15.3.2 Projected atmospheric changes
The areally averaged warming in the Arctic is projected to range from about 2°C to about 9°C by the year 2100, depending on the model and forcing scenario. The projected warming is largest in the northern autumn and winter, and is largest over the polar oceans in areas of sea-ice loss. Over land, the projected warming shows less seasonal variation, although regions such as the Canadian Archipelago are not well resolved.
In contrast to the unanimity of the models in predicting a north-polar amplification of warming, there are differences among the model projections concerning polar amplification in Antarctica, especially over the continent (Parkinson, 2004). However, in several simulations, the warming is amplified over a narrow Southern Ocean band from which sea ice retreats.
Global precipitation is projected to increase during the 21st century by 10 to 20% in response to the SRES A1B emissions scenario of the IPCC Fourth Assessment Report (AR4) simulations. However, the seasonality and spatial patterns of the precipitation increase in the Arctic vary among the models. Similar results have emerged from other IPCC AR4 simulations (Kattsov et al., 2007). In addition, the partitioning among snow and rain will change in a warmer climate, affecting surface hydrology, terrestrial ecosystems and snow loads on structures. The ratio of rain to snow should increase especially in those seasons and Arctic sub-regions in which present-day air temperatures are close to freezing. The difference between precipitation and evapotranspiration (P-E), which over multi-year timescales is approximately equivalent to runoff (river discharge), is also projected to increase over the course of the 21st century. The projected increases of runoff by 2080 are generally in the range of 10 to 30%, largest in the A2 scenario and smallest in the B1 scenario. Of the major river basins, the largest increases are projected for the Lena River basin. Additional information on projected changes is presented elsewhere (Meehl et al., 2007). The effects of these freshwater changes on the thermohaline circulation are uncertain.