Sea levels are rising - but how quickly?

Glacier Sea level rise remains one of the big unknowns of climate science

Scientists are warning that the level of the sea may rise by slightly more than previously forecast - but they also say that the very worst predictions look much less likely.

Confused? If so, you're not alone.

The future of sea level rise is one of the most important questions in climate science because so many millions around the world live beside coasts - but it's also one of the most difficult to answer.

One of the authors of a new study, published today, even described the uncertainties of the field with that memorable phrase first coined by the former US defence secretary Donald Rumsfeld: "unknown unknowns".

Not everyone is always so open about this.

I'll never forget a video screened at the opening session of the UN's climate talks in Copenhagen in 2009 - the conference that got nowhere - which portrayed this issue at its most simplistic.

One scene showed a vast flood that left a little girl hanging from a tree screaming her heart out.

The message to delegates was crude: "If you don't negotiate a climate deal, the kid gets it".

Start Quote

The headline is that forecasts of the changes to the ice-sheets and glaciers in the Arctic and Antarctic suggest their melting could contribute between 3cm-36cm to sea level rise.”

End Quote

I reported at the time that nothing in the scientific literature was specific enough to justify this lurid threat.

In the event, none of the major players was paying attention to this climate horror show anyway and there was no treaty.

Then and now

So what did the science say back then and what does it say now?

The last major report by the UN's Intergovernmental Panel on Climate Change (IPCC), its Fourth Assessment Report published in 2007, concluded that the global average level of the sea may rise by between 18cm-59cm.

Much of that was expected to come from what's called "thermal expansion" - the physical process by which the volume of a body of water expands as it warms, as the oceans are.

But, crucially, the IPCC said that not enough was known about the other great potential contributor to sea level rise - the melting of the polar ice sheets and glaciers.

Polar melting has been a kind of wild card in these calculations: a sudden acceleration in melting could cross some hidden threshold and trigger runaway melting that would cause an unexpected surge. Or not.

To try to fill that gap in the knowledge, a leading British polar researcher, Professor David Vaughan of the British Antarctic Survey, and teams of colleagues came together in a consortium called Ice2sea.

Start Quote

One conclusion is that the very scariest scenarios look less likely.”

End Quote

So what does the new study tell us? The headline is that forecasts of the changes to the ice-sheets and glaciers in the Arctic and Antarctic suggest their melting could contribute between 3cm-36cm to sea level rise.

This would add another 10cm to the upper range of the last IPCC forecast.

This perhaps does not sound like much of a change - and the range in numbers is still pretty large spanning from "not very much at all" to "quite a bit".

But Prof Vaughan and others say the estimate is much more robust than earlier ones and provides more clarity for policy-makers.

In any case a rise on slightly larger bigger scale would of course be serious for communities in low-lying countries. I saw for myself how most the tiny island nation of Tuvalu is already only 2-3m above sea-level.

Assumptions made

The challenge with all forecasts - especially on this question - is that they rely on a whole series of factors and assumptions.

Tuvalu Tuvalu fears rising tides

The Ice2sea researchers started with a global climate model that assumed a particular scenario for the rise in greenhouse gases. It then used further models to investigate the potential change in climate in regions such as Greenland and Antarctica.

Another layer of computer simulations explored how the ice might respond. Final stages investigated what that could mean for sea level not just globally but also on particular coasts.

Hence the large range of possible outcomes.

On top of that, the ice-sheets and glaciers themselves are often inaccessible and hard to measure. One research effort even involved firing hi-tech javelins at a glacier in Antarctica.

And satellite data only stretches back a few decades.

Interestingly, one conclusion is that the very scariest scenarios look less likely.

There is only a one in 20 chance, they reckon, that by 2100 coastal towns in the Thames Estuary, Holland and Ireland may get hit by extreme surges about one metre higher than now.

At a media briefing, I asked what that meant and whether we could take it that there's a 95% chance that this won't be what happens. That's right, I'm told. The very gloomiest warnings look far less plausible.

The most certain piece of knowledge is that the global average sea level has been rising by about 3mm a year.

But what really matters is whether this will accelerate - and by how much. And all that's still a work in progress.

David Shukman Article written by David Shukman David Shukman Science editor

The largest vessel the world has ever seen

An enormous ship is being built to collect and liquefy gas at sea for the first time.

Read full article


This entry is now closed for comments

Jump to comments pagination
  • rate this

    Comment number 90.

    @2 and 4 (two lowest rated)

    *head desk*

    thats right up there with "its cold today, the globe cant possibly be warming"

  • rate this

    Comment number 89.

    The rising sea levels may not be alarming, but it is confirmation of global warming. MORE WATER, MEANS LESS ICE.

    The real problems are droughts in the Amazon, China and Africa.
    You worry about paying tax?
    You can pay for a clean energy transformation now, or for millions of refugees later.

  • rate this

    Comment number 88.

    acid rain as reduced enormously, but co2 is a more worldwide problem, the west is dealing with the problem on a personal scale, the building trade is far better and more adequatley prepared for energy efficiancy than ever before, cars and transport in general are better, but the emerging economies are the fly in the ointment, and how do we tell them not to do what made some of us rich?

  • rate this

    Comment number 87.

    I can't give you the link because it's subscription only, but New Scientist has a very interesting article 'Where melting ice means lower seas' 6th May 2013.
    The whole business is a lot more complex than uniform sea level rise.

  • rate this

    Comment number 86.

    @78 DSA

    The acid rain you talk of just vanished with time you mean? By itself?
    It took lots of countries much coordinated effort to curb sulpher and nitric oxides.
    They reduced these emissions by more than 95% That is why it stopped raining acid. And that is how we should be handling the CO2 emissions! Thank you for reminding us.

  • rate this

    Comment number 85.

    Surprise, surprise.

  • rate this

    Comment number 84.

    Perhaps Nigel Lawson, in a somewhat different context, although not entirely, deserves the credit for rolling a million words of debate into the devastatingly erudite encapsulation- 'inconsequential'.

  • Comment number 83.

    This comment was removed because the moderators found it broke the house rules. Explain.

  • rate this

    Comment number 82.

    Glad that 'the very scariest scenarios look less likely' so we can just carry on messing things up? But maybe we should do something - better safe than sorry, eh? And the human race is renowned for getting things at least 95% wrong - err - look at the eu. And OMG! of course this is the only planet we've got - so far - and the signs on Perranporth beach keep getting undermined and washed away...

  • rate this

    Comment number 81.

    "The last major report by the UN's Intergovernmental Panel on Climate Change (IPCC), its Fourth Assessment Report published in 2007, concluded that the global average level of the sea may rise by between 18cm-59cm.

    But, crucially, the IPCC said that not enough was known about the other great potential contributor to sea level rise"

    Poor reporting, IPCC excluded ice dynamics.

  • rate this

    Comment number 80.

    Even giving the modellers the benefit of the doubt - that their results are not merely artifacts of their methods - sea levels appear to rise and fall in regular cycles, as does temperature. In context of the historical record the current cyclical changes are completely unremarkable. Historical global data is sparse, which means the modellers have to use interpolation.

  • rate this

    Comment number 79.

    Have any of you guys ever seen the devastation caused when the insides of our planet rupture in a volcanic event that spews out so much ash and co2 that summers become winter. It happened last century and it'll likely happen this century. We are but ants on this planet,so don't get hung up about this climate nonsense.

    The BBC has brainwashed you.

  • rate this

    Comment number 78.

    Back in the early 70's a college lecturer informed us that greenhouse gases would raise the global temperatures and a half degree rise would flood many low lying coastal areas.
    Also, at about the same time were had doom and gloom forecasts about acid rain.
    To date, not a lot has happened except I am 40 years older and have paid a lot of tax to stop out non events.

  • rate this

    Comment number 77.

    Amazed by some peoples stubborn refusal to acknowledge man made global warming & that the impacts of this could be serious. Tell me what facts do you have problems with. CO2 is a 'green house' gas (its a scientific fact), burning coal, gas & oil creates massive amounts of CO2 , levels of CO2 are higher than for millenia (its a fact) & in parallel global temperatures have increased (its a fact) ?

  • rate this

    Comment number 76.

    Meanwhile the world has the more immediate threat of economic meltdown and militant religious barbarism to contend with.In the context of 2008 melting ice is low on peoples agendas, particularly as all scientists can give is maybe,possibly,likely,potentially.The bulk of populations have already maxed out their stress debit cards on other items-this vague 'science' is way down the list.

  • rate this

    Comment number 75.

    73. feedbackloop
    "This range of uncertainty is not a good platform for policy."

    Neither unfortunately is it a good platform for inactivity.

    Whether you get hit when crossing the road without looking is subject to huge uncertainty; I would not suggest that there is no need/policy to look first. It's the precautionary principle.

    If we lose the North Atlantic Drift, the consequences are horrible.

  • rate this

    Comment number 74.

    @63 - Pakgila yes, it's like the 'Day of the Triffids' in here!

    Also, why is it that .., erm.., forgot what I wanted to say now. OH Yes.., why is it that the thing with the wotsit that tells us when the atmospheric pressure goes um, y'know, oh I dunno, erm, yeah that, and yet no-one is able to accurately determine the flux capacities of the meteorologically aligned satellite instrumentation that

  • rate this

    Comment number 73.

    The key conclusion from this piece is the confirmation that the range of uncertainty (what scientist call standard deviation) in the prediction is very very large. This is true of a large part of climate science. With the result that predicted impacts are either very limited or very very serious. This range of uncertainty is not a good platform for policy.

  • rate this

    Comment number 72.

    The snootiness of 8, 26, 16 against prima facie evidence from 2 & 4 is staggering. You provide evidence from £1b satellites that we can't possibly confirm whilst we have NO budget to challenge your scaremongering, taxing us back into the Stone Age over problems that barely exist. The real problem is macroeconomics & social unrest -- we cannot afford to fiddle over a millimetre while Rome Burns.

  • rate this

    Comment number 71.


    Here is the comment I was arguing against:

    'The only part of East Anglia that is 50 feet above sea level is a mound near Norwich, currently occupied by a TV antenna.'

    Now, is that statement true or untrue?

    Pakgila, I am surprised your questions haven't been removed. The BBC spent a fortune of our money trying to keep the attendance of the '28gate' meeting secret.


Page 1 of 5



BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.