Antarctic Changes Associated with Global Warming towards Equilibrium for Different Levels of Stabilisation of Greenhouse Gases

To examine the Antarctic changes associated with global warming, over the past century and possibly developing in the future, the Australian CSIRO coupled atmosphere-ocean global climate model was used for a number of simulations with forcing from the increasing greenhouse gases over the past century based on observations (from 1880 AD) and with projected scenarios to different levels of stabilisation and then continuing into the future, with those levels fixed. The future atmospheric equivalent CO₂ concentration followed the IS92a (IPCC, 1996) radiative forcing scenario to 2000 (the present, or `P'), 2033 (two times the value of 1880, or `2x') and 2082 (three times the value of 1880, or `3x'), then continuing with those respective values for the rest of the simulation, until near to equilibrium at the surface was reached in each case. This time ranged from over 1000 years for the `P' case to over 2000 years for the `3x' case. Although global changes are simulated in this work we focus particularly on the simulation over the Southern Ocean and the Antarctic region where for many features the changes can be much larger than for the global average. When compared to the observations available, the modelling results show reasonable agreement for the changes over the Southern Hemisphere and the Antarctic region up to the present.

For the future changes the reduction of Antarctic sea-ice (in both thickness and area coverage) is associated with strong surface and atmosphere warming, and increased net radiation received at the surface, but it is also accompanied by increases in precipitation (P), evaporation (E), and P-E. These changes occur most strongly in both the simulations for the `2x' and `3x' stabilisation cases but with much less change for the `P' stabilisation case. The warming from the initial conditions over the (fixed) Antarctic maximum sea-ice region, is 1.3 K for `P', 2.4 K for `2x' and 3.7 K for `3x' by the next 100 years. By the next 800 years from the present the corresponding simulated changes are 2.1 K, 4.7 K and 9.7 K respectively for the `P', `2x' and `3x' cases. The changes over the Antarctic continental area are similar. As a result of the surface freshening in the Antarctic sea-ice zone (due to the sea-ice reduction and the increase of P-E) the Antarctic Bottom water formation was largely reduced by the end of 21st century, which leads to a reduced deep ocean circulation, even for the `P' case, but to a less extent.