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Friday, 25 September 2015

Uncertain World: Understanding past and future sea level rise

A recent study published in Science Advances suggests that if we burn all attainable fossil fuels (up to 12,000 gigatonnes of carbon), the Antarctic ice sheet is likely to become almost ice-free within 10,000 years. However, what does this mean in terms of sea level rise? To illustrate this we have designed an infographic which shows the likely extent of sea level rise under a range of different scenarios. We have chosen to use the Wills Memorial Building as an example and assume, for the purpose of this exercise, that it resides at sea level (Figure 1).

1) Sea level rise over the next century:

The most recent report by the Intergovernmental Panel on Climate Change (IPCC AR5) indicates that if we continue emitting greenhouse gases under business-as-usual scenarios (i.e. no reduction in emissions), it is likely that global mean sea level will rise between 0.52 and 0.98 m by the year 2100. If we are more optimistic, and we allow greenhouse gas emissions to peak in 2040 and decline thereafter, the range of likely global mean sea level rise is lower, but not insignificant (0.36 to 0.71 m). Both of these estimates are illustrated below and shown alongside the Wills Memorial Building. 

Figure 1: An infographic showing the approximate height of sea level rise depending upon a range of different scenarios (Fretwell et al., 2013; IPCC AR5). This assumes the Wills Memorial Building resides at sea level

Although ~30 to 100 cm of sea level rise may seem insignificant, it is worth considering what this means for other regions. For example, "...since 80% of its 1,200 islands are no more than 1m above sea level", sea level rise has the potential to impact up to 360,000 citizens and lead to widespread migration.

The reason that scientists provide a range of values for sea level rise is that the climate system is very complex. For example, under low emissions scenarios, there is expected to be an increase in moisture content around Antarctica, leading to increased snowfall along the ice sheet margins. However, under higher emissions scenarios, ice sheet discharge overcompensates for an increase in snowfall, leading to a net sea level rise.

2) Sea level rise over 10,000 years:

The variations between these two emission scenarios are less important when looking over longer timescales. Winklemann et al. (2015) have recently simulated changes in the Antarctic ice sheet over the next 10,000 years using a combination of climate and ice sheet models. From these experiments, it is clear that ice loss is driven by two key feedback mechanisms. The first begins with warming and subsequent retreat of the grounding line (Figure 2). The grounding line is the region where ice transitions from a grounded ice sheet to a freely-floating ice shelf. When the grounding line retreats to a point where the ice sheet falls below sea level, then ice sheets can become unstable.

Figure 2: A schematic of an ice sheet showing the position of the grounding line (bottom right). Image credit:

Winklemann et al. (2015) argue that the West Antarctic Ice Sheet (WAIS) becomes unstable when cumulative carbon emissions reach 600 to 800 gigatonnes of carbon (this is equivalent to a 2 degree rise in temperature by 2100). If this part of the Antarctic Ice Sheet becomes unstable, we can expect ~4 m of global sea level rise (Figure 1).Once a specific temperature is reached, a second feedback then kicks in. This destabilises the rest of the Antarctic ice sheet via the so-called surface elevation feedback. On the timescale of 10,000 years this will eventually lead to an almost ice-free Antarctica (Winklemann et al. 2015).

Figure 3: Predicted ice-sheet loss on Antarctica under different carbon emission pathways (Winkelmann et al., 2015: Science Advances).

3) Sea level rise over millions of years:

Palaeoclimatologists can provide insights into the fate of ice sheets over longer timescales. For example, the last time Antarctica was ice-free was during the early Eocene (~56 to 48 million years ago). During this interval, carbon dioxide concentrations were much higher and allowed the development of lush, tropical rainforests along the ancient coastline (Figure 4). Gradual cooling over millions of years eventually culminated in the sudden and rapid establishment of ice-sheets on Antarctica. This occurred ~34 million years ago and was likely driven by a reduction in carbon dioxide (and perhaps some other feedback mechanisms). Although Antarctica has fluctuated in size since then, it has never been completely ice-free since the Eocene. However, under rising carbon emissions, we are rapidly returning to a world that has not been seen for at least 34 million years.

Figure 4: This may be what the East Antarctic coastline looked like during the early Eocene (Pross et al., 2012). 

Further reading:
  • Fretwell et al. 2013. Bedmap2: improved ice bed, surface and thickness datasets for Antarctica. The Cryosphere. v. 7.
  • Winkelmann et al. 2015 Combustion of available fossil fuel resources sufficient to eliminate the Antarctic Ice Sheet. Science Advances, v.1. 
  • Bamber et al., 2009. Reassessment of the Potential Sea-Level Rise from a Collapse of the West Antarctic Ice Sheet. Science. v. 324
  • Church et al. 2013.  Sea Level Change. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA (see Chapter 13; Table 13.5, p. 1182 for 21st Century sea-level rise estimates).
  • Pross et al., 2012. Persistent near-tropical warmth on the Antarctic continent during the early Eocene epoch. Nature. v. 488.
n.b. As with the IPCC, we occasionally use the following terms to indicate the assessed likelihood of an outcome or a result. These are noted in italics: Virtually certain 99–100% probability, Very likely 90–100%, Likely 66–100%, About as likely as not 33–66%, Unlikely 0–33%, Very unlikely 0–10%, Exceptionally unlikely 0–1%.


Correction: the original post incorrectly stated that "... more than 80% of the Maldives lie one metre below sea level". This has since been amended. Thanks to @radicalrodent for spotting this.

This blog was written by Gordon Inglis (@climategordon), a palaeoclimatologist working in the Organic Geochemistry Unit within the School of Chemistry. The infographic was created by Catherine McIntyre (@cathmci), an organic geochemistry PhD student working in same group.


  1. It might be an idea to ensure that you get your most basic facts right, should you want people to take your publications seriously: the source for your “… more than 80% of the Maldives lie one metre below sea level…” actually states: “...80% of its 1,200 islands are no more than 1m above sea level…” (my bold).

    Mind you, your strapline ending, “…how we live in a changing and uncertain world.” does indicate that either you are afraid of any change, or (perhaps worse) you wish to engender fear in others about change. As change is one of the few constants in the universe – even rates of change can, erm, change – it does seem a strange thing to try to generate fear over. It has to make a rational person ask: why are you doing this?

    The implications in your first paragraph are that any change is being brought about by humans burning “fossil” fuels: “…if we burn all attainable fossil fuels…” (over what time period, by the way – today, or over a few decades, centuries or millennia?). Apply some simple observations: while the human consumption of fossil fuels has risen exponentially, the rise in CO2 concentrations in the atmosphere has been more or less linear; while atmospheric CO2 levels have been rising steadily, temperatures have not. Quite how you can state that each or any of these has a direct effect upon the others is yet another mystery: why are you doing this? Perhaps you should remind yourself about Richard Feynman’s ideas about theories and reality – if reality does not match the theory, it is the theory that is wrong!

    1. Not to mention the fact that so endangered by sea level rises (author doesn't seem to know that atolls rise with the sea - consult Darwin (a REAL scientist) regarding that) are the Maldives that they can't stop building on the islands. However, the CAGW cult is a very good way of getting monry out of suckers.

      Would you do THIS?

      if you thought you were sinking into the sea?

      No, of course you wouldn't,


  2. Hey Radical Rodent,

    Thanks for your comment re: the Maldives and spotting my mistake. The article has now been corrected and this change has been noted at the bottom of the article.

    Thanks for reading!


    1. Thank you for your acknowledgement, Gordon. It would also be interesting, though, to have some response to the rather more contentious issues I raised, for which “Elby the Beserk” seems to show somewhat over-enthusiastic support.

      Are you genuinely trying to establish facts, and possible outcomes (in which case, what are the alternatives?), or are you just trying to generate fear?

    2. The aim of this blog (and the infographic) was: 1) to provide the facts about sea level rise and 2) convey the magnitude of sea level rise, both in the future (short- and long-term) and in the past. I was hoping that this type of exercise would help to convey sea level rise in a way that numbers alone cannot. Whether that worked or not is for others to decide.

      Also, you noted that: "“…if we burn all attainable fossil fuels…” (over what time period, by the way – today, or over a few decades, centuries or millennia?)."

      This is a good question which is discussed in the Winkelmann et al 2015 (which is open access btw). They state that:

      "Although the interval of carbon release into the atmosphere does not last more than 500 years in any scenario, elevated CO2 concentrations persist for millennia".

    3. This comment has been removed by the author.

    4. (With no facility for editing, I deleted to correct a few typos. Below is the same comment, properly corrected – hopefully)

      “The magnitude of sea-level rise”? I am not sure a questionable 1mm a year is a magnitude of any concern (and is certainly a lot lower than it has been) – for a start, how can sea-level be measured to 1mm? Have you tried measuring the level in the bath, while you are in it? That would give a good indication of how feasible that degree of accuracy is (much the same as temperatures measured to 0.02°C). Also, the Maldives, and similar coral atoll islands, have grown from the sand eroded off the coral being trapped on those reefs; storms and rough weather are a requisite for this, but might cause some distress to the inhabitants. I would not be surprised if islands like the Maldives are actually growing – except, of course, the fact that the Maldives could be causing their own problems, mining the coral for the construction on those islands.

      As for “…elevated CO2 concentrations persist for millennia” … so what? There is a certain historical precedent for that, with most of Earth’s past having considerably higher levels than today. As I have pointed out, while CO2 concentrations are steadily rising, temperatures are not; why should there be any concern about CO2 in the atmosphere? It is not a pollutant, as is occasionally being claimed, as it is essential to ALL life on the planet, and may actually be helping to green the drier parts of the world.