Toward a seasonally ice-covered Arctic Ocean: Scenarios from the IPCC AR4 model simulations

The sea ice simulations by the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) models for the climate of the twentieth century and for global warming scenarios have been synthesized. A large number of model simulations realistically captured the climatological annual mean, seasonal cycle, and temporal trends of sea ice area over the Northern Hemisphere during 1979-99, although there is considerable scatter among the models. In particular, multimodel ensemble means show promising estimates very close to observations for the late twentieth century. Model projections for the twenty-first century demonstrate the largest sea ice area decreases generally in the Special Report on Emission Scenarios (SRES) A1B and A2 scenarios compared with the B1 scenario, indicating large multimodel ensemble mean reductions of -3.54 +/- 1.66 X 10(5) km(2) decade(-1) in A1B, -4.08 +/- 1.33 X 10(5) km(2) decade(-1) in A2, and -2.22 +/- 1.11 X 10(5) km(2) decade(-1) in B1. The corresponding percentage reductions are 31.1%, 33.4%, and 21.6% in the last 20 yr of the twenty-first century, relative to 1979-99. Furthermore, multiyear ice coverage decreases rapidly at rates of -3.86 +/- 2.07 X 10(5) km(2) decade(-1) in A1B, -4.94 +/- 1.91 X 10(5) km(2) decade(-1) in A2, and -2.67 +/- 1.7107 X 10(5) km(2) decade(-1) in B1, making major contributions to the total ice reductions. In contrast, seasonal (first year) ice area increases by 1.10 +/- 2.46 X 10(5) km(2) decade(-1), 1.99 +/- 1.47 X 10(5) km(2) decade(-1), and 1.05 +/- 1.9247 X 10(5) km(5) decade(-1) in the same scenarios, leading to decreases of 59.7%, 65.0%, and 45.8% of the multiyear ice area, and increases of 14.1%, 27.8%, and 11.2% of the seasonal ice area in the last 20 yr of this century. Statistical analysis shows that many of the models are consistent in the sea ice change projections among all scenarios. The results include an evaluation of the 99% confidence interval of the model-derived change of sea ice coverage, giving a quantification of uncertainties in estimating sea ice changes based on the participating models. Hence, the seasonal cycle of sea ice area is amplified and an increased large portion of seasonally ice-covered Arctic Ocean is expected at the end of the twenty-first century. The very different changes of multiyear and seasonal ice may have significant implications for the polar energy and hydrological budgets and pathways.