Arguments
Climate scientists make a bold prediction about sea level rise
Posted on 10 August 2016 by John Abraham
One of the great things about science is that it allows you to make predictions. Three top climate scientists just made a very bold prediction regarding sea level rise; we should know in a few years if they are correct.
As humans emit greenhouse gases, it’s causing the Earth to warm. That’s indisputable and proven. We can actually measure the amount of extra heat. Since most of it ends up in the oceans, we can also measure other changes in the oceans.
For instance, the oceans are rising. We know that’s indisputable. Measurements taken from physical gauges and from satellites confirm sea level rise. The cause of the rise is more complex.
Part of the rise is from ocean warming – warm water is less dense so the sea level rises as temperatures increase. Another part of the rise is from melting ice, especially ice that is currently on land (like glaciers and ice sheets). As this ice melts and flows into the oceans, the water levels rise. A third reason for sea level changes is from alterations of where water is stored on the planet. For instance, changing rainfall patterns and storage of water underground, in lakes, or in the atmosphere can affect sea levels.
The three ways we know sea levels are rising are from physical tide gauges, from satellites that measure the water height, and from satellites that measure where ice is stored across the globe. While tide gauge measurements go back many years, they only measure water levels at their location. Many tide gauges have to be in place to get an accurate sense of what is happening globally.
Satellites, on the other hand, are much more capable of taking global measurements. The problem with satellites is they have only been taking measurements since approximately 1993 (not nearly as long as tide gauges). So scientists try to combine these two measurements to get a long-term and global picture of what is really happening.
A very recent paper published in Nature has evaluated the history of sea level rise, and what they find is really interesting. The lead author (John Fasullo from the National Center for Atmospheric Research) and his colleagues tried to determine if the rate of sea level rise is changing. That is, are the water levels rising linearly, the same amount each year? Or, is the rate increasing (faster and faster each year)?
Using satellite data, the authors found little evidence of an acceleration. However, they show that this is because the satellites began measuring in 1993, right after a large volcanic eruption (Mount Pinatubo). This eruption temporarily reduced global warming because particles from the eruption blocked sunlight. Just by coincidence, the timing of the satellites and the eruption has affected the water rise so that it appears to be linear. Had the eruption not occurred, the rate would have increased.
This allows the scientists to make a prediction:
I haven't read (Fasullo 2016) but this explanation:
is confusing, or even defies my logic. If the starting point of SLR curve in question is 2 years fter Pinatubo (which happened in 1991), which means it's a local minimum, then the tread calculated from such cherry picked start should be larger than the average trend after smoothing the variability. It's like cherry picking a strong LaNina starting year for the temp trend: the measured trend appears larger than the average. Opposite happens when deniers of statistics try to pick a strong ElNino year (e.g. 1998) as the starting year and to claim that global warming has "stopped".
In our case herein, cherry picking 1993 (a local minimum) overestimates the trend. So how the statement "Had the eruption [local minimum] not occurred, the rate would have increased" makes sense?
The new paper offers an explanation why the acceleration of sea level rise, as measured by satellites, from the 90's (post '93) through the 2000's was less than expected.
The answer is that the rate of sea level rise in the 90's (post '93) was boosted by the recovery following the Pinatubo eruption. Had the eruption not occured, the rate of sea level rise in the later 90's would have been smaller, the rise in the 2000's would have been the same, and the acceleration of sea level rise over those two decades more clearly evident.
Hope that clears things up a bit. Predictions of future sea level rise depend not so much on the current rate of rise, but on the rate of the acceleration of the rise over time. A small diference in the acceleration rate has a dramatic effect on the estimate of sea level rise for the year 2100 and beyond.
As a persistent (pest?) pro-nuclear person on this blog, this is probably a good time for me to say that I would like to see some kind of worldwide declaration (or even international regulation) adopted that all future nuclear power plants should be constructed a miniumum of 30m above sea level. I actually said so even pre-Fukushima (yes, really). I wasn't thinking of tsunamis at the time, but rather that rising sea levels and increased typhoon activity could result in record-breaking storm surges, and thus coastal floods. It was actually the Hurricane Katrina disaster in 2005 that started me thinking along those lines. The point was demonstrated again in 2008 by Hurricane Ike, which put Galeveston, Texas, under water.
Of course, it's not just nuclear power plants we have to be concerned about - any critical infracture that is now being planned in coastal areas should be taking global sea level rise into consideration. I make no predictions just how fast and high the seas will rise, but it's better to err on the side of caution.
A paper reassessing the satellite altimeter estimates of global mean sea level rise was published in May 2015 by the Uni of Tasmania, CSIRO and others in Nature Climate Change.
This graph from the paper shows that when possible biases in the early satellite record are taken into account, firstly the sea level rise becomes about 10 mm less over the last two decades. Secondly there has been an acceleration of the rate of sea level rise over this period. They say ‘our revised record indicates that the rate of rise has increased over the last 2 decades (independent of how we estimate the vertical land motion), consistent with other observations of the increased contributions of water and ice from Greenland and West Antarctica.’
The two papers taken together presumably provide more confidence in the conclusion that sea level rise has accelerated over the last two decades and give an even higher estimate of this acceleration.