UPDATED INFORMATION ON*

FUTURE CLIMATE CHANGE OVER THE ANTARCTIC

for

The International Antarctic Weather Forecasting Handbook:

IPY 2007-08 Supplement

by

Tom Bracegirdle

Natural Environment Research Council,

High Cross,

Madingley Road,

Cambridge, CB3 0ET, United Kingdom

tjbra@bas.ac.uk

May 2008

Climate models

Information on how climate may evolve in the future mainly comes from climate models. The current generation of climate models are coupled ocean/atmosphere/sea-ice models. This is important for representing the complex interactions that occur between these systems. For instance, the pattern of surface warming of the ocean that occurs in response to atmospheric forcing can feedback on the atmosphere by altering the behaviour of large-scale weather patterns. 

To make assessments of future climate change, assumptions about contributions from anthropogenic factors must be taken into account. Since the future of human activity is difficult to predict, model runs simulating the future are often repeated a number of times with a range of different evolutions of anthropogenic emissions. The most widely used set of such ‘scenarios’ are taken from the Special Report on Emissions Scenarios (SRES), which was prepared for the Intergovernmental Panel on Climate Change (IPCC). These scenarios take into account factors such as population growth, wealth, energy use, land use and technologies. Simulations forced by a given scenario are considered as projections rather the predictions, since no one scenario is considered to be more likely than another.

Most of the projected changes shown in this section are based on climate model data that were provided by various modelling centres as part of the IPCC Assessment Report Four (AR4) process. The projections shown are from the SRES A1B scenario. In this scenario atmospheric CO₂ concentrations reach approximately 720 ppm by the year 2100. The global mean warming simulated with this scenario is about in the middle of the range of the SRES scenarios. The projected values are an average of all (~20) of the AR4 models and a measure of the spread (standard deviation of the projections from the various models).

Temperature

By the end of the 21^st century a surface temperature increase of 2.7 ± 0.8 °C is projected over the Antarctic continent compared to current conditions. This warming is about the same magnitude as the surface warming projected over other large land regions around the globe.  The consensus over the magnitude of the warming is stronger over the interior of Antarctic than for coastal regions that are more strongly influenced by the ocean. Differences between the models in their projected and equilibrium sea ice distributions are a key source of uncertainty for coastal regions. In locations where large decreases of sea ice are projected climate models show a large near-surface warming in winter, when sea ice insulates the atmosphere from the relatively warm ocean surface. For instance, the AR4 models show surface temperature increases of 4.1 ± 2.1°C off East Antarctica for winter (June through August).

Sea ice

Changes over the 21^st century show the annual average total sea-ice area is projected to decrease by 2.6 ± 0.73 ´ 10⁶ km² (33 ± 9%). Other measures of sea ice amount, volume and extent, show decreases of 34% and 24% respectively. Most of the ice retreat occurs for winter and spring when the sea ice extent is largest. In the summer and autumn the main region of decrease is in the Weddell Sea. As for temperature there is a relatively strong consensus between the models on Antarctic-wide changes, but far less consensus on the scale of regions such as the Weddell and Ross seas.   

Precipitation

There is a projected increase of net precipitation rate (i.e. precipitation minus evaporation, hereafter referred to as P-E) averaged over the continent of 2.9 ± 1.2 mm a^-1 dec^-1. When integrated over the 21^st century this represents an increase of 20% on current conditions. This increase is a consequence of the warmer and more humid atmosphere that will transport moisture more effectively to high latitudes [ Noone and Simmonds , 2002]. 

The surface mass balance over Antarctica has a strong influence on sea level, with an annual water turnover of 6 mm sea level equivalent [ Budd , 1991]. In terms of sea level rise, the ensemble mean P-E projection represents a negative contribution of around 5 cm by the year 2100. Positive contributions are possible through basal melting of ice shelves in contact with the ocean and the acceleration of glacial ice export into the ocean as ice shelves break away.  However, at present there is a large uncertainty over these other effects due to lack of observations with which to constrain ocean and ice-sheet models.

Precipitation trends show a large spatial variation. Absolute increases of precipitation over the continent are largest near to the coast and increases relative to current conditions are largest (up to 25%) over the interior region of East Antarctica. 

near-surface Winds

Little work has been done on future changes of wind speed. The main reason for this is that accurate wind speed calculations require a good representation of cyclonic storms, which are often too small to be captured by most of the current generation of climate models. Here we show results of changes to the monthly-mean 10-meter wind vectors, which are less dependent on the representation of cyclones. Changes to the magnitude of the wind vectors are nearly equivalent to changes of the mean wind speed if the directional constancy (ratio of the average wind vector magnitude to the average wind speed) is large. Over the steep orography of the Antarctic continent the directional constancy is very large [ van den Broeke et al. , 1997].  Over the ocean around Antarctica the directional constancy is small and changes of the mean wind speed will in general differ from monthly-mean wind vector magnitude. 

The annual and zonal mean wind vector magnitude at 60S is projected to increase by 0.73 ± 0.36 m s^-1 (21 ± 10 %) over the 21^st century. The largest increase is in the autumn season, for which the magnitude of the autumn maximum of wind increases at 60S is 1.13 ± 0.49 m s^-1 (27 ± 12%). The poleward shift and increases of the westerly wind component result in decreases of the coastal easterlies around Antarctica in summer and autumn, but do not penetrate sufficiently far south to influence coastal Antarctica in winter and spring.  There is a strong consensus between the models for a decrease of the coastal easterlies. The average wind vector magnitude trend over the interior of Antarctica is close to zero, with cancellation between regions of increase and decrease. The increases (up to 10% in autumn) consistently occur in a region that extends over Queen Mary Land and Wilkes Land. 

References cited by Bracegirdle

Editors’ note: yet to be provided by author.