any negative contribution to sea level from increased snowfall may therefore be countered by faster rates of ice flow and increased discharge of inland ice to the ocean.
Challenges in estimating future climate
To account for the difficulties inherent in predicting human behaviour, the general approach taken in the climate science community is to consider a range of plausible ‘what if’ scenarios for anthropogenic greenhouse gas emissions, with no explicit judgement over which might be more likely . The climate change estimates based on these scenarios are therefore referred to as ‘projections’ rather than predictions [e.g. see Figure 1].
Changes up to the mid 21st century will not necessarily follow the long-term warming trend . A key consequence for policy makers is that regions that have been warming very rapidly in recent decades could potentially switch to a period of cooling on time scales of a few years, before background warming takes over. Currently there is significant research effort being directed at seasonal and decadal prediction in an effort to bridge the gap to longer-term climate-change timescales .