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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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How much is sea level rising?

What the science says...

Select a level... Basic Intermediate

A variety of different measurements find steadily rising sea levels over the past century.

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.

AR6 WGI Chapter 2 Figure 2_28c

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

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Further viewing

From Peter Hadfield (potholer54 on YouTube) published on Dec 5, 2021

Compare two photos 130 years apart and it looks as though sea levels haven't moved. So why all the fuss about rising sea levels and evacuating islands? This video closes the yawning gap between internet myths and science.


 

From Peter Sinclair (greenman3610 on YouTube) published on Sep 24, 2009

Denial101x lecture

Comments

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Comments 176 to 200 out of 322:

  1. Steve I'm not sure I understand your concern. How do you uncouple ocean volume from sealevel rise at time scales of a few decades, Steve? And why would we be interested in the component of sea level rise that is not due to eustatic changes in ocean volume? I would think the component of sea-level change that is most relevant to understanding linkages to climate are those realted to ocean volume.
  2. #176 Stephen Baines Where do you want to measure sea level from? The shore line? Or the center of the Earth? If your answer is either one of those, adding in the 0.3 mm/yr GIA correction will give an erroneous answer. Besides all that, if the scientists are really interested in how much ocean water there is and run a time line to represent that, then plotting mass instead of volume would be the most accurate way to go. They're not doing that.
    Response:

    [DB] "if the scientists are really interested in how much ocean water there is"

    You tread dangerous ground here.  If you are going down the path you are implying (fraud/conspiracy) then take this whole conversation to a more appropriate venue (many dozens exist).  This forum is about the science of climate change.  No room exists for implications such as this.  FYI.

  3. I've been drawn off the topic of my original post which was to point out that over time, the estimates of the rate at which sea level rises have changed over time. I don't know what the justification for some of those changes are. In particular, the 2004 - 2008 time line. A plot of the current data that CU provides with their estimate for the period 2004 - 2008 doesn't show an increase in rate for that period.
    Response:

    [DB] Steve, we have all been through this before.  You focus on insignificant timescales (while ignoring the contextual greater picture) and fixate on minutiae without a good grounding and background.  Perhaps more time researching, reading and studying would bear more useful results than the current approach.

  4. [DB] You know perfectly well what my point is, but I can't post it on this forum because it will be deleted. But I can put up data and graphs and I can state that I don't see any justification for changes that I see. If you or any other posters here know what that justification is and how it relates to the data and graphs, then by all means put it up.
    Response:

    [DB] Then we are at impasse.  Whether due to a lack of background in climate science and statistics (I give you more credit than that) or an ideology discordant with the science, you continually prosecute an agenda of conspiracy and fraud rather than discussing the science itself.

    I reiterate:  if you wish to have an open dialogue about the actual science of climate change, then you are welcome to participate here.  Your present course is unacceptable. 

    Your choice on how this plays out.

  5. Yes, we are at an impasse, I would think that you or some one would be curious to find out what the reasons for those increases from 2.8 mm/yr to 3.3 mm/yr over that period of time are when graphically it looks like it didn't happen.
  6. [DB] I missed your comment on measuring mass instead of volume as the best way to determine how much water is in the ocean. Measuring how much water there is is more a function of mass than volume since temperature affects volume. I stated the obvious, and you tell me I'm on dangerous ground. ????????
    Response:

    [DB] "I stated the obvious, and you tell me I'm on dangerous ground."

    Re-read my response to you above.  If you are again implying that fraud & conspiracy are obvious then we are done.

  7. Steve, I'm not interested. If I was, I'd doublecheck my own methodology and then carefully read the CU pages on processing and steric effects. Then I'd go through their bib carefully. If I still didn't find an answer to my question, I'd ask CU. What you're effectively doing here is backing into an argument for the difficulty of measuring sea level rise, but I can't figure out what you want to do with this argument. Scientists who study sea level rise know that it's difficult. Are you attempting to use sea level rise as a proxy for measuring the energy in the Earth system? If so, why not use something more direct and less subject to gravitational shift, currents, cycling, sedimentation, local uplift/subsiding, crustal movement, incomplete historical coverage, and instrumental error? Or do you think you've found the Golden Hoax?
  8. #181: But volume is the measure that is relevant to sea level rise, not mass. Concentrating on very short time periods (such as 5 years) will very often lead you to the wrong conclusions.
  9. Steve Case, How did you come up with the trend line in your graph? Why is it above the actual from 2006 to mid year 2007, and then below the actual from mid year 2007 on? You stop at 2008. Here is a current, full data set: Here is that same image/data, with your "trend line" overlaid where it belongs. Can you see the problem here? And can you see the problem with using too short of a dataset to try to infer a trend? Or with paying too much attention to the tail in what is clearly very noisy data?
    Response:

    [DB] In addition, per Willis and Leuiette (2011):

    "Because of both uncertainties in the observational systems and interannual variations, it has been estimated that a minimum of 10 years is necessary to meaningfully interpret global trends in sea level rise and its components (Nerem et al., 1999)."

    "Despite efforts to maintain them, there are still limitations to the current observing systems. Coverage of the ice-covered and marginal seas is not possible with the current generation of Argo floats, and there is no systematic network for measuring steric changes in the deep ocean. Challenges also remain for altimeter measurements poleward of the 66° turning latitude of the reference missions and in regions covered by sea ice."

    Also from Hamlington et al. (2011, J.Climate):

    "However, from the 12-yr and full time series, we can see that the SNR patterns begin to converge, and an increase of areas with SNR greater than one occurs from 12 to 16 years."

    [Emphasis added to each.]

    Santer et al, DelSole et al and Hamilton et al. (and probably others) all point toward a minimum sample period of 16 years or so increase the SNR.  Anything less than 10 years is meaningless, a chasing after the wind.

    H/T to the mighty Albatross...

  10. # 184 Sphaerica at 03:54 AM on 19 September, 2011 Nice job on the graphic overlay, I was about to do that one myself. And what does it show? It shows that over that short time, and yes it is short, that sea level did not increase in rate. Yet over that very same short time CU did report in it's estimates, that it periodically updates and posts, that the rate did increase. By 0.5 mm/yr I might add. By the way, CU made a +0.1 mm/yr adjustment between 2011 Release #1 and Release #2 and they explained it: ..the rate increased slightly from 3.1 to 3.2 mm/yr due to the improvements to the TOPEX SSB model and replacement of the classical IB correction with the improved DAC correction... For the 2004 - 2008 time line there would have to have been a whole series of corrections like that. Much like the GIA 0.3 mm/yr correction they would have to have been applied to the entire time series. So let's add it up. The 2004 - 2008 time frame doesn't show a rate increase, but the reported rate increased by 0.5m/yr. The May 2011 GIA correction is 0.3 mm/yr and the most recent correction is 0.1 mm/yr. That adds up to a 0.9 mm/yr increase due to corrections and improvements in methodology of one kind or another since 2004. Do you agree with that assessment? What I'm doing is painting a picture of what it is that needs to be explained to those of us in this world that are skeptical of what we are told about climate change.
    Response:

    [DB] "What I'm doing is painting a picture of what it is that needs to be explained to those of us in this world that are skeptical of what we are told about climate change."

    Then paint that picture to those at UCAR.  I'm very certain that they will be able to explain things to "skeptics".

  11. Steve, I don't have time at the moment, but have you addressed your concerns to the guys and gals at the University of Colorado? If not, why not?
  12. Rob No, It isn't that I haven't ever e-mailed big names in the "Global Warming" debate, but I don't do it to argue or open some sort of a dialog with them, although it did happen once. I figure that a website like Skeptical Science ought to be able to make a good argument regarding the questions I bring up.
  13. 185, Steve Case,
    What I'm doing is painting a picture...
    Yes, exactly. You're using fingerpaints when what you need to understand science. You've already been caught in a ridiculous misrepresentation of the data, and it has been explained to you that you cannot compute a sea level trend with just a few years of data.
    So let's add it up. The 2004 - 2008...
    And yet, there you go again.
    Do you agree with that assessment?
    No, because your time period is far, far to short to be arguing about.
  14. # 188 Sphærica Yes the time period is short and Colorado U adjusted the rate of sea level rise at least three times over that stretch.
  15. 189, Steve Case, So what? What's your point? That scientists make adjustments as data and techniques improve? Did CU adjust their rate using a short range, or did they adjust their rate based on new data that extended a reasonably long range for even longer?
    Response:

    [DB] "What's your point?"

    The point Steve has been trying to make is one of conspiracy and fraud on the part of UCAR.  However, he is reduced to making them via veiled insinuations, because he knows any more open accusations won't survive moderation.

    It has become very tiresome.

  16. Was my post deleted or did I just forget to press submit?
    Response:

    [DB] The moderator deleted your comment because:

    1. You have continued to focus on inconsequentially short time periods despite repeated advice to the contrary.
    2. You have still not indicated what your point in posting on inconsequentially short time periods is.
  17. Since the 2004 earthquake, the Earth's rotation has increased. So, is there a slight increase in the bulge at the equator? If so, the sea level will go down (except along the equatorial seas) until this effect is overtaken by ocean expansion and other factors. Or do the satellite measurements null this effect out?
  18. Henry Justice, satellite measurements are just that--measurements. They measure what is there, so they cannot "null this effect out." Do they measure with sufficient spatial resolution to detect the differences in sea level between the equator and other regions? Yes. When someone wants a single statistic that summarizes the sea level across all regions of the Earth, that statistic (e.g., a mean--an average) necessarily will collapse across the equator versus other regions.
  19. Henry J#192: "So, is there a slight increase in the bulge at the equator?" No. From NASA 2005: They found Earth's oblateness (flattening on the top and bulging at the equator) decreased by a small amount. It decreased about one part in 10 billion, continuing the trend of earthquakes making Earth less oblate. The article details that there is a long term trend and thus cannot be responsible for recently observed change.
  20. Here is an updated TOPEX and Jason data time series from CU. Hope it posts OK as I have not tried here before. Seems completely consistent with previous results from CU. It seems that CU AVISO, CSIRO and NOAA all have converged on a 3.1 mm/yr rate. Source
    Response:

    [DB] Fixed image. Html posting tips are here.

    As a tip for images, place your URL address desired between both sets of "" in the string below:

    <a href="">< img width="450" src=""></a>

  21. Sorry clearly an amateur at this here are the unembellished links.... http://sealevel.colorado.edu/files/2012_rel4/sl_ns_global.png http://www.aviso.oceanobs.com/fileadmin/images/news/indic/msl/MSL_Serie_MERGED_Global_IB_RWT_GIA_Adjust.png http://www.cmar.csiro.au/sealevel/images/alt_gmsl_seas_rem.jpg The NOAA trend seems to be a bit lower @2.8mm/yr http://ibis.grdl.noaa.gov/SAT/SeaLevelRise/slr/slr_sla_gbl_free_txj1j2_90_500.png
  22. If sea level was falling, Earth would be in an ice age and that wouldn't be nice for any of us.

  23. Earthling, does that fact somehow make rapid sea level rise somehow benign?

  24. DSL,

    What Earthling is saying is that because starvation is bad for you, obesity must be good. Or because drought is bad, flooding must be good. This seems like the appropriate place for that argument.

    If he's worried about ice ages, perhaps he should ponder the selfishness of squandering what could have been a useful geoengineering resource to future generations faced with an imminent glaciation; carefully burning fossil fuels to enhance the greenhouse effect just enough to maintain temperatures in the face of declining northern-hemisphere insolation due to the Milankovich cycles may well be the most cost-effective method for them to do so, if those resources are still around at the time.

  25. Gads - Joe D'Aleo is on a The Weather Channel comment stream trying to convince people that Morner is a "the top world expert" in sea level.  I quote:

    "Also see http://www.jcronline.org/doi/pdf/10.2112/JCOASTRES-D-10-00157.1 where after studying tide gauges (instead of relying on failing models) they conclude: Our analyses do not indicate acceleration in sea level in U.S. tide gauge records during the 20th century. Instead, for each time period we consider, the records show small decelerations that are consistent with a number of earlier studies of worldwide-gauge records. The decelerations that we obtain are opposite in sign and one to two orders of magnitude less than the+0.07 to+0.28 mm/y2 accelerations that are required toreach sea levels predicted for 2100 by Vermeer and Rahmsdorf (2009), Jevrejeva, Moore, and Grinsted (2010), and Grinsted, Moore, and Jevrejeva (2010)." 

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