While we unilaterally agree that we are in for some serious rise in sea levels, the actual figures have been difficult to predict. Determining how past sea levels have varied can help us better understand how sensitive they are to climate changes and their consequences.
It has been held that through the last natural rise in temperature that happened between 117,000 to 128,000 years back, past sea levels rose between 20 and 30 feet (6 – 9 meters) when compared to current levels. And the temperatures during that period were a mere 1 to 2 C higher than during the pre-industrial times. That is a figure that we are set to cross in a few decades.
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The figures could only have been possible with the total collapse of the polar ice cover. Yet future models predict that future sea levels will rise only around a meter with the corresponding rise in temperature that has been predicted.
Past Sea Levels Studies Did Not Factor In The Rise And Fall Of Land Mass
Recent studies at Columbia University have suggested that experts did not accurately factor in land ups and down over the long term with past sea levels. Fresh studies indicate that the rise in the sea levels in the previous interglacial was in line with current estimates. They believe that the seas rose a maximum of 4 feet (1.2 meters). That is in step with the prediction for the hundred years.
The higher levels seem of over 17 feet seem unlikely. To reach the maximum projected figure of 30 feet (9 meters), huge parts of the polar regions of Antarctica and Greenland would have to melt. But it appears that it did not occur during the last warm period. But the conservative estimates are bad enough, while the upper estimates predict terrible consequences.
Other Factors Influenced Past Sea Levels
A new factor that had been previous ignored was the depression of land under the influence of layers of ice sheets. Landmass depresses under the pressure of ice that builds up gradually. The ice cover over much of North America at the time of the last glaciation that ended around 15,000 years ago, pushed down the land. It was even hundreds of meters at some places.
Such corollary deformity has not been properly studied by scientists. Outside the polar regions, it is difficult to come up with an estimate. The area covered would be hundreds of thousands of miles, covering large periods. So even as ice pressure depressed the landmass, they rose again once the pressure eased.
These movements are called glacial isostatic rebound and climatologists have still not got around to studying them. This effect in North America was felt as far south as the Bahamas archipelago. But it is difficult to put figures to such variation inland due to ice pressure.
Scientists found that islands in the north sank around 10 meters (32.8 feet) through the interglacial period, while it was only 6 meters in the south. Future predictions are modeled on past observations and it has been estimated that ice sheets will respond to warming, but not at as dramatic a level as previously estimated.
But there has been evidence of a higher rise in past sea levels from the Indian Ocean region, Australia, and the Mediterranean. The North American ice sheets are now being held to be smaller than previously assumed.
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Another reason for the last interglacial period was the orientation of the sun varied and both poles were not affected simultaneously. So even as one polar region lost ice, the other pole saw again in ice cover that eventually balanced out the loss, maintaining past sea levels.
But current models are bound to be different global heating has been caused by carbon emissions and the rate of heating is quicker and moves even. So comparing past sea levels results to current models is difficult as conditions varied then. Robert DeConto, a scientist at Massachusetts University says that there remains the possibility of a higher rise in sea level than 1.2 meters (3.94 feet). But the glacial rebound is at work. for instance, the Atlantic Ocean has seen a rise of 1-2 millimeters every year. and the US East Coast is also sinking at the same amount as ice sheets further north influence the shift. It has meant double trouble along the eastern coast.