Sea Level Rise

Sea Level Rise

Determining the amount of sea level rise in the past, and predicting it for the future is a tricky proposition at best, and may possibly be something that’s not currently possible to do.

The media touts recent studies as proof of global warming and of a disastrous sea level rise in the future.

Examining the basic components of sea level rise can provide a better perspective on the issue. They are:

  • The amount of glacier melt or growth
  • Increase or decreases in ocean temperatures
  • Subsidence
  • Rebound from past glacier activity
  • Geologic and topographic features along shorelines

It may well be that satellite measurements of sea levels are the most accurate, but these have only been available for the past twenty-five years, since approximately 1992. 

These measurements show an average sea level rise of approximately 3.2 mm/year or about 12 inches per 100 years. IPCC data show that sea level rise from 1930 to 1950 was equal to or greater than the rise shown by satellites from 1992 to the present. This would indicate there hasn’t been a recent acceleration in sea level rise.

Sea Level Rise from IPCC AR5
Satellite altimetry readings are shown with 90% confidence levels as an error bar.

Here are the variables involved in making estimates about past sea level rise, and predictions about future sea level rise:

  1. Sea levels have risen since the last ice age due to melting glaciers. What happens over the next 100 years depends on whether currently existing glaciers melt, or increase in size.
  2. Over the past 10,000 years, global temperatures have increased and decreased. As a result, oceans levels have risen and fallen. 
  3. Tidal gauges around the world have been used to measure sea level rise locally and were the primary method for measuring sea level rise globally prior to the use of satellites. In modern times, tidal gauges have become very sophisticated, replacing gauges mounted on pilings etc. with electronic measuring devices. Tidal gauge measurements were inherently difficult to use for measuring global sea level rise because they were affected at different locations by:
  • Subsidence. Locally, the ground subsided so that the gauges indicated a rise in sea level that was actually a lowering of the ground level.
  • Rebound from the last ice age. Glaciers deformed the Earth where they stood during the last ice age. Areas where the glacier was thickest caused the Earth to sink, while at the edge of the glacier the Earth tended to rise. With the loss of glaciers the Earth rebounded where the glacier was heaviest, and the tidal gauges, such as around Sweden, would indicate a lowering of the sea level. Obversely, such as in Venice, the tidal gauges would indicate rising sea levels.
  1. Geologic processes, such as described above, land use and coastal or river engineering and erosion can influence how sea level rise affects specific shorelines.
National Geographic Cover

The recent dramatic National Geographic Magazine cover typifies how sea level rise is misunderstood and dramatized by the media. It so happens that the area around New York City has been sinking, by 1 to 2 mm per year, or at least 4 inches per century, which offsets the 11 inch rise claimed by the National Geographic article, bringing sea level rise close to what was considered normal over the past century, or approximately 7 – 8 inches.

Recent studies reach varying conclusions, but those cited by the media show sea level rise below that shown by satellites. More detailed information on recent studies can be found at http://bit.ly/1plW9y8 

Media histrionics need to be kept in perspective. Understanding the basic variables and what satellite data has shown thus far, would indicate no dramatic disasters are awaiting us from sea level rise caused by global warming.

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