A tale of three models

Speaking truth to power: 1 & 2

Andrew Zolnai
Nov 5, 2019 · 6 min read

Sea level rise models show ins&outs of climate change science

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East Anglia coastal inundation models +5m sea level rise: CReSIS 2005, Firetree 2006 & ClimateCentral 2019

A tale of two maps contrasted CO2 emission by country and over time, in simple yet visually compelling bubble animations. Tech posts on my pro channel detail the means to do so: here we’ll post open data web maps, and the contrast will be in the modelling used to derive three maps.

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http://bit.ly/9Y9g6Z

I have blogged on East Anglian geo-history for almost a decade, initially to provide open source maps based on open data for the now defunct Fenlands Heritage Environment Project.

East Anglia is a contrast of of a flat expanse at or near sea level south of the Wash, and uplands of low cliff-forming coastlines that are rapidly eroding in Norfolk to the east.

Coastal inundation is thus very topical and I continue web maps using available open data.

Then

Part of the climate change risk & mitigation efforts revolve around coastal inundation. I first mentioned this almost a decade ago , when I highlighted flood.firetree.net sea level rise map based on NASA SRTM digital elevation data — more recent example using same is shown on Climate Central Surging Seas Risk Zone Map — Firetree used NASA data to map various sea elevations at 1, 3, 5 etc. m. above current sea level, and every point above each level is left white, while those below are posted blue: Simple yet compelling coastal maps.

I could not, however, source the original data — an issue around openness and web services discussed five years ago — but I did find a version on Esri’s ArcGIS Online. Toggle on the WWF / NASA 4 and 11 m. sea level rise below left, and half of Cambridgeshire in pale yellow is inundated!

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Coastal inundation models by Firetree and CReSIS, view original (click all images to enlarge)

When I was doing Arctic sea ice maps, however, I found NGDC-based CReSIS sea level maps. In contrast to Firetree, here a flood model was applied to ETOPO2 digital elevation data in coastal areas (see Overview and credits under here). Desktop mapping showed me a far more conservative coastal inundation of East Anglia: measured from 1 to 6 m contrasts with the 4 and 11 m above sea level. Luck would have it that Kenneth Field had already posted the 1 and 5 m levels of the same data. Toggle those two on in the map above, and you’ll only see the north tip of Cambridgeshire inundated!

Now

CReSIS and Firetree maps were created in 2005 and 2006 respectively, before this became a hot topic. Climate Central’s Risk Zone Maps are today very much in the news. They report NASA’s re-calibration of digital elevation data, released as CoastalDEM 1.1, which is 1–2 m lower than previously thought. They offer also a handy tool to compare old and new inundation models.

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Firetree (NASA) and ClimateCentral (legacy & CoastalDEM) flood models for Cambridge & villages north

While the difference doesn’t appear to be huge in the banner illustration, it’s quite apparent in localised maps above. I used the left one to inform Extinction Rebellion locally and the public in general in Cambridge UK: the purported shoreline would lie along the villages where I live just North of Cambridge, or in Cambridge itself depending on the modelled sea level rise. These raised outrageous speculations locally, which I was keen to correct with facts.

Next

These three maps highlight the issues in modelling data to document, interpret and predict the effects of climate change. Continuing to create maps created from open data, I posted as a story map East Anglia Coastal Inundation — Opportunities in Climate Change Modelling. In it are contrasted the original dataset referenced to local high tide lines (specifically, the mean higher-high water tidal datum, which matched the maps above at coastal.climatecentral.org), against the same data referenced to the more standard geoid vertical reference datum, EGM96 (to allow for easier comparisons with other DEMs, such as SRTM or lidar-derived models).

Sure enough this proved to be true! Compare the baselines as follow:

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CoastalDEM1.1 ref. high tide line (black, L) and CoastalDEM90 ref. standard datum (R)
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CoastalDEM (L) and OS OpenData (R), below sea level blue, 0–2 m. SLR white, > 2 m. elev. the rest

This is fully explained in the story map above, & shows the importance of datums.

Time

That story map concludes with timelines published by Climate Central in Nature Communications. They say “experts now estimate there is a 5 percent chance 21st century sea-level rise will exceed 2 m.”

One of my friendly Climate Deniers, whose friend in turn cherry-picked one data point to counter Climate Central’s whole premise (!), did raise a point I’m quite familiar with as a geologist: Bangkok like many coastal cities is subsiding — sinking into the adjacent gulf, which in fact would exacerbate coastal inundation, much to Deniers chagrin — and it’s not clear if coastal inundation models take that into account? In my East Anglia story map above, we may actually see the opposite: Are there remnant effects of glacial rebound I mentioned in a historic context in my original blog?

Well right on cue, NASA / JPL scientists came up with a model studying Antarctic ice sheets that include some of the above: They “ included solid Earth processes — such as the elastic rebound of the bedrock under the ice, and the impact of changes in sea level very close to the ice sheet”. And they “ estimated that projections for the next 100 years are within about 1% of previous projections for that same time period. ” So statistically no significant change then. Whew!

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Five years ago in Anthropocene Review, I wrote “map stories provide dynamic visualisations and broaden factually based public understanding”. Posting a couple of years ago on flood data interpretation further showed how complex an area this is, as is anything around climate change, as said recently on Medium here. All of these are, however, based on relatively simple maps that can be derived by you, concerned citizens, to help you sort fact from fiction. And the story map above shows a reasonable 2 m. sea level rise immediately north of Cambridge UK, with CoastalDEM atop and OS Open Data at bottom.

Following ye olde “think globally, act locally”, in addition to contributing maps above, I’m blending such concerns in my Cottenham Open village outreach project with Terry Jackson, and will cooperate with local initiatives such as Cambridgeshire Climate Emergency. Do seek out your local initiatives to address sustainability, community engagement, simple living etc.

Update: this is further developed in my blog on Cottenham Open, as coastal inundation, temperature anomalies and river vs. sea inundation play a big part of community engagement — to inform the public with open and authoritative data — as well as a story map on East Anglia Fenlands coastal inundation. A wrap-up blog post estimates the affected areas from Wikimedia settlement data.

Update 2: here is the definitive answer to any Climate Denier:

Climate models are often attacked, but most of the time they're remarkably good

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Andrew Zolnai

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