NOAA Sea Level New York City
In recent years there have been several projections regarding sea level rise, including claims that the oceans could rise up to 10 feet by 2100. It is time to consider just how much weight we should give these projections as they would suggest that vast areas of the world’s coastline could be flooded before there was time to fully react.
What we know
Global warming
As per the National Oceanic and Atmospheric Administration (NOAA), the global temperatures are rising at a rate of about .07C per decade (0.7C per century) and currently are about 0.8C higher than around 1900. The warming has not been continuous; however, as there was a cooling trend between 1880 and 1910 followed by a moderate warming of about 0.25C from about 1910 to 1945. Temperatures remained stable or possible cooled slightly between 1945 until about 1978 followed by another period of stronger warming of about 0.6C from 1978 to present.
NOAA Global Surface Temperature Anomaly
Sea Levels
During the 20th Century, sea level rose at a rate of about 1.8 mm/yr based on actual tide gauges. More recent data based on satellite observations suggests, however, that since 1993 the rate was between 2.9 mm/yr and 3.4 mm/yr.
The accuracy of direct sea level measurements via satellites is about +/- 2 cm (20 mm) so we may need more time to judge the accuracy of the trends calculated from these measurements vs. those made from long-term tide gauge observations. See: Sea Level Trends – NOAA Tide and Currents page
Studies of sea level rise rates during the past century suggest that short-term (18-yr trends) of sea level rise rates have ranged between 0 and 4 mm/yr, even prior to 1950 (before most of CO2 was added to the atmosphere). This suggests that the current higher rates may not be solely driven by greenhouse gas emissions and adds uncertainty to the various sea level projections.
18 year sea level rates
Source: Is sea level rise accelerating? Judith Curry Blog post February 23, 2016
The Future
Bottom line, sea level is rising and the rate has increased but there is significant uncertainty regarding predicting future rates and thus sea level estimates for the next 50-100 years are problematic. If the rate of sea level rise would double, for example, over the next century from the current satellite estimates, we would expect a total sea level rise of about 1.2-1.4 ft. by 2100
By contrast, global sea level rose by a total of about 120 meters over a period of about 8,000-9,000 years as the vast ice sheets of the last glaciation melted away. This equates to an average rate of sea-level rise during this period of roughly 1 meter (3.3 feet) per century or about 10mm per year (Some studies suggest the rate was up to 13 mm per year). Even if somehow we could get to the rates encountered after the last glaciation of 10-13 mm/yr over the next century then by 2100, the total rise will be about 1.8-2.2 feet.
So then the question is how could we get to a 6 to 10 foot sea level rise so quickly? A recent study suggests that a rapid collapse of the Western Antarctic Ice Sheet is possible. The western half of the Antarctic has its base lying below sea level so as global temperatures rise; warmer ocean water will melt the underside of the ice sheets.
The modeling suggests significant sea level rise is possible from Antarctica ice melt alone by 2100. The key assumption here is that greenhouse gas emissions will boost the planet’s temperature by about 4 degrees C (7 degrees F) which is considerably more than the present 0.8C of observed warming. The paper admits that under the model the timing and pace of Antarctica’s ice loss is “really uncertain”.
Without the vast continental ice sheets of the last glaciation, it is doubtful that we will reach the extreme prediction of up to 10 feet. In order to see a sea level rise of 10 ft by 2100, the average annual rise would have to be about 37 mm/yr, more than 10 times the current rate!. In addition, these new estimates are also far above the estimates by the Fifth Assessment Report from the United Nations’ Intergovernmental Panel on Climate Change. Here, they claim that the most likely amount of global sea level rise would be about 1 foot to slightly more than 3 feet by 2100.
Fred Pickhardt
Ocean Weather Services.
