More
Ripples in the Sea Level Debate
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 Earths 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 Gophysique de lEnvironnement of Domaine
University, St Martin dHres, 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
continents 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
models 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.