Sea level rise 101

Based on the NC legislature’s decree about the science of sea level rise projections and some of the related propaganda we have seen from climate change deniers, I get the sense there is a lot of confusion about sea level rise. So here is a primer on what we know about sea level and climate change.

1) Sea level fluctuates naturally by 10s to 100s of meters but has been relatively stable for the last few thousand years. Changes in sea level are controlled in large part by atmospheric temperature, which in turn controls how much water is locked up on the continents as ice. Cool temperatures = lots of ice and low sea level. When the earth’s temperature increases (by just a few degrees C), water from the melting glaciers drains into the oceans and increases sea level – sometimes very rapidly and by tens of meters.

The graph below depicts global change in sea level since the end of the last “ice age” or “glacial period“. During this 15,000 year period, sea level has increased by a whopping ~125 meters or ~400 feet! You can also see some periods when the earth was transitioning to an interglacial period when sea level rose very rapidly, such as during “Meltwater Pulse 1A” when it appears to increase at least one to two feet per decade. These rapid increases in sea level are – as you might have guessed – caused by the rapid melting of glaciers. So the link is real and far from subtle; the earth warms slightly, the glaciers melt, and sea level rises.

Figure 1. This figure shows sea level rise since the end of the last glacial episode based on data from Fleming et al. 1998, Fleming 2000, & Milne et al. 2005. (Source and details)

During natural glacial cycles global temperature trends are influenced by a number of factors including slight changes in the earth’s obit around the sun and atmospheric composition. However, this time the earth’s warming isn’t due to an increase in heat coming in from the sun, but instead because greenhouse gases are letting less of that heat back out, i.e., the greenhouse effect.

2) Greenhouse gas emissions are causing sea level to rise (Fig. 2) via “thermal expansion” (warming a liquid increases it’s volume) and by melting mountain glaciers. Until human activities increased the concentration of greenhouse gases in the earth’s atmosphere, global sea level had been relatively stable for several thousand years (which can be seen in Fig. 1 above and with greater detail here).

Figure 2. Change in annually averaged sea level at 23 geologically stable tide gauge sites with long-term records as selected by Douglas (1997). The thick dark line is a three-year moving average of the instrumental records. This data indicates a sea level rise of ~18.5 cm from 1900-2000.  (Source and details).

3) The rate of sea level rise appears to be accelerating, i.e., non-linear. The indented text in this section below is from Skeptical Science.

The blue line in Fig. 3 below clearly shows sea level as rising, while the upward curve suggests 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.

Global mean sea level (e.g., the global average height of the ocean) has typically been calculated from tidal gauges. Tide gauges measure the height of the sea surface relative to coastal benchmarks. The problem with this is the height of the land is not always constant. Tectonic movements can alter it, as well as Glacial Isostatic Adjustment. This is where land which was formerly pressed down by massive ice sheets, rebounds now that the ice sheets are gone.

To construct a global historical record of sea levels, tide gauge records are taken from locations away from plate boundaries and subject to little isostatic rebound. This has been done in A 20th century acceleration in global sea-level rise (Church 2006) which reconstructs global sea level rise from tide gauges across the globe. An updated version of the sea level plot is displayed in Figure 3:

Figure 3: Global mean sea level from 1870 to 2006 with one standard deviation error estimates (Church 2008).

Tidal estimates from sediment cores go even further back to the 1300’s. They find sea level rise is close to zero in the early part of the sedimentary record. They then observe an acceleration in sea-level rise during the 19th and early 20th century. Over the period where the two datasets overlap, there is good agreement between sedimentary records and tidal gauge data (Donnelly 2004Gehrels 2006).

What we’re most interested in is the long term trends. Figure 4 shows 20 year trends from the tidal data. From 1880 to the early 1900’s, sea level was rising at around 1mm per year. Throughout most of the 20th century, sea levels have been rising at around 2mm per year. In the latter 20th century, it’s reached 3mm per year. The five most recent 20-year trends also happen to be the highest values.

Figure 4: The linear trends in sea level over 20-year periods, with one sigma error on the trend estimates shown by the dotted lines. From 1963 to 1991, there were a series of volcanic eruptions which caused cooling and hence contraction of the upper ocean. This temporarily slowed the rate of sea level rise.

In summary, the historical record suggests that the rate of sea level rise appears to have increased since the late 19th century, however, we don’t know whether this acceleration will continue, i.e., whether this non-linearity is a short or long-term trend. My money is on the latter given the current work in Greenland and Antarctica indicating a frightening degree of melting.

4) There is a lot of variation in the rate of sea level rise. This is a point frequently missed or ignored by all kinds of people (including some scientists) talking about sea level rise.  Take a look at the figure below (from NASA). Changes in sea level clearly vary from place to place. In some parts of the western Pacific ocean, sea level rise has been nearly 1cm per year. In others, sea level is falling. This variability is caused by several factors and nature cycles. This variability is not surprising and does not in any way challenge the fact that globally, average sea level is rising. It just means that the impacts of sea level rise, like very other aspect of global climate change, are variable. For some people they are trivial, to others they are catastrophic.

5) How much sea level rise should we expect this century? (some of the text in this section is from Skeptical Science)

We have a pretty good sense of how much sea level rise to expect from the two mechanisms mentioned above (thermal expansion and the melting of mountain glaciers) but a third important mechanism is a lot harder to deal with: the melting and calving of ice from the massive glaciers covering Greenland and Antarctica. Recent work suggesting that both processes may be accelerating.

Calving is accelerated by warming (in some really interesting ways) but the processes are not well understood. Therefore, scientists cannot predict what will happen to the massive amounts of water locked up in the glaciers on Greenland and Antarctica (although we suspect that calving will accelerate). Therefore, IPCC did not include the effects of what it calls “dynamical processes” like this, arguing they couldn’t be modeled: “Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude.”

In 2001, the IPCC Third Assessment Report (TAR) projected a sea level rise of 20 to 70 cm by 2100. In 2007, the IPCC Fourth Assessment Report (AR4) gave similar results, projecting sea level rise of 18 to 59 cm by 2100. How do the IPCC predictions compare to observations made since these two reports?

Figure 6: Sea level change. Tide gauge data are indicated in red and satellite data in blue. The grey band shows the projections of the IPCC Third Assessment report (Allison et al 2009).

Observed sea level rise is tracking at the upper range of model predictions (just as many scientists argued they would).

Why do climate models underestimate sea level rise? The main reason for the discrepancy is, no surprise, the rapid (likely accelerating) loss of ice from Greenland and Antarctica, i.e., the “dynamical processes” that are not considered in the models. Even East Antarctica, previously considered stable and too cold to melt, is now losing mass.

An alternative way to predict future sea level rise is from the known relationship between sea level and global temperature from the recent and distant past (Vermeer 2009). For example, Fig. 7 shows the strong correlation between observed sea level (red line) and reconstructed sea level (dark blue line with light blue uncertainty range) from 1880 to 2000.

Figure 7: Observed rate of sea-level rise (red) compared with reconstructed sea level calculated from global temperature (dark blue with light blue uncertainty range). Grey line is reconstructed sea level from an earlier, simpler relationship between sea level and temperature (Vermeer 2009).

Using this relationship, the amount of global sea level rise can be predicted based on different IPCC “emissions scenarios”, which make different assumptions about the future global economy and how much carbon dioxide it will emit (and thus how much the earth will warm).

The range of projected sea level rise by 2100 is 75 to 190 cm. As you get closer to 2100, the contribution from ice melt grows relative to thermal expansion. This is the main difference to the IPCC predictions which assume the portion of ice melt would diminish while thermal expansion contributes most of the sea level rise over the 21st Century.

Figure 8: Projection of sea-level rise from 1990 to 2100, based on IPCC temperature projections for three different emission scenarios. The sea-level range projected in the IPCC AR4 for these scenarios are shown for comparison in the bars on the bottom right. Also shown in red is observed sea-level (Vermeer and Rahmstorf 2009).     

Fig. 8 shows projected sea level rise for three different emission scenarios. These predictions for sea level rise are roughly 3 times greater than the IPCC predictions. Note the IPCC predictions are shown as vertical bars in the bottom right. For the lowest emission rate (B1), sea levels are expected to rise around 1 metre by 2100. For the higher emission scenario, which is where we’re currently tracking, sea level rise by 2100 is around 1.4 m. Grim.






17 responses to “Sea level rise 101”

  1. Wonderful resource, John!

  2. Nicely done. My only quibble is with figure 1 – the ongoing melt up to present day, as suggested by Fleming, makes the formation of coral atolls nigh impossible. The sea level highstand in the equatorial basins thousands of years ago (which formed the atolls) is only possible when global sea level rise grinds to a virtual standstill once the vast continental ice sheets have melted away. The subsidence of the ocean floor adjacent to the continents slowly drains water away from the equatorial basins over time, lowering sea level and subsequently exposes the reef formations above current high tide. This would not happen under Fleming’s modeling.

    1. Thanks Rob! (especially if you wrote some of the text I lifted from SkS). Could you point me to a holocene SLR graph you are more comfortable with?

  3. Bill Price

    The NC CRC’s  Science Panel & scientists:
    – Used the Least Reliable Tide Gauge Data in NC.
    – Used Obsolete Reports
    – Used only One-sided Sea Level Rise  reports
    – Used only  One-sided Global Warming reports,
    – Admitted they did no science, only a “Literature Search”.
    – No 4 miles inundation of NC Tidelands over 150 years is visible .
    –  Ignored  US Coast Survey , and US Fish Services Tide Gauge Data 1850 1950.
    – Said, ‘What’s the Big Deal, Let’s wait 5 Years and see what happens.’ 
    – Reports confuse Erosion due to dredging, winds, waves and currents with Inundation. 
    – Have not answered  questions about the above concerns.
    Thankfully, it looks like NC Legislators want verifiable Science upon which to base multi billion dollar public policy decisions that could harm tens of thousands of local, taxpaying property owners.
    This is serious business, and it’s good NC doesn’t base decisions on a comedy skit.
    Bill Price    Pine Knoll Shores
    PS: It might be helpful to urge the Science Panel to participate in an  Open Public Forum to answer above and other questions about their science.
    So far they have declined.

  4. I just rewrote the text in section 5. – JB

  5. Hey, I did say it was only a quibble didn’t I? (LOL). I’ll see what I can track down.

  6. […] Sea level rise 101 by John Bruno: Based on the NC legislature s decree about the science of sea level rise projections and some of the related propaganda we have seen from climate change deniers, I get the sense there is a lot of confusion about sea level rise. So here is a primer on what we know about sea level and climate change… […]

  7. […] Sea level rise 101 by John Bruno: Based on the NC legislature s decree about the science of sea level rise projections and some of the related propaganda we have seen from climate change deniers, I get the sense there is a lot of confusion about sea level rise. So here is a primer on what we know about sea level and climate change… […]

  8. There are quite a few errors in this article.
    I’ve identified five of them here:

    Dave Burton

    1. Thanks for your engagement Dave, but you are wrong (Ill explain why ASAP). There are not errors in my post. Simply denying disturbing science and facts will not make them go away.

    2. Dave Burton, there are a number of errors in your blog reply:

      1. Although most of the Holocene sea level rise did result from the melt of the Laurentide & Fennoscandian icesheets, it is quite nonsensical to suggest that Greenland & Antarctica were not involved. The rebounding of these regions (crustal uplift) is evidence that ice mass was lost, and of course we would expect this to be the case based on physics – the Last Glacial Maximum was globally much cooler than present, so a great deal more ice would have grown on Greenland & Antarctica.

      However the warming of the Earth reached a peak in Holocene Climatic Optimum, and by around 4000 years ago global sea level rise had ground to a standstill. The most obvious example is that provided in my earlier comment to this blog – the Holocene sea level highstand in the equatorial regions would not exist. 1-3 metre highstands are endemic to the equatorial regions- just as the modeling predicts.

      2. Current concern of future sea level rise is not only based on physics and climate modeling, but also because of observations of past globally warm periods. For instance, in the last interglacial (The Eemian) global sea level reached a peak some 6-9 metres higher than now, even though global temperatures were thought to be only 0.5-1°c warmer than now. Crucially this implies that perhaps both the Greenland and West Antarctic ice sheet underwent partial collapse.

      3. Sea level rise in the 20th century not only accelerated, but it was anomalous within the context of the last, at least 7000 years. This is a point conveniently neglected in the blogosphere quibbling over the Medieval Period. This sea level trend consistent with the temperature proxy data from the likes of Mann etc. i.e – current global temperatures are indeed anomalous.

      4. Yes, sea level rise is not globally synchronous, nor do experts ever expect it to be. Your claim is known as a strawman fallacy. The global average, however, is a different story. Scientific experts expect it to increase as the oceans warm and further land ice melts and finds its way to the sea. Quite clearly this is indeed what is taking place. No real surprises here.

      5a. Your confusion is again evident. Both tidal gauges and satellite observations show a similar trend – rising global sea levels.

      The bottom line: The Antarctic and Greenland ice sheets have sufficient water contained within them to raise global sea levels some 65-70 metres. Current warming poses a substantial risk of destabilizing them and perhaps setting in motion a partial disintegration of either ice sheet. We could see up to a metre, maybe more, of sea level rise this century. And much more in the coming centuries. The recent acceleration of sea level contribution by the Greenland and Antarctic ice sheets shows that this risk is very real.

      Pretending that sea level rise will not rise any faster than it has in recent times may be reassuring for people unable to accept the scientific evidence, but it will not prevent sea level rise from accelerating if we humans continue to warm the planet.

      1. Rob, I did not suggest that Greenland & Antarctica were unaffected when the great ice sheets melted. But John B. implied an equivalence between those melting ice sheets and current conditions, implying that similarly dramatic changes in sea level are plausible now (they aren’t). Greenland is a slightly less implausible source for significantly increased meltwater contributions than Antarctica, but Greenland apparently didn’t cause a catastrophic increase in sea level 8-12 centuries ago, when it was much warmer than now, and the best measurements have detected only a very tiny increase in meltwater production from Greenland, sufficient to cause less than 0.1 mm/year of sea level rise.

        The point that is conveniently neglected by Climate Movement alarmists who still insist that rising CO2 levels are going to cause sea level rise to accelerate is this: sea level rise in the 20th century ceased accelerating around the time that mankind began significantly driving up CO2 levels.

        Do you understand that means the CO2 increase had nothing to do with the 20th century’s sea level rise acceleration?

        Satellite and tide gauge data do, indeed, show a similar trend: slightly decelerating sea level rise. Frankly, I don’t think the satellite data is trustworthy, but the tide gauges are, and they prove beyond any doubt that all those GHGs we’ve been emitting have caused no measurable acceleration in the rate of sea level rise.

        Rob, I think you know that you’re just being silly when you raise the specter of the East Antarctic ice sheet (where most of the world’s frozen water is) “destabilizing” and causing 65-70 meters of sea level rise. It is the coldest place on earth, and hasn’t melted in millions of years. There’s no significant “recent acceleration of sea level contribution by the Greenland and Antarctic ice sheets,” either. The last numbers I saw for Greenland were that, in the last decade, it might have contributed the equivalent of 0.07 mm/year more meltwater to the oceans than it is guessed to have averaged over the last half of the 20th century; Antarctica even less, if any at all.

        The last 3/4 century of GHG emissions haven’t caused any increase at all in the rate of sea level rise. That doesn’t mean it is absolutely certain that the next 3/4 century will also see no increase at all, but it is certain that there will be no dramatic increase. A prediction of 39″ of sea level rise by 2100 is a prediction for an average rate of sea level rise over the next 88 years that is 5.7 times the rate we’ve seen at the southern NC coast over the last 77 years. It is completely ridiculous.

        If you really believe that CO2 & other anthropogenic GHGs will cause such an enormous increase in the rate of sea level rise in the future, then why do you suppose they’ve caused none in the past?

        1. Craig Thomas

          “Greenland apparently didn’t cause a catastrophic increase in sea level 8-12 centuries ago, when it was much warmer than now,”

          In which parallel universe is this statement factual?

          Longer-term temperature comparisons simply do not include very recent temperature increases: the “now” that the MWP is supposedly higher than is generally 1900, or maybe even 1950. Nobody seriously asserts that the MWP was hotter than 2000-2012. Nobody.

          ” and the best measurements have detected only a very tiny increase in meltwater production from Greenland, sufficient to cause less than 0.1 mm/year of sea level rise.”

          Greenland’s melt has accelerated by a factor of 5 within the last 20 years. The o.1mm/year is the long-term figure.

          The bottom line is that sea level rise is accelerating, warming is continuing, and coastal communities will suffer the consequences.

          Meanwhile, people who cherry-pick little chunks of the available data in order to announce, “Look! Sea level rise is decelerating”, are dishonest buffoons. The irony being that this cherry picked “deceleration” you are relying on, (at 2.3mm rise per year) actually clearly demonstrates an acceleration over the long-term sea level rise of 1.6mm per year.

          So, you say it’s decelerating and point to evidence of acceleration.

          What is this called?

      2. Dave, there was no catastrophic increase in sea level 8-12 centuries ago because it was globally cooler than it is now. The warming of the Medieval Period (MWP) was largely confined to the mid & north Atlantic. That’s why, for instance, MWP shows up in the South American and Greenland ice cores, but not in the High Asian Mountains (Himalayas). A number of paleoproxy sea level indicators, Earth’s rate of rotation vs eclipse historic observations, and the sea level highstand in the equatorial ocean basins are all consistent – 20th century sea level rise is anomalous compared to the last 7000 years (at least). By pointing out that no major sea level rise occurred along with the MWP you are actually shooting yourself in the foot – the sea level trend amply demonstrates that the MWP was not a global phenomenon. Pretty much what Mike Mann and fellow researchers concluded long ago.

        The take-home message here is that with 65-70 metres of sea level locked up in the Antarctic and Greenland icesheets, the potential for large increases in sea level rise exists. We know this because it has happened before, most recently in the Eemian interglacial, some 115-130,000 years ago. The high rates of sea level rise near the apex of the Eemian
        highstand, and that sea level rise may have been up to 6-9 metres higher than now, imply the Greenland & West Antarctic ice sheets largely collapsed.

        I find it quite ironic that residents of North Carolina should be retreating away from the best available scientific advice on sea level rise, because it is one of the regions on Earth that is going to affected the most by sea level rise, should the West Antarctic icesheet begin to disintegrate.

        Anyway, I’m putting together a series of posts on this topic for publication at Skeptical Science. It’s only by looking at the broader picture that readers will realize how wrong people like yourself and John D are. And it’s the truth we are interested in here, because nature functions according to physical laws, not legislative ones.

  9. […] Sea level rise 101 by John Bruno: […]

  10. […] There is a nice article in the NYT today by Justin Gillis about new research designed to answer this question.  The team is identifying fossil beaches from the pliocene (~ 3 millions years ago) that were formed when the atmospheric concentration of carbon dioxide was (naturally) what it is now (not naturally).  Then, as now, the relatively high CO2 concentration led to global warming and thus sea level rise.  The preliminary results are shocking – in short, as we’ve been warning, we are in for a heap of new oceanfront property.  (note, Iv’e got a primer here, Sea Level Rise 101) […]

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