Sea Levels Rising

A friend of mine, JP, started all of this when writing:

If you were to ask me 2 years ago what my key understandings were about climate change, I would have said the following:

• Sea ice is rapidly shrinking (summer arctic sea ice to be gone by 2015)
• Sea levels are rising and accelerating
• Polar bear populations are under stress (have increased in the last 20 years)
• The levels of glacial retreat around the world are unprecedented (similar retreats have been seen in the last century)
• 97% of scientists agree that global warming is real and an urgent problem
• Any scientist who is skeptical about the claims made about climate change is a "denier" and is funded by oil/resource companies
• We are seeing an increase in extreme weather events (they are actually getting less common)
• Climate models are accurate in their predictions 

Every one of those things is either totally false, or a largely exaggerated claim.

My response, which was split over a few emails, serve as a basis for the following, in which I discuss sea levels along with an earlier article about sea ice (in which I provided a little more context about JP).  I may post about the other issues separately in more depth later, or they might be touched on in a series of articles that I intend to put together on the nature of climate denialism.  Please also note the caveat.

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JP’s Claim: The statement “sea levels are rising and accelerating” is either totally false or largely exaggerated

No timescale is provided for this claim so it’s difficult to say over what period there isn’t acceleration.  I’m not going to make things difficult for myself, so I will look at the past century or so.  I will defend the entire claim – that sea levels are rising, and that the rate of rise is accelerating.

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If you are a denialist and believe that the land ice on Greenland and the land ice at Antarctica, and various glaciers, are not melting, then there seems to be little reason to expect that sea-levels would be rising.  Global warming would make ocean water expand but not by a huge amount, [pure] water is most dense at 4C [1 kg per litre] expanding both as it turns into ice and as it warms, reaching a bit more than 0.99 kg per litre at 40C and closer to 0.96 kg per litre as it approaches boiling point – and it’s not going to be hugely different with salt water.  Naïvely you could think that the ocean levels wouldn’t rise by much, what’s one per cent going to do after all?  Also, surely only upper layer of the water, that exposed to the sun and the atmosphere, would be warming.  True enough, there is some mixing between the upper and lower layers but not that much, right?  So, it could seem that the sea level wouldn’t rise much due to thermal expansion.  Nevertheless, NASA associated scientist, Steve Nerem, said in 2018 that about half of the sea level rise up until that point was is understood to be due to thermal expansion.  The team he was associated with also reported that sea level rise is accelerating.  This would make sense given that the rate at which the Antarctic Ice Sheet is melting is accelerating.

The top 200m of the ocean is relatively smooth temperature-wise, at an average of about 13C.  So, even if we ignored the rest (which cools progressively to 4C at more than a kilometre down), then we can say that we have a 200m layer of water that warms and cools with the climate.  Making a rough estimate that between the thermal expansion of water between 4C and 40C is such that it expands by the same amount for each degree, that means that there is about 0.028% expansion, from 4C, per degree.  (Note, you can calculate this in at least two ways, either as the 1/36th root of 1.01, giving you the factor 1.0002764, ie an increase of 0.02764%, or by dividing 1% by 36, giving you 0.02778%.  Since we’re being rough here, I am happy with two significant places.)  If 200m of water is contained and expands by 0.028% per degree, that gives you an increase of 5.5 centimetres per degree.  In the past 25 years, according to EPA records, we’ve had an increase in average global sea surface temperature of about 0.75F, or 0.4C, which would equate to a rise in sea level of about 2.2 centimetres.

Add about 50% of that to account for shifting of the thermocline down to 1000m, and you have 3.3 centimetres, which is close to what Steve Nerem was implying (3.5cm).  So that seems to check out – sea levels are rising.

The other part of the claim is that sea level rise is accelerating.  Below is my annotation of CSIRO Global Mean Sea Level chart (note that there are links available to the data used on that chart):

What I was attempting to do, and admittedly it’s a bit messy, is show that if you take various chunks of time, you can see that there has been an acceleration in the rate at which sea levels have been increasing.  A steeper line indicates a greater rate at which sea levels were rising.  They are lowest between 1880 and 1930 (50 years) at about 1mm/year, although you can cherry pick a period from 1962/3 and 1984/5 to get an equally low rate of rise.

The coloured lines from 1880 basically tell the story, the 50 year line gives you 1mm (it’ll all be per year), the 75 year line is 1.3mm, the 100 year line is 1.4mm, the 125 year line is 1.6mm, and the 132.5 year line is 1.8mm.  If we look at 25 year chunks, which are useful given the magnitude of year on year variation, we also get increases (although there was an apparent hiatus, acceleration wise, between 1960 and the mid-1980s).  From 1980 to 2005, the rate was 2.4mm/year.  In the 25 years 1994 to 2019 (including a fair chunk of overlap), the rate was 3.3mm/year.

There has been an acceleration.  Whether that acceleration is continuing today is an open question, but I think it’s reasonable to say that, in terms of the recent past, the rate of sea level rise is accelerating.

Now JP did provide data in support of his claim, namely tide gauge data from Fort Denison in Sydney Harbour:

It is indeed difficult to see any acceleration in the rate of sea level rise in that chart.  However, I went to source of the data and generated my own charts (another based on the same data set is mentioned in Sea Level Denialism).  The first is a scatter diagram with a trend line from 1993 to 2018 (this is not just a 25 year period, it was also the period in which satellite telemetry was available and thus a one to one comparison could be made):

With a rise from 6980 to 7070 (90mm) in a period of 25 years that gives us a rate of sea level rise of 3.6mm a year or about the global average for the period.  What is interesting though is to look back another 25 years and see the period 1968-1993:

That’s about 6989 to 7015 over 25 years, or 1mm a year, again about the same as for the global average.  So, when you take a closer look at the data, it does in fact show that sea level rise has been, over the recent period, accelerating.

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The claim made by JP is that the statement that “sea levels are rising and accelerating” is either entirely false or largely exaggerated.  The evidence however, both globally and locally (Fort Denison), supports the statement – sea levels are rising and the rate at which sea levels are rising is increasing, thus there’s an acceleration involved.  The statement is neither entirely false nor largely exaggerated.

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I was going to touch on climate denialism with respect to sea levels in this article as well, but it got too long, so I’ve broken that out into Sea Level Denialism (which will appear after On Climate Denialism).