Arguments
How much is sea level rising?
What the science says...
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Basic
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A variety of different measurements find steadily rising sea levels over the past century. |
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Climate Myth...
Sea level rise is exaggerated
"We are told sea level is rising and will soon swamp all of our cities. Everybody knows that the Pacific island of Tuvalu is sinking. ...
Around 1990 it became obvious the local tide-gauge did not agree - there was no evidence of 'sinking.' So scientists at Flinders University, Adelaide, set up new, modern, tide-gauges in 12 Pacific islands.
Recently, the whole project was abandoned as there was no sign of a change in sea level at any of the 12 islands for the past 16 years." Vincent Gray).
At a glance
You'd think it would be obvious, wouldn't you? If ice (or snow) melts, you get water. Water flows downhill through gravity and collects wherever it can be retained. In areas that see regular winter snowfalls, the processes involved are familiar. Snow-capped mountains look photogenic but along comes the eventual thaw and the river levels rise sharply with all the meltwater.
Now apply the same basic principles to glaciers and ice-caps. It should not come as a surprise that exactly the same thing happens and where that meltwater collects is ultimately the oceans. Note here that we're talking about land-based ice, not sea-ice: sea-ice is already part of the ocean so does not affect sea levels as it forms and melts every year. But melt enough land-ice and you get very significant change indeed.
What do we mean by very significant? Well, let's look at the transition out of the last ice-age that dominated the last 20,000 years. It began with ice-caps over parts of Europe and North America and ended not so long ago with much of that ice gone but with sea levels having risen by more than 120 metres. If that's not significant, what is?
There's not enough ice left on Earth to raise sea levels by that whopping amount now, but there is enough to raise the oceans by more than 60 metres. Over what sort of time-frame? Well, we know that the current rate of sea level rise is some 3.7 mm a year, or nearly an inch and a half per decade. A lot of that is due to the expansion of the oceans - as things are warmed up they expand. But the rate is accelerating. How fast do we think it can get?
We do have the past to consider: during the glacial meltdown of the past 20,000 years, there was a period ominously named Meltwater Pulse 1A that began some 14,700 years ago. During this enhanced period of melting, sea levels rose by between 16 and 25 metres in about 400–500 years. That's roughly 40–60 mm per year or 16-23 inches a decade.
Could such drastic rates of sea level rise happen again? Probably not but nevertheless it shows what is possible as ice-sheets collapse in a warming world. But even if sea level rise stays at its current rate (it won't), that's getting on for a two-metre increase over the coming 300 years and a one-half to one-metre increase over the next 100 years. Now go anywhere affected by tides and think about all the communities of people that live and work along the shore. Pick the biggest spring tides, take a look at where they reach at high water, maybe watch the waves and surge when a storm occurs, then imagine an extra two metres of water on top of that.
And try to imagine being the decision-makers in the coming decades and centuries, who will have to work out what best to do. What would you think of the people all those years ago, who went around pretending this was not happening? Not favourably, for sure - because of such behaviour, that is how history will remember them.
Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!
Further details
The climate myth set out in the coloured box above gives an insight into the minds of climate change deniers. Why? Because it's entirely made-up. It annoyed the Realclimate blog's Gavin Schmidt sufficiently for him to write an eloquent debunking in 2012 that is well worth reading because it demonstrates so clearly what we, the scientific community, are up against.
The claim that tide gauges on islands in the Pacific Ocean show no sea level rise is nonsense: the data presented in the Realclimate link above show a variably rising sea level trend at each station. But what about global sea level rise?
Sea level rises as ice on land melts and as warming ocean waters expand. As well as being an existential threat to coastal habitation and environments (think about many of the world's capital cities here), sea level rise corroborates other evidence of global warming
The black line in the graph below (fig. 1) clearly shows sea level is rising; its upward curve shows how sea level is rising faster as time goes on. The upward curve agrees with global temperature trends and with the accelerating melting of ice in Greenland and other places.
Because sea level behaviour is such an important signal for tracking climate change, the misinformers seized on the sea level record in an effort to cast doubt on this evidence. As fig. 1 clearly demonstrates, sea level bounces up and down slightly from year to year so it's possible to cherry-pick data and falsely suggest the overall trend is flat, falling or linear. You can try this yourself. Starting with two closely spaced data points on the graph below, lay a straight-edge between them and notice how for a short period of time you can create almost any slope you prefer, simply by being selective about what data points you use. Now choose data points farther apart. Notice that as your selected data points cover more time, the more your mini-graph reflects the big picture. The lesson? Always look at all the data rather than being fooled by selective presentations.
Fig. 1: sea level change, from IPCC AR6 WGI Chapter 2 section Climate Change 2021: The Physical Science Basis. Tide-gauge and, more latterly, altimeter-based estimates since 1850. The full image with all four panels and IPCC caption is available here.
Other denialist arguments about sea level concern the validity of observations, obtained via tide gauges and more recently satellite altimeter observations.
Tide gauges must take into account changes in the height of land itself caused by local geological processes, a favourite distraction for deniers to highlight. It will come as no surprise to learn that scientists measuring sea level with tide gauges are aware of - and compensate for - these factors. Confounding influences are accounted for in measurements and while they leave some noise in the record they cannot account for the observed long-term upward trend.
Various technical criticisms are mounted against satellite altimeter measurements by deniers. Indeed, deriving millimetre-level accuracy from orbit is a stunning technical feat so it's not hard to understand why some people find such an accomplishment unbelievable. It's astonishing that in another breath they are happy to jump aboard an airliner, parts of which are engineered to a similar tolerance!
In reality, researchers demonstrate this height measurement technique's accuracy to be within 1 mm/year. Most importantly there is no form of residual error that could falsely produce the upward trend in observations.
As can be seen in an inset of the graph in fig. 1, tide gauge and satellite altimeter measurements track each other with remarkable similarity. These two independent systems mutually support the observed trend in sea level. If an argument depends on skipping certain observations or emphasises uncertainty while ignoring an obvious trend, that's a clue you're being steered as opposed to informed. Don't be misled by only a carefully-selected portion of the available evidence being disclosed. Look at it all.
Current sea level rise is not exaggerated, in fact the opposite case is more plausible. For one, sea level rise is not the same everywhere. Many areas around the world already experience much faster rates of sea level rise than the average global rate shown in Fig 1. As well, observational data and changing conditions in such places as Greenland suggest if there's a real problem here it's underestimation of future sea level rise. Past IPCC synthesis reports offered rather conservative projections of sea level increase based on assumptions about future behaviour of ice sheets and glaciers, leading to estimates of sea level roughly following a linear upward trend mimicking that of recent decades. In point of fact, observed sea level rise is already above older IPCC projections - and accelerating - while at the same time it appears the mass balance of continental ice once envisioned by the IPCC was overly optimistic (Rahmstorf 2010; Otosaka et al. 2023).
Fast-forward to 2023 and the recent IPCC AR6 Synthesis Report is a bit less nuanced:
Limiting global surface temperature does not prevent continued changes in climate system components that have multi-decadal or longer timescales of response (high confidence). Sea level rise is unavoidable for centuries to millennia due to continuing deep ocean warming and ice sheet melt, and sea levels will remain elevated for thousands of years (high confidence). However, deep, rapid and sustained GHG emissions reductions would limit further sea level rise acceleration and projected long-term sea level rise commitment. Relative to 1995–2014, the likely global mean sea level rise under the SSP1-1.9 GHG emissions scenario is 0.15–0.23 m by 2050 and 0.28–0.55 m by 2100; while for the SSP5-8.5 GHG emissions scenario it is 0.20–0.29 m by 2050 and 0.63–1.01 m by 2100 (medium confidence).
The report goes on to state, however:
The probability of low-likelihood outcomes associated with potentially very large impacts increases with higher global warming levels (high confidence). Due to deep uncertainty linked to ice-sheet processes, global mean sea level rise above the likely range – approaching 2 m by 2100 and in excess of 15 m by 2300 under the very high GHG emissions scenario (SSP5-8.5) (low confidence) – cannot be excluded.
If they cannot exclude such risks - and they know what they are talking about - can you?
Last updated on 20 August 2023 by John Mason. View Archives
[JH] Recommended supplemental reading:
Does NASA Data Show That Global Warming Isn’t Causing a Sea Level Rise? by Alex Kasprak, Snopes, Aug 1, 2017
I guess Manhattan must be elevating to actually show a decrease in sea level since 2010. And certainly no acceleration in sea level rise for more than 150 years.
https://tidesandcurrents.noaa.gov/sltrends/sltrends_station.shtml?stnid=8518750
[PS] Precisely what point are you trying to make here from the basis of a single tide station? The relevance to the article is not clear.
George @277 , the tide gauge records collated by NOAA, for The Battery, on Manhattan, show a 400 mm rise in sea level during the past 160 years. (Strangely, the record shows a gap from about 1879 - 1893.) As you say, the reference no. is 8518750.
Mean sea level rise approx 2.8 mm per year, over that 160 years.
Possibly Manhattan Island is developing a strong tilt in recent times, with the northern end of the island lifiting high? I must look more closely at the Empire State Building, to see if it appears out of plumb. My previous impressions were that it was nowhere like as bad as that skyscraper in Pisa (fortunately).
George,
While there is no rise since 2010, there is a large rise since 2008. The long term rise is clear. Do you have a reason for selecting 2010 or was it just to cherry pick your claim?
Have you ever seen the escalator?
Do you think you could make a similar graph from the tide guage in New York?
Here are a couple more around the U.S.
https://tidesandcurrents.noaa.gov/sltrends/sltrends_station.shtml?stnid=9413450
https://tidesandcurrents.noaa.gov/sltrends/sltrends_station.shtml?stnid=8449130
There is certainly no acceleration in sea level rise and in fact a drop in sea level since 2010.
My belief is that there has been additional snow and ice buildup causing the drop in sea level.
"Cherry Pick since 2010"? Eight years of opposing data is not enough to make you question your hypothesis?
[DB] Multiple comments using the same failed rhetoric are unhelpful. The facts are, global sea levels have risen since 2010. Attempting to find a few areas where sea levels show different results is like using the existence of smaller waves at the oceanside following larger ones in an attempt to show that the tide has somehow stopped coming in. It's a fallacy.
We also know which of the components of SLR contribute to it most (mass increases vs thermal expansion):
And, from Hsu and Velicogna 2017, which areas will see more resultant SLR than others:
No amount of effusive prestidigitation (handwaving) can make that go away:
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George,
Your first link (I cannot get the graph to copy here) is from Monterey Bay. While it has a lower long range trend, the trend from 2010-present is very high, the opposite of your claim. Your second graph, from Nantucket, shows a high long range trend but lower from 2010-present. You must konw that individual locations have high noise and also land height change issues. If that is the best you can find in the entire USA that indicates that sea level is rising rapidly.
If you get some guages from Alaska they will actually show a long term decrease in sea level since there is strong isostatic lift (increase in land height) due to the melting of the glaciers from the ice age. That does not mean the sea level is not rising, just that the land is rising faster.
https://www.youtube.com/watch?v=EhW-B2udhQw
7:00 no sea level rise in La Jolla, CA
12:00 Topex/Poseidon Satellite measurements are crude
12:43 ENVISAT, European satellite with higher resolution, much lower sea level rise, decrease since 2010
How do you explain Manhattan? Has the land elevated there?
https://tidesandcurrents.noaa.gov/sltrends/sltrends_station.shtml?stnid=8518750
[Rob P] - Not surprisingly, sea level at New York has risen over the long-term. In the short-term, fluctuations in the ocean circulation are having an effect. Now that ice mass loss from the Greenland ice sheet is fully underway it will be interesting to see how that alters sea level rise along the North American east coast. There's a recent paper on the subject here: Causes of accelerating sea level on the East Coast of North America (Davis & Vinogradova [2017])
George,
Manhattan is a very noisy record. There are many short periods where it appears to not rise while the long term (actual) trend is increasing. Since land elevation is a long term issue that would primarily afect the long term trend and not the noise. Tamino has a lot of sea level analysis where he explains the statistics.
The noise is affected by the weather and by issues like El Nino/La Nina. Longer term processes like the Arctic ocellation would also affect the noise.
George,
I noticed that you did not link the graph for La Jolla. Here it is. Long term trend 2.17 mm/yr, not no sea level rise. Read some of Tamino's stuf and you will be better able to determine if there is a trend from a graph.
FFGeorge,
The ENVISAT satalite has been shut down since April 2012. How can you possibly suggest that it has more accurate data for the period 2010-2017 when it has not collected data for most of that time? The video you linked is unreliable.
You are choosing a time period that is too short to measure the sea level rise. You are focusing on the noise and not the signal.
Find a reliable source of information. Read Tamino.
I started at about post #7 with sea level rising at 2.46 mm per year. Eight years later, that may be 3 mm per year (3.46 +/- 0.4) or so ... so about 0.1 in per year ... less than a foot by 2100.
California has accepted a new building code, expecting 10 feet by 2100.
Does this make any sense? Should we all be investing in Dutch dike companies?
[DB] And yet, per Nerem et al 2018:
Chez Provence: You wrote:
California has accepted a new building code, expecting 10 feet by 2100.
Please document and link to the source of this claim.
w6g-9-2018-exhibits.pdf
prepare-for-10-feet-of-sea-level-rise-california-commission-tells-coastal-cities/
Thanks for the Nerem reference. This may be a local politics thing ... In the draft report of the California Coastal Commission, they acknowledged the 800mm rise shown by, Nerem ... added their own sense, suggesting that the most likely will be 1.2 m, worst case 2 m ... and proposed regulation for 3 m. Starting point is fig 4 on page 46 of the w6g reference above.
ChezProvence: Thank you for providing the documentation per my request.
The headline of the Scientiic American article:
Prepare for 10 Feet of Sea Level Rise, California Commission Tells Coastal Cities
The sub-headline of the article qualifies the headline:
Though an extreme scenario, it should be factored in to coastal infrastructure planning, new guidance suggests
That is quite different than your statement:
California has accepted a new building code, expecting 10 feet by 2100.
ChezProvence: Furthermore, the head-line and sub-headline of the SA article do not comport with the facts set forth in the first three paragraphs of the article:
As Yogi Berra was wont to say, "It ain't over until the fat lady sings!"
I encounter some extreme fringe RWNJ deniers on various investor sites. They literally don't care if the West Coast or Northeast have issues with SLR "because Democrats live there" (no, I'm not kidding). So I would like to put it in terms that they might care about. At approximately what year does SLR become "a problem" for Florida (that big swing state needed to win elections)? The Houston Ship Channel (which would be supremely ironic)? Has there been any science done at that kind of level?
I think Miami is already struggling with sealevel rise. Cant help with Houston Ship Channel sorry.
AFT: Try https://sealevelrise.org/states/florida
The deniers are saying the satellite equipment that can only measure distances in centimeters cannot produce sea level measurements in millimeters. Can someone address this or provide a link where this is addressed. I can’t find anything on this. Thanks.
Hank: Sea level is a population parameter that is estimated by combining multiple sample measurements of it. The Law of Large Numbers explains that the precision of the combined estimate increases with the number of sample measurements. Look up Law of Large Numbers in a textbook or in Wikipedia, then prove it yourself using a spreadsheet. There is a tutorial on satellite measurement of sea level linked right above the image at the bottom of This NOAA page, though currently it does not work, probably due to the government shutdown.
Thanks Tom. As an engineer I’m familiar with the Law of Large Numbers and thought that might be at least part of the explanation. I look forward to reading the tutorial if the government ever opens.
[DB] The Internet Archive has a backup of the tutorial page, here.
Just what I was looking for. Thank you.
Do an Earth mean sea level has any real meaning ?
https://www.psmsl.org/products/trends/
[DB] As a professional nautical cartographer, I can assure you that mean sea level has a concrete meaning. If you have an actual question, please be more specific.
Reduced image width (keep image widths below 500 to avoid breaking page formatting).
Sealevel rise is not even across the globe.
Source:
Note that sealevel measured from satellite altimetry is based on height change with respect to the reference ellipsoid, avoiding the issue of subsidence or tectonics which plague tide guages (though both measurements systems yield comparable results).
Sealevels are falling close to melting ice thanks to reduced gravitional attraction and isostatic rebound. See here for more detailed explanation.