Swallowed Islands: Getting Sea Level Rise Out of Variability

Guest Essay by Kip Hansen

Damien Cave, the NY Times Australia bureau chief, writes yet another heart-rending story with great personal touches about how the Solomon Islands are being “swallowed by the sea”.  His latest offering is titled: Solomon Islands Dispatch — His Pacific Island Was Swallowed by Rising Seas. So He Moved to a New One.

The story is about a seaweed “farmer” who grows a particular ” wiry breed that’s exported across Asia.”   The seaweed grows in the shallow water around his “island”.  Calling it an “island” is a bit of a stretch — it is really a sand spit or sand bar, “At mid-tide, it’s 24 steps across at its widest point, and 58 steps long.”   [ Note:  this is not a coral atoll — just a pile of sand, maybe over some bit of rock.] Of course, The Sea Weed King’s first sand spit got washed away by the currents, so he moved to a new one.  Here’s what it looks like:

We’ll have to guess what the elevation above Mean High Water is, the Times doesn’t tell us, but is certainly isn’t more than a foot or two.

Now, you can read the whole story in the NY Times (linked in the first paragraph) — but here’s the thing — the Times repeats this oft-echoed falsehood:

“Scientists call it a global hot spot. The surrounding seas have risen about 7 to 10 millimeters per year since 1993, roughly three times today’s global average — and what scientists expect across much of the Pacific by the second half of this century.”

They get that impression from someone looking at graph like this:

That’s right — my golly — 225 millimeters in 25 years — that’s 9 mm/yr!

Oh my — I won’t say it — I won’t.  I will not call this F… News.

I will, however, supply a more pragmatically annotated graph of the NOAA-listed  Tide Gauge in the Solomons:

Experienced readers will see right away (have already seen in the first graph) how a big rise has been made out of highly variable data.  The data is extremely  variable — 18 inches of variability, which is about the same as the normal daily tides in the Solomons.  In fact, over the last 25 years or so, Mean Sea Level at Honiara has soared and fallen over an 18 inch total range, ten times or so.  Even if we insist on using the apparently somewhat outlier date of 1993 (far left) we can only squeak out a 4 inch rise comparing to range for 2015-2016 (a 6 month peak in 2017) and 2018.

A reverse-bias view would be something like this:

The truth however, is the bare raw data for Honiara-B:

When we have the raw data graph (this is monthly MSL) a trained observer can see what the real situation is.  We suspect that an equipment move or change made the break in 1997 sometime.

Drawing “trend” lines over times series graphs is itself a form of bias, constructed from the choice of start and end dates.  I have already demonstrated that the “alarming rise in sea level” in the Solomons is a construct of such choices and presents an entirely false image of the situation.  Likewise, selecting mid-1990s to early-2018 is equally biased, but in the opposite direction.

The graph immediately above is what we really know about sea level changes at Honiara-B — there has been a very wide range of variability over the last 25 years.  Mean Sea Level at Honiara-B is currently running low in the range, not high.

How one finds out about all this is to go to the PSMSL website and find the PSMSL page for Honiara-B station in the Solomon Islands.  There is a lot of information there, but the data authority for Honiara is actually the National Tidal Centre in Kent Town, South Australia.  The National Tidal Centre page for Honiara offers downloads of yearly data.

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Of Note:  The New York Times article quote used above contains this:  sea level has “risen about 7 to 10 millimeters per year since 1993, roughly three times today’s global average — and what scientists expect across much of the Pacific by the second half of this century.”   This links to a very interesting 2015 paper titled: “Coastal vulnerability across the Pacific dominated by El Niño/Southern Oscillation” in NATURE Geoscience by Patrick L. Barnard et al.  It is a mystery to me why they used this as a reference as this paper does not make any predictions for mean sea level rise for the Pacific, none, at all.   It does, however, say this:

“Upper-end sea-level rise scenarios [IPPC projection scenarios] could displace up to 187 million people by the end of the twenty-first century1, with flood losses exceeding US$1 trillion per year for the world’s major coastal cities by 2050 (ref. 2). However, prior studies typically omit key oceanographic components of water level elevations during storms that drive severe beach erosion and flooding of coastal communities, and can be highly temporally and spatially variable. As the climate system evolves nonlinearly, so too will the spatial distribution of mean and extreme wind speed, wave height, period and direction, water level anomalies, and resulting coastal response, as is evident from trends observed over the past two decades.”

and

“We find that observed coastal erosion across the Pacific varies most closely with El Niño/Southern Oscillation, with a smaller influence from the Southern Annular Mode and the Pacific North American pattern. In the northern and southern Pacific Ocean, regional wave and water level anomalies are significantly correlated to a suite of climate indices, particularly during boreal winter; conditions in the northeast Pacific Ocean are often opposite to those in the western and southern Pacific.”

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Author’s Note:

Journalists should do their job.  They should check the most pertinent facts for themselves — in this case:  Is sea level really rising 7-10 mm/yr in the Solomons?

Finding out that it hasn’t and isn’t makes a much more interesting story than “yet-another-alarmist-talking-point”.

Do note that while coral atolls are generally self-regenerating, sand spits/sand bars are not — they are at the mercy of the currents and waves.

Thanks for reading.

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Quick Links:

Damien Cave

Solomon Islands Dispatch — His Pacific Island Was Swallowed by Rising Seas. So He Moved to a New One.

expect across much of the Pacific

PSMSL page for Honiara-B station

National Tidal Centre page for Honiara

expect across much of the Pacific

Coastal vulnerability across the Pacific dominated by El Niño/Southern Oscillation

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