February 23, 2007

More Ripples in the Sea Level Debate

Filed under: Sea Level Rise

The debate over future sea level rise from melting ice in a changing climate has raged for years. It is widely believed that the expression of climate change is amplified across the polar regions of Earth, and there exists some supportive evidence in historical climate records. However, much of the worry about rapid polar climate change stems from model predictions of inordinate warming in the high latitudes during the 21st century. The chain of events, then, is warming, glacial melt, and sea level rise, which is a logical sequence that makes doomsday scenarios of sea level rise easy to sell. The dogged pounding of this drum by the global warming crusaders is enough to raise suspicion that they are actively purchasing all of the high-elevation real estate on the face of the Earth!

During recent decades - a period during which alarmists claim that Earth’s lower atmosphere warmed more than at any point during the past two millennia - trends in Antarctic temperature are rather ambiguous. Several research efforts have documented rapid warming over the Antarctic Peninsula, while others have shown a cooling trend over the eastern coastline of the continent over the last few decades of the 20th century. In the interior of Antarctica, a significant increase in the surface mass balance has recently occurred despite no significant increase in precipitation. Linking future global sea level rise to Antarctic melt has been hard work, and a recent piece of research has thrown more straws on the back of the crusaders’ Antarctic camel.

The recent research article, “Simulated Antarctic Precipitation and Surface Mass Balance at the End of the Twentieth and Twenty-First Centuries,” contains conclusions that further contradict the once-popular theory of eroding Antarctic ice mass and associated global sea level rise. The research, conducted by Gerhard Krinner of the Laboratoire de Glaciologie et de Géophysique de l’Environnement of Domaine University, St Martin d’Hères, France, and colleagues, and published in the February 2007 issue of Climate Dynamics, employed an atmospheric general circulation model with high resolution over Antarctica to simulate the continent’s climate over the last two decades of the 20th and 21st centuries. The model used by the researchers, enhanced specifically for simulating polar climates, generates precipitation and the energy fluxes necessary to simulate snow or ice melt. After one year of spin-up, the model simulated the Antarctic climate and surface mass balance for the 1981-2000 and 2081-2100 periods. The model’s Antarctic polar amplification of temperature is 16%, meaning that the air temperature change over Antarctica is 0.16 times greater than the global temperature change. Impressive, huh? This places it close to the average of the models within the 4th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). The simulated surface mass balance accumulation generated by the model during the period 1981-2000 was well within observed estimates. The two mass balance components of precipitation and melt were in very good agreement with estimates and satellite observations, and the researchers concluded that ±20% of the true value of the surface mass balance was represented by the general circulation model.

At the end of the 21st century the model simulated a mean surface mass balance of the Antarctic ice sheet of 183 kg m-2 year-1 (Figure 1), and an increase of 32 km m-2 year-1. This is in agreement with recent observations of increasing mass balance in the interior of the continent that have been attributed to increased precipitation from a warmer atmosphere that is capable of holding a greater amount of moisture. The model predicts that snow and ice melt will increase by more than a factor of three, but it remains very small compared to precipitation. Melt is important in some regions of the continent within the model, but across Antarctica at the end of the 21st century 80% of the melt water re-freezes. The increase in Antarctic surface mass balance equates to a decrease in sea level of 1.2 mm year-1 at the end of the 21st century compared to the end of the 20th century. Furthermore, Krinner et al. state that if we assume that the rate of change is linear for the next 100 years, the surface mass balance increase “would lead to a cumulated sea level decrease of about 6 cm.”


Figure 1. Simulated surface mass balance (kg m-2 year-1) for the Antarctic ice sheet for the years 2081-2100.

Krinner et al. concede that melt in certain regions of the Antarctic continent could have impacts on future sea level changes; however, their model indicates that much of that melt water refreezes, even under the warmth of the end of the 21st century. It seems that future mass balance changes on Antarctica are mostly dependent upon an increase in continental-scale precipitation in association with continental-scale warming. Krinner et al. make it a point to note that the warming-precipitation relationship is indeed complicated. However, their findings approximate those by Wild et al. (2003), van Lipzig et al. (2002), and Huybrechts et al. (2004).

Evidence is quickly mounting against the idea that Antarctic melt will contribute to global sea level rise during the 21st century. In fact, the real debate might be concerning the degree to which an increase in mass balance on Antarctica will counterbalance melt elsewhere across Earth.

References:

Huybrechts P., Gregory J., Janssens I., Wild M., 2004. Modelling Antarctic and Greenland volume changes during the 20th and 21st centuries forced by GCM time slice integrations. Global Planetary Change, 42, 83–105.

Krinner, G., Magand, O., Simmonds, I., Genthon, C., Dufresne, J.-L., 2007. Simulated Antarctic precipitation and surface mass balance at the end of the twentieth and twenty-first centuries. Climate Dynamics, 28, 215-230.

van Lipzig N.P.M., van Meijgaard E., Oerlemans J., 2002. Temperature sensitivity of the Antarctic Surface Mass Balance in a regional atmospheric climate model. Journal of Climate, 15, 2758–2774

Wild M., Calanca P., Scherrer S., Ohmura A., 2003. Effects of polar ice sheets on global sea level in high-resolution greenhouse scenarios. Journal of Geophysical Research, 108, 4165.




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