Saturday, March 4, 2017

Has Global Sea Level Rise Increased to 5 mm Per Year?

There has not been a great deal of media coverage about the past 3 years of increase in the rate of sea level rise. Global sea levels have risen by an average of about 5 mm (0.2 inches) in each of the past 3 years. This is a significant increase versus in 3.3 mm (0.13 inches) increase per year during the satellite era (1993 to present). As shown below, the rate of sea level rise jumped up above the trend line in late 2014 and continued to rise in 2015. Sea level rise has remained above the trend, with another big jump in the most recently posted result (the period ending November 30, 2016)

Is sea level rise accelerating? There are a couple of valid reasons to question whether the past 3 years signals an increase in the rate of sea level rise. Time will tell whether an increase in the rate of sea level rise began in 2015 or not.
  1. The increase in sea level rise during the last three years may be due to El Nino. The warm global temperature and other weather impacts of El Nino may have produced the increase in the annual sea level rise. Sea level rise may revert back to 3.3 mm per year in non-El Nino years. 
  2. Three years of higher sea levels is probably not a long enough period to confirm a change in the trend of sea level rise. Although the increase in sea level rise during the satellite era is on a fairly linear trend, the annual results are rather noisey. However, if the past three years does indicate that sea level rise is now increasing at a faster pace, the consequences for coastal areas are dramatic. 
Whether or not an acceleration in the rate of sea level rise has begun has enormous implication for the future of coastal areas. Thirty years of 0.13 inches of sea level rise would result in a 4 inch increase. However, thirty years of 0.2 increases would lead to a 6 inch increase. Tidal flooding is already a serious problem. An additional 2 inches of sea level rise (6 instead of 4) would lead to billions of dollars in damage.

And of course, many climate researches are predicting that the rate of sea level rise is going to go parabolic, and that 6 inches of sea level rise in the next 30 years is an unrealistically optimistic scenario. As shown below, the NOAA intermediate prediction (green line) is for about 1 1/2 feet of sea level rise within the next 30 years.