Sea Rise!

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Mon, 10/17/2011

It has often and continually debated on how much the level of the sea rise in the next few decades and centuries. Sea level changes is actually old hat and has been happening (up and down) for millenia. New research from several international research groups, including the Niels Bohr Institute at the University of Copenhagen provides independent consensus that IPCC predictions of less than a half a meter rise in sea levels is around 3 times too low. The new estimates show that the sea will rise approximately 1 meter in the next 100 years in agreement with other recent studies. The results have been published in the scientific journal, Geophysical Research Letters.

Current sea level rise potentially impacts human populations (e.g., those living in coastal regions and on islands) and the wider natural environment (e.g., marine ecosystems). Global average sea level rose at an average rate of around 1.8 mm per year over 1961 to 2003 and at an average rate of about 3.1 mm per year from 1993 to 2003. It is unclear whether or not the increased rate observed between 1993 and 2003 reflects an increase in the underlying long-term trend.

There are two main factors that have contributed to observed sea level rise. The first is thermal expansion: as ocean water warms, it expands. The second is from the contribution of land-based ice due to increased melting. The major store of water on land is found in glaciers and ice sheets.

Since IPCC (Intergovernmental Panel on Climate Change) published their predictions in 2007, that the sea would rise less than half a meter (500 millimeters) in the next 100 years, it became clear that there was a problem with the prediction models as they did not take into account the dynamic effects of the melting ice sheets. The estimates were therefore too low.

However, the new model estimates, from international research groups from England, China and Denmark, give independent support for the much higher predictions from other recent studies.

”Instead of using temperature to calculate the rise in sea levels, we have used the radiation balance on Earth — taking into account both the warming effect of greenhouse gasses and the cooling effect from the sulfur clouds of large volcanic eruptions, which block radiation”, explains Aslak Grinsted, PhD in geophysics at the Center for Ice and Climate, the Niels Bohr Institute at the University of Copenhagen.

The research is based on observations of sea levels from the 1700s to the present and estimates of the radiation balance through approximately 1000 years.

The sun’s heat varies periodically and currently there is a solar minimum, but even if solar radiation were to reach its lowest level in the past 9300 years, it will have only a minimal impact on sea levels. Some have suggested that you could inject sulfur into the atmosphere and get a kind of artificial volcanic eruption cooling effect, but the calculations show that it would only slow down the rise in sea levels for 12-20 years. What are important are greenhouse gasses like CO2, the research shows.

The results are that the sea level will rise between 0.7 and 1.2 meters during the next 100 years. The difference depends on what mankind does to stop the emission of greenhouse gases into the atmosphere. If we seriously reduce the emissions of CO2 globally, the sea will only rise 0.7 meters, while there will be a dramatic rise of 1.2 meter if we continue to increase CO2 emissions with the current use of energy based on fossil fuels.

In the calculations the researchers assume that we continue to emit CO2, but that we move more towards other energy supplies and reduce our use of fossil fuels and with that reduce the emission of greenhouse gases. This scenario would give a rise in sea levels of around 1 meter.

Even a one meter rise in sea levels would have a big impact in some places in the world with low lying areas, which will become much more susceptible to extreme storm surges, where water could easily sweep over the coasts.

For the past 6,000 years (many centuries before the first known written records), the world's sea level has been gradually approaching the level we see today. During the previous interglacial about 120,000 years ago, sea level was for a short time about 6 m higher than today, as evidenced by wave-cut notches along cliffs in the Bahamas. There are also Pleistocene coral reefs left stranded about 3 meters above today's sea level along the southwestern coastline of West Caicos Island in the West Indies. These once-submerged reefs and nearby paleo-beach deposits are silent testimony that sea level spent enough time at that higher level to allow the reefs to grow (exactly where this extra sea water came from—Antarctica or Greenland—has not yet been determined). Similar evidence of geologically recent sea level positions is abundant around the world. So the rise in the seas is not totally unexpected.

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