Sunday, August 25, 2013

Rising seas - maybe one or up to two metres this century

Sou | 9:40 AM One comment so far. Add a comment

What do "a lot of scientists" think about sea level

If we're going to plan infrastructure for the next few decades, should we plan for the minimum projected sea level rise or the maximum?  I'd argue for the maximum.  It's fine if it doesn't happen but if it does we don't want to get caught out.

There has been quite a bit of interest in sea levels lately.  On WUWT today Anthony Watts has posted an article by Bjørn Lomborg who is scoffing at Justin Gillis on NPR. Bjørn Lomborg wrote the following, presenting it as if it were a direct quote (which it wasn't) as:
Justin Gillis tells NPR how much sea levels will rise:
experts believe sea levels will rise at least 3 feet in the next century, and that number could be as much as 6 feet.
The links in the WUWT article all went to some facebook pages but I figured it would be worthwhile seeing what Justin Gillis actually said in that interview.  So I googled  and found that the supposed quote was from a summary of an interview on NPR from 21 March this year - and it wasn't a direct quote.  Here is what NPR reported:
Gillis says experts believe sea levels will rise at least 3 feet in the next century, and that number could be as much as 6 feet.

A pretty serious problem

So then I went to the interview transcript to see what Justin Gillis actually said and came up with with this.  Note the section I've printed in bold italics.
DAVIES: So the melting of the land ice will contribute to sea level rise, unlike the melting of the ice in the ocean?
GILLIS: No question about that, and in fact the ocean is rising already. Many people know this. It's gone up about eight inches or so in the last century. That doesn't sound like much, but if you can imagine a very gently sloping shoreline, even eight inches of sea level rise has meant a whole lot of erosion. And in fact people have spent billions of dollars along the coastlines of the United States battling erosion already.
Now we're trying to understand, well, how much more sea level rise are we going to get over how long a period? The essential question is really how fast will this unfold. And a lot of scientists lately have been coming to the conclusion that we could fairly easily see three feet or so of sea level rise in the coming century and, you know, possibly as much as six feet.
So if we get that much, that's going to start to become a pretty serious problem.
The context: "The essential question is really how fast will this unfold."  and "we could fairly easily see three feet or so...". There is no "at least three feet" in the transcript.  That was what NPR said, not what Justin Gillis said.  See how in the various repeats a nuanced but informed speculation is turned into a bald certainty by NPR and then presented by Lomborg as a direct quote from Justin Gillis?  And the denialists call people who accept mainstream science "alarmists"!  It does pay to do your own research.

Now Lomborg insists that Justin Gillis is wrong (in what he didn't say) -  and refers readers to a recent AR5 draft, which I think he's arguing Justin Gillis should have seen back in March when he did the interview.  Time travel is no barrier to contrarians and lukewarmers.  Lomborg writes:
So, Gillis tells us the one end of the spectrum is 3 feet and the highest 6 feet, while the the UN says 1 foot to 2.7 feet. His *lowest* estimate is higher than the *highest* of the UN Climate Panel’s new, higher estimate.
Well, no he didn't tell us that.  You are wrong there, Dr Lomborg.  What Justin Gillis reported was "a lot of scientists have been coming to the conclusion that we could fairly easily see three feet or so of sea level rise in the coming century..."

The one bit of progress at WUWT is that they are touting the IPCC as the 'bible' on climate, for a change!

CSIRO confirms that we could see one to two metres by 2100, but probably no higher

What does Australia's CSIRO have to say on the subject?  I found this report, which reports "recent progress in understanding sea-level rise and also clarify confusion around interpretations of the IPCC sea-level projections".  The CSIRO report is not definitive.  For example:
Recent interpretations of geological data suggest that at the time of the last interglacial (~125,000 years ago), when sea level was close to today’s value, there was a period when “...the average rate of sea-level rise [was] 1.6 m/century.”14 This demonstrates that sea-level rise of 1 m or more by the year 2100 is plausible. (page 8)
...There is increasing concern about the stability of both the Greenland and the West Antarctic Ice Sheets leading to a more rapid rate of sea-level rise. While our understanding of the relevant processes is limited, it is important to recognize that the uncertainties are essentially one-sided. That is, the processes can only lead to a higher rate of sea-level rise than current projections. (page 9)
The CSIRO booklet has a lot more information and sets out the factors affecting sea level quite well.  The main message is that ice will melt and seas will rise and they won't stop rising at midnight on 31 December 2099.  The other main message is that we have the power to limit the sea level rise, should we feel inclined to do so.  There is more on this page on the CSIRO website:
The AR4 explicitly states that larger rises cannot be excluded and its projections for sea-level rise do not give a best estimate or an upper bound. Note that since publication of the AR4, Pfeffer et al. (2008) have argued that a rise in excess of 2 metres is "physically untenable," and that a maximum rise of 0.8 metres (near the upper end of the IPCC AR4 projections) is more plausible.
So based on the best information I can find from CSIRO, seas may well rise by 3 feet (one meter) and could go as high as six feet (two metres) by 2100 but are not likely to go higher than that this century.  But they will keep rising over the coming centuries.

Many of you will have read the following on The Conversation or at RealClimate.org or elsewhere.  It is more confirmation that we cannot be complacent about sea levels.  Dr Levermann also talks of an upper limit this century of around two metres, same as Justin Gillis reported the 'experts' as saying and the same as reported by the CSIRO.

The inevitability of sea level rise

By Anders Levermann, Potsdam Institute for Climate Impact Research

Small numbers can imply big things. Global sea level rose by a little less than 0.2 metres during the 20th century – mainly in response to the 0.8 °C of warming humans have caused through greenhouse gas emissions. That might not look like something to worry about. But there is no doubt that for the next century, sea level will continue to rise substantially. The multi-billion-dollar question is: by how much?

The upper limit of two metres that is currently available in the scientific literature would be extremely difficult and costly to adapt to for many coastal regions. But the sea level will not stop rising at the end of the 21st century. Historical climate records show that sea levels have been higher whenever Earth’s climate was warmer – and not by a couple of centimetres, but by several metres. This inevitability is due to the inertia in the ocean and ice masses on the planet. There are two major reasons for the perpetual response of sea level to human perturbations.

One is due to the long lifetime and warming effect of carbon dioxide in the atmosphere. Once emitted carbon dioxide causes warming in the atmosphere over many centuries which can only be reduced significantly by actively taking the greenhouse gas out again. This is because both the amount of heat and carbon dioxide the ocean can absorb is reduced, and so the temperature stays up for centuries or even millennia. Of course, not cutting emissions would exacerbate the problem even further.

The other reason is that both the ocean and the ice masses are very big and a warming of the surrounding atmosphere will only penetrate slowly, but inevitably, into them. As a consequence their sea level contribution continues even if the warming does not increase. Sea level rise over the last century has been dominated by ocean warming and loss of glaciers. Our recent study indicates that the future sea level rise will be dominated by ice loss from the two major ice sheets on Greenland and Antarctica – slumbering giants that we’re about to wake.
Sea level rise contributions over 2000 years from: 
ocean warming (a), mountain glaciers (b), 
Greenland © and Antarctic (d) ice sheets. 
The total sea level commitment (e) is about 
2.3m per degree of warming above pre-industrial.

It is easier to understand a future world that has adjusted to a new equilibrium of higher temperatures than it is to understand the dynamic (perhaps rapid) transition from today’s world to a warmer one. That is why we used physical models for the ocean, the mountain glaciers and the big ice sheets to compute how the systems would be different if the world was warmer.

What we found was that for each degree of global warming above pre-industrial levels the ocean warming will contribute about 0.4 metres to global mean sea-level rise while Antarctica will contribute about 1.2 metres. The mountain glaciers have a limited amount of water stored and thus their contribution levels off with higher temperatures. This is over-compensated for by the ice loss from Greenland, so that in total sea level rises quasi-linearly by about 2.3 metres for each degree of global warming (see figure).

How fast this will come about, we do not know. All we can say is that it will take no longer than 2,000 years. Thus the 2.3 metres per degree of warming are not for this century. They need to be considered as our sea level commitment – the sea level rise that cannot be avoided after we have elevated global temperatures to a certain level.

Ben Strauss of Climate Central has considered the different possible future pathways that society might take and computed which US cities are at risk in the long-term. He poses the question as to what year, if we continue with greenhouse emissions at current rates, we will have caused an inevitable sea level rise that puts certain cities at risk.

According to his analysis, within the next few years Miami in Florida will be committed to eventually lie below sea level, while our future actions can still decide on whether we want to one day give up cities such as Virginia Beach, Sacramento, Boston, Jacksonville or New York City.

This is a decision society has to take for future generations. We will need to adapt to climate change in any case, but some things we will not be able to adapt to. Society needs to decide whether we want to give up, for example, the Tower of London, or to put the brakes on climate change so that we don’t have to.

Anders Levermann does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.

This article was originally published at The Conversation. Read the original article.

And if you're not sick and tired of reading about sea levels, there was another article by Justin Gillis recently in the NY Times - you can get it here.  In it he talks about a new paper by O'Leary et al on the possible catastrophic collapse of ice sheets in the last interglacial - as a warning of what might happen this time around - though not immediately.

And if you want more still, you may be interested in this new article by Andy Revkin on Dot Earth.

1 comment:

  1. IMO, Watts is doing the simple calculation that the timescale for AGW effects are slow enough for him to make a living before the case is undeniable. Personally, I don't know how he sleeps at night. I know I couldn't.


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