Posts Tagged ‘sea level rise’

Antarctica: ice gain or loss?

November 25, 2015

Guest post by Dr. Jan Wuite, Enveo, Innsbruck

A new study released by NASA scientist Jay Zwally and colleagues in the Journal of Glaciology, receiving wide coverage in the media last month, reports an 82 Gigatons per year increase of land ice in Antarctica during the period 2003-2008. The study received much skepticism from other leading scientists in the field, as there are many indications that point at the contrary: ice loss, possibly irreversible. How does this new study fit in that picture, what are the consequences for expected sea level rise and are these numbers correct? Glaciologist and polar scientist Jan Wuite, working at Enveo in Innsbruck and involved in various international studies related to Antarctica explains.

One of the adverse consequences of climate change is global sea level rise. At more than 3 mm per year, the current sea level rises twice as fast as during the 20th century. The expectations are a rise of at least 70 cm by the turn of this century. The principal causes are clear: global decline of land ice (mountain glaciers & ice sheets) and thermal expansion of ocean water (water expands as it becomes warmer). To clarify, land ice is resting on land and can reach a thicknesses of up to several kilometers, in contrast to seasonally restricted sea ice (mainly just frozen ocean water), that is typically only a few meters thick and has no direct influence on sea level. Studies have indicated an increasing contribution of the two largest ice sheets, the Greenland and Antarctic ice sheets, to sea level rise.

The largest unknown for future sea level rise is caused by uncertainty in the predicted response of the Antarctic ice sheet to global warming. As warmer air can hold more moisture, it is possible that increasing snow accumulation compensates part of the sea level rise. On the other hand it is also possible that ice drains faster to the oceans accelerating it.

There is a lot of ice in Antarctica; in some places the ice thickness reaches well over 4 km. There is enough ice to, when melted completely, raise global sea levels with roughly 58 m. But even if only a small part of that melts it could have a significant impact on coastal communities, or ocean circulation. For this reason scientists are very interested in mass changes of the ice sheets: the mass balance.

Figure 1. An illustration of key processes determining ice sheet mass balance. Source: Zwally et al figure 1.

(more…)

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EGU General Assembly: The Arctic, Models, and Data

June 7, 2013

Guest post by Heleen van Soest

In April, the annual European Geosciences Union conference was held in Vienna, Austria. Heleen van Soest, MSc student Climate Studies at Wageningen University, attended the conference, and shares some thoughts and tweets (@Hel1vs).

The opening reception, April 7, reveals that geoscientists are fond of beer. I get to talk to some nice people and hand out my first business cards. Yay! I talk with Walter Schmidt,  President of the Division on Geosciences Instrumentation and Data Systems, about observations and data. Lesson learned: data are important, but never take them for granted. Especially from satellites: they basically measure counts and voltages. To interpret the numbers and get something useful, we already need models, i.e. algorithms. Usually, model skill is tested against data. Disagreement between them is often blamed on model errors, assumptions, etc. Keep in mind that data might be wrong, too. Fortunately, raw data is increasingly archived as such, together with the algorithms used to interpret them. In that way, data can still be used if the algorithms are updated. I dedicate my first #egu2013 tweet to this conversation and go home. I am happy to find a Va Piano (Italian restaurant) in ‘my’ street. Together with Sherlock Holmes (the book, that is), I eat my pasta.

Tweet At #egu2013 opening reception, interesting conversation about models and data: “important, but never take them for granted” (Walter Schmidt)

Monday, 8 April

Permafrost day. An important issue, as permafrost contains about half of the world’s soil carbon. If permafrost thaws, the organic carbon becomes available for microbes to degrade. Greenhouse gas (methane) emissions are a result, further increasing temperatures. This positive feedback is sometimes compared to a time bomb. Modelling studies of permafrost do show it will degrade under further warming. For example, Greenland permafrost south of 76°N will disintegrate this century. However, see RealClimate before you start to worry that this bomb is about to explode.

But today is not only permafrost; I’ve also got something on ice observations.

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Eric Wolff on areas of agreement and on the public debate about climate science

May 25, 2011

Dr. Eric Wolff is spot on (see also further below): 

as an outsider to the blogosphere, it surprises me that so many people, presumably mostly with even less knowledge and training than me, seem absolutely convinced they have mastered every area of climate science.

A peculiar line-up of speakers assembled recently at the Conference on the Science and Economics of Climate Change in Cambridge: Phil Jones, Andrew Watson, John Mitchell, Michael Lockwood, Henrik Svensmark, Nils-Axel Morner, Ian Plimer, Vaclav Klaus and Nigel Lawson. Bishop Hill reports that

Dr Eric Wolff of the British Antarctic Survey tried valiantly to find a measure of agreement between the two sides.

This proved an interesting exercise and resulted in a useful list (also reproduced by Judith Curry) on what we can all agree on (perhaps excluding those too far out on the fringes; links added by me):

  1. CO2 does absorb infrared radiation
  2. The greenhouse effect (however badly named) does occur in practice: our planet and the others with an atmosphere are warmer than they would be because of the presence of water vapour and CO2.
  3. The greenhouse effect does not saturate with increasing CO2
  4. The CO2 concentration in the atmosphere has risen significantly over the last 200 years
  5. This is because of anthropogenic emissions (fossil fuels, cement production, forest clearance). [Ian Plimer disagreed, but as Curry stated: “the anthropogenic contribution is (should be) undisputed”]
  6. If we agree all these statements above, we must expect at least some warming. [Bishop Hill noted “broad agreement” with this point]
  7. The climate has warmed over the last 50 years, [as is evident from] land atmospheric temperature, marine atmospheric temperature, sea surface temperature, and (from Prof Svensmark) ocean heat content, all with a rising trend.
  8. We probably don’t agree on what has caused the warming up to now, but it seemed that Prof Lockwood and Svensmark actually agreed it was not due to solar changes, because although they disagreed on how much of the variability in the climate records is solar, they both showed solar records without a rising trend in the late 20th century. [Excellent point and ironically hitting Svensmark with a stick of his own making. Note the distinction between variability and trend.]
  9. On sea level, I said that I had a problem in the context of the day, because this was the first time I had ever been in a room where someone had claimed (as Prof Morner did) that sea level has not been rising in recent decades at all.  I therefore can’t claim we agreed, only that this was a very unusual room.  However, I suggested that we can agree that, IF it warms, sea level will rise, since ice definitely melts on warming, and the density of seawater definitely drops as you warm it.
  10. Finally we come to where the real uncertainties between scientists lie, about the strength of the feedbacks on warming induced by CO2 [i.e. climate sensitivity]

Eric Wolff also chimed in with a substantial comment over at Bishop Hill’s, rebutting some commonly heard arguments and making some very spot-on remarks:

(…)

I should first state the rationale for the summary I made at the Downing event. The meeting was about the science and economics of climate change, and I was asked to lead a discussion that came between the science talks and the two economics talks. I therefore felt the most useful thing I could do was to try to summarise what we had heard, as a basis for the discussion of whether society should do anything in response to that, and if so what. In particular I did hear a surprising number of things on which almost everyone in the room could agree, and it seemed worth emphasising that, rather than rehearsing old arguments.

I notice in the thread here several comments about who sets the “terms of the debate”, and about the “context of the debate”. While such phrasings may make sense in discussing energy policy, it is a strange way to discuss the science. Our context is the laws of physics and our observations of the Earth in action; our aim as scientists is to find out how the Earth works: this is not a matter of debate but of evidence. I think some of the comments on this blog come dangerously close to suggesting that we should first decide our energy policy, and then tell the Earth how to behave in response to it.

Regarding Plimer’s proposal that volcanic emissions were more important than we thought, (…) if volcanoes were “causing” the recent increase, then around 1800 their emissions would have had to rise above their stable long-term rate, and then stayed high. This is however a rather hypothetical discussion because the change in the isotopic composition of CO2 in the atmosphere over the industrial period is not consistent with an increased volcanic source anyway.

Regarding the idea that the “temperature increase stopped in 2000”: my point is that we know there are natural variations (due eg to El Nino) that cause runs of a few years of temperature colder than the average or a few years warmer. Look on the record at the decade around 1910 for example when there was a long run of cold years on a flat background.

Such a period superimposed on a trend would look like the last decade (but more so). The point of my analogy is that you can’t determine the trend over several months by measuring the gradient in a run of a few days. Similarly, you simply can’t determine a multidecadal trend by measuring the gradient over a few years because all you get is the “noise” of natural variability.

Professor Morner claims that globally sea level has not risen at all; he dismisses the evidence, from both satellites and the global tide gauge network, that it has. (…) There are numerous reasons why a single site can show a sea level signal, either real or apparent, that departs from the global mean.

I am not the best person to discuss models and feedbacks in detail (see comment on expertise below). However, I could not let two issues pass. Firstly, when models are run out for a century into the future, they do indeed show runs of years with flat temperatures amidst a trend (…) (because of El Nino and other natural factors). I am therefore not clear why this is evidence that something is missing. Regarding positive feedbacks: a positive feedback implies amplification, but not a system out of control; this is only the case if the sum of the gain factors is greater than 1.

Finally, a few specific issues that interested or worried me.

(…) CO2 emits infrared as well as absorbing it. (…) indeed, this property is precisely why its effects do not saturate (but fall logarithmically), because it allows the height from which the emitted radiation finally escapes to rise into regions with less and less air.

Geckko accidentally made an important point. S/he did not like the statement: “We can agree that if it warms the sea level will rise”, because it was too simplistic. Well, as a scientist I always like to boil things down to a statement that my brain can grasp, but that contains the essential explanation of an observation or process. And this one does, for example being demonstrably what was observed in going from a cold ice age world, with sea level 120 metres below the present level, to the present. However, you are right: there are factors that could make this statement false, such as increased snowfall when it warms, adding more ice into ice sheets. As soon as several such competing processes have to be taken into account, our brains cannot predict the outcome, and so we have to resort to putting all the “millions of assumptions” into a numerical model and seeing which of them “win”. An argument for models?

Coldish made a good point about expertise, and this is where I am going to go into a slightly more challenging area. I freely admit that I am not an expert on all, or even most, aspects of climate. When I reach a topic that I have not previously studied, I go to those who are experts, either in person or by reading their work. I maintain scepticism about some of their conclusions, but my working assumption is that they are intelligent and that they have probably thought of most of the issues that I will come up with. Can I observe as an outsider to the blogosphere, that it surprises me that so many people, presumably mostly with even less knowledge and training than me, seem absolutely convinced they have mastered every area of climate science.

and more so, convinced that they are right and almost all of the experts are wrong. That must be the height of hubris.

However, Coldish specifically mentioned IPCC, and I think there is also an interesting point to make about that. At the Downing event, there seemed to be two IPCCs in the room. To some it was a huge plot, masterminded by some mysterious power that manipulates troublesome scientists. To me and the scientists in the room, it is (at least in WG1) simply a set of well-researched review papers, describing the present state of the peer-reviewed literature. I mention this only because I think the former view is a type of groupthink where, because people form an extreme opinion in their private space, they think it is widely held, or even true.

Alas, Wolff’s forray into the blogosphere was short, as is evident from a short comment a while later:

Just in case anyone thinks they are addressing me with their remarks:

I thought this might indeed be a chance for a civilised discussion, and some of the respondents seem happy to have that. However there are also a lot of remarks on here that are frankly rude and aggressive, and I won’t be returning. Now I remember why I hate the blogosphere.

Sea level versus temperature

January 19, 2011

The previous post featured some graphs of changes in historical sea level. Obviously, sea level depends on temperature (due to thermal expansion of water and due to melting of glaciers and ice sheets). Let’s see how the relation between sea level and temperature has been over the Earth’ past. An often seen figure is the one from Archer (2006):

Future projections of sea level rise are much lower than what this graphs seems to suggest, based on the past. There are a few reasons for this:

These values refer to situations where sea level could be assumed to be close to equilibrium with respect to temperature, a situation we’re very far removed from at the moment, and that takes very long (milennia) to achieve. Also, past periods become worse as climate analogues for the future the farther in time they’re from. E.g. during the Eocene the continental configuration was different from today, which has implications for climate.

The Eemian period (the last Interglacial, 125,000 years ago) is probably a more useful analogue for the future. Global average temperature was about 2 degrees higher than now, whereas sea level was 4-6 metres higher than now (“with individual maxima up to +7 or +9m“). However, Rahmstorf pointed out that there’s also a …

problem here: this is forced by orbital changes, i.e. highly regional and seasonal insolation anomalies, not a global mean forcing.

Perfect analogies don’t exist. But all timeperiods from the past point to a very strong dependence of eventual sea level to the earth’ temperature. I wrote about this before, with a home-made figure of sea level versus temperature, including the Eemian.

Grinsted made another graph of sea level versus global average temperature, based on the analysis by Rohling (2009):

Rohling et al based their analysis on data from the past 5 glacial cycles (covering half a million years), to which they fitted an exponential (their figure 2b, reproduced as the thick blue line above, here scaled to global average temperatures. Values in red are ‘recent’ sea level values and a sea level projection for 2100). Grinsted provides the following explanation for this relation being sub-linear:

During glacials where ice volume was large the sea level response to a [change in temperature] was also large. In interglacials, with much less ice volume, the sea level response is much smaller.

Based on Rohling’s data, he concludes that in a warming climate, equilibrium sea level rise would be 6-10 m/degC of global average temperature change. That’s a helluva lot more than projections for the next 100 years suggest (due to long equilibration timescales), though less than the 20 m/degC from Archer’s figure. That’s good news I guess, though as the saying goes: “When it’s bad, it’s really bad. When it’s good, it’s still pretty bad”.

Quoting myself:

We know relatively little about dynamical processes that influence the breaking up and melting of land ice. But apparently large changes in sea level are possible if the temperature remains long enough above (or below) a certain value. The examples from the past may give a sense of what order of magnitude sea level rise we could eventually expect for a given temperature increase. The rate of sea level rise is the most uncertain. Most scientific literature concludes that sea level rise won’t be more than one or at most two meters by 2100 (but it will continue to rise thereafter). That is quite a strong increase for large parts of the world to adapt to, and uncertainty in the rate and level of the rise is not really comforting. The examples from the past are even less so.

Past, current and future sea level rise

January 18, 2011

What’s there to worry about sea level rise; it’s going very slowly, right? Let’s put current sea level rise in a historical perspective.

Here’s a graph of sea level since the last ice age. As the ice from the last ice age was melting, sea levels rose by some 120 metres over the course of about 8000 years, before it flattened out ~6000 years ago. On the top right, I drew a black line with an approximate slope of 3 mm/year, which is the current rate of sea level rise (over the past 20 years or so). This is much faster than the relatively stable sea level during the ~6000 years before, though not as fast as the sea level rise at the end of the last ice age.

Let’s zoom in on the last 9000 years (covering most of the Holocene epoch). The strong sea level rise at the end of the last ice age is still visible on the left hand side, slowing down 7000 years ago and even more so 4000 years ago. Until recently: Current sea level rise represents a clear increase. For the future, most recent estimates of sea level rise fall between 0.5 and 1.5 metres in 2100. It won’t stop thereafter, since there’s a lot of inertia involved in warming up the oceans and in melting (parts of) the large ice sheets (Greenland and Antarctica).

 

Under a business as usual scenario sea level will likely rise by multiple metres over the course of the next few centuries (see e.g. the Dutch DeltaCommittee and German WBGU reports). The vertical red line attempts to give an indication of these long term projections (which are uncomfortably uncertain). With increased warming, sea level rise will likely also increase, so the black line denoting 3 mm/yr can be seen as a lower bound (until the temperature goes down again and the climate system re-equilibrates). An upper bound is harder to predict, because that depends e.g. on tipping points being surpassed, such as parts of the great ice sheets becoming unstable. The fact that in the past sea level has risen with multiple metres per century means that such rates are physically possible. Whether that will happen again in the next millennium depends partly on nature’s forces (uncertainty in the physics, i.e. the response of ice sheets) and partly on what we do to poke nature around (uncertainty in our future emissions).

Quoting from a  recent book “Climate Change: Global Risks, Challenges and Decisions” (eds. Richardson, Steffen and Liverman), the chapter on sea-level rise and ice-sheet dynamics:

The period of relative stability of sea level over the past 6000–7000 years (Harvey and Goodwin, 2004 ) has now ended, and sea level is undoubtedly rising in the post-industrial period (IPCC, 2007a ). Given the massive heat capacity of the ocean, the Earth is already committed to many more centuries of sea-level rise due to thermal expansion alone. The dynamics of the large polar ice sheets and the rapid retreat of glaciers and ice caps will significantly add to the magnitude of sea-level rise. The critical questions are: how much and how fast?

(…) 

Several lines of evidence point towards a sea-level rise by 2100 of perhaps a metre higher than 1990, which is at the upper limit of the IPCC ( 2007a ) range of projections. First, the mid-range of projections of semi-empirical models is centred around 1 m (Rahmstorf, 2007; Grinsted et al ., 2009 ). Second, observed sea-level rise is currently tracking at or near the upper limit of the IPCC projections (Rahmstorf et al ., 2007 ; Domingues et al ., 2008 ). Third, recent observations show an increasing rate of mass loss from both the Greenland and West Antarctic ice sheets (Cazenave, 2006 ; Rignot and Kanagaratnam, 2006 ). Finally, an analysis of the kinematic constraints on dynamical ice loss suggests a plausible increase in sea level of 0.8 m by 2100 (Pfeffer et al ., 2008 ).

Nevertheless, considerable uncertainty surrounds many of the processes that contribute to sea-level rise, especially concerning the dynamics of the large polar ice sheets. All of these uncertainties point in the same direction – towards more rapid and severe sea-level rise. Thus, rises higher than 1 m by 2100 cannot be ruled out.

(Graphs based on Robert Rodhe’s Global Warming Art)

Tipping points in the climate: Melting ice

January 7, 2009

(Dutch version here)

 

James Hansen has put the concept of ‘tipping points’ on the agenda. It is not a strictly defined term, but at a tipping point, a relatively small change has a relatively large consequence, and the climate could end up in a different equilibrium state than before. (Compare it with the concept of meta-stability, with the classical example of a ball on a hill, which needs only a minor push to end up in the valley – the new equilibrium state.)

 

Ice-albedo feedback

Large scale melting of ice could cause such a tipping point. Ice reflects a much larger part of the incoming solar radiation (i.e. it has a higher albedo) than land or water surfaces do. Therefore, when ice melts and the underlying land or water surfaces become exposed, much more sunlight will be absorbed than was previously the case. This causes more warming, which causes more melting, and the circle is closed.

 

Sea ice

The amount of Arctic sea ice at the end of summer has dramatically decreased over the last 30 years. The last two summers (2007 and 2008) ended with even smaller amounts of ice than would be expected based on the long term downward trend. It is too early to tell whether this means that the trend has changed (i.e. having passed a ‘tipping point’). Melting sea ice has no direct consequence for sea level, and if the warming trend is halted or reversed, the sea ice is expected to return to its ‘normal’ state. It is therefore a reversible tipping point.

 

arctic-sea-ice-trend-nsidc1 

Decrease of Arctic sea ice extent over the last 30 years. Data are for September, when the ice extent reaches its minimum. Source: NSIDC

 

Land ice

The melting of land ice, on the other hand, does lead to sea level rise, and is practically irreversible on human time scales. The melting of Greenland would lead to a globally averaged sea level rise of about 6 meters. The West Antarctic ice sheet is good for about 7 meters, whereas the remainder of Antarctica has enough ice for over 50 meters global sea level rise. But that’s not gonna happen any time soon, is the expectation. No major changes are happening in the Antarctic, and in some places in the interior ice mass even seems to be increasing, due to increased snowfall. This is a predicted consequence of slight warming, because it leads to more water vapor in the air. As warming continues, melting will at some point start to outperform the effects of increased snowfall.

 

Sea level rise

According to Hansen et al, “equilibrium sea level rise for today’s 385 ppm CO2 is at least several meters, judging from paleoclimate history.” This seems predominantly based on the fact that in the previous interglacial, 125,000 years ago, sea level was about 6 metres higher than now, while the average temperature was about 1 degree higher. Even when the CO2 concentration would stop increasing, the Earth would still continue to warm up by another 0.5 degrees, mainly due to the thermal inertia of the oceans. So we’ll approach the same global average temperature of 125,000 years ago, even with current CO2 levels.

 

It’s not evident to what extent the relation between temperature and sea level is linear. Over ‘short’ timescales, when thermal expansion is the main influencing factor, it is probably close to linear. Several equilibrium situations from the distant past also show a strong relation over longer timescales, mainly influenced by the amount of land ice. The whole idea of ‘tipping points’ is of course that changes happen stepwise, rather than smoothly.

 

sealevel_vs_temp_paleo 

Relation between sea level (relative to today) and global average temperature based on different epochs. LGM stands for Last Glacial Maximum, Eocene is also known as PETM (Pleitocene-Eocene thermal maximum), Eemian is the previous interglacial. YBP stands for years before present. Numbers are from multiple sources and are associated with a ‘certain’ degree of uncertainty.

 

To what extent can we translate relations between climate variables from the past to the current situation? Melting of polar ice mainly depends on the regional temperature, and its relation with the global average temperature is not necessarily constant. We know relatively little about dynamical processes that influence the breaking up and melting of land ice. But apparently large changes in sea level are possible if the temperature remains long enough above (or below) a certain value. The examples from the past may give a sense of what order of magnitude sea level rise we could eventually expect for a given temperature increase. The rate of sea level rise is the most uncertain. Most scientific literature concludes that sea level rise won’t be more than one or at most two meters by 2100 (but it will continue to rise thereafter). That is quite a strong increase for large parts of the world to adapt to, and uncertainty in the rate and level of the rise is not really comforting. The examples from the past are even less so.

 

Does the Deltacommission exaggerate?

October 12, 2008

(Nederlandse versie hier)

 

In the Dutch media there is some noise that the Deltacommission consciously exaggerated the dangers of sea level rise.

 

Fear mongering? 

On one of the Dutch news channels it was claimed that the commission wants to scare the citizens to make them accept the huge infrastructure project. Did they engage in fear mongering? That depends on whether the danger is being exaggerated. Pointing out a danger is not by definition fear mongering.

 

The commission is very clear that they take as a starting point a “plausible upper bound” to sea level rise. They did not present this upper bound (on the basis of scientific input) as being the most likely (as was claimed by eg Hans von Storch according to a Dutch newspaper); that would indeed have been an unfair representation.

 

‘Delta dictator’??

The same newsflash also contained the charge that the communication advice to the commission suggests that a charismatic leader, a so called ‘Delta dictator’, should quickly put the plans in motion. I couldn’t find any information about this charge on either the commission’s or the communication firm’s websites. A lot of the internet buzz refers back to the website of the news channel instead.

 

Comparison with WMD???

The newsflash ended with a comparison with the situation in the US just before the Iraq war. The Bush government tried to gain public support for invading Iraq by charging that “the Iraqi dictator has weapons of mass destruction”, whereas the evidence for this claim was very weak. It stands in no comparison with the scientific foundation on which estimates of sea level rise are based. Even though the uncertainties regarding its precise rise and timing are large, the evidence that sea level will continue to rise is very strong indeed. This is a totally different cup of tea compared with the unfounded charge of WMD. A bad ending of a tendentious newsflash.

 

Risk

The discussion should really be about what risk we are willing to take. Is it enough to protect ourselves against the most plausible sea level rise (according to current wisdom), or should we take into account a “plausible upper bound”? Should our horizon end at 2100, or should we also take into account what may happen thereafter? Should we only adapt to what may happen, or should we also try to prevent the worst (and bad) case scenarios? These are normative questions that should be discussed, on the basis of scientific knowledge with her inherent uncertainties. The Deltacommission clearly exposes her view on these issues (with the notable exception of mitigation). Instead of accusing them of fear mongering, the critics could perhaps have said that they are willing to take a higher risk. Okay, let’s discuss that then.

Sea level rise and the Dutch Deltacommission

September 9, 2008

(Nederlanse versie hier)

 

The Dutch Deltacommittee released their report on how to protect the Netherlands against the rising waters. The advice goes quite far, from the inevitable heightening of dikes to increasing the waterlevel of the major lake by 1.5 meters.

 

Starting point

The starting point is an estimate of sealevel rise of 0.55 to 1.1 metres by 2100 and of 2 to 4 metres by 2200. To their credit, the prognosis doesn’t stop in 2100, and neither does sealevel rise. In a business as usual scenario the sealevel will continue to rise (long) after 2200. The report mentions that they use a “plausible upper bound” of sealevel rise. A comparable range (0.5 to 1.4 metres by 2100) is found by extrapolating the observed correlation between temperature and sealevel rise.

 

The consequences of ice dynamics (e.g. mechanical instability of large ice sheets due to  more meltwater, associated with large uncertainties) is included in their estimate. That is the main reason that they are higher than the KNMI and IPCC estimates for 2100 (40 to 85 cm and 25 to 76 cm for 2100, respectively). A new article in the journal Science gives 80 cm as the most likely sealevel rise for 2100, and 2 metres as the upper bound (including uncertainties related to ice dynamics). Besides uncertainties regarding ice melt, the future greenhouse gas emissions are of course an important variable, and one that we can influence (for better or for worse).  Realclimate has a discussion of sealevel rise here and a thorough review of the IPCC estimates here.

 

Adaptation…?

Can the Netherlands adapt to such high sealevels? The Deltacommitte is very positive about that, at least up to a 4 metre (!) sealevel rise. I think that at some point the possibilities for adaptation become limited. For example, can the Netherlands continue to exist amidst a sealevel that is 6 metres higher than now? That’s what sealevels were 125,000 years ago, when the global average temperature was “only” 1 to 2 degrees higher than now. Such a massive change in sealevel probably takes centuries, if not millennia, to achieve, but it makes sense to me that we’d try to avoid it from happening nevertheless. Therefore we cannot afford to keep the earth very long or very high above the 1-2 degrees higher temperature. For how long or how high can we, before the big icesheets (Greenland and the Antarctic) start melting irreversibly? Nobody knows. But it’s not worth an experiment, I’d say.

 

Mitigation…!

The emphasis in the committee’s report is strongly on adaptation to rising sealevels, although emission reduction (mitigation) is of course also necessary. Otherwise it’s like mopping while the tap is running, as we say in Dutch. Is it perhaps better to relocate certain economic activities to safer grounds instead of pumping large sums of money into keeping them in the current low lying and vulnerable area? One comment I heard is from a member of parliament, who wondered whether our (grand)children would be happy  to pay for the measures we’d be taking now (because part of the money would be borrowed is the plan). I think that if anything, they will complain that we did too little for the problems facing us than too much.

 

To what extent should we anticipate (and thus adapt to and trying to prevent) future problems, that haven’t manifested themselves to their full extent? That’s a debate that’s worth having. Science doesn’t tell us which risks are worth taking and which are not; that’s very much an individual choice. Science does have an important role in informing us about the chances of a certain outcome; in other words, about the risk. The difficulty is that an individual’s choice in this matter has consequences for the risks of others, but their own risk is only marginally affected: the “tragedy of the commons”. Governments also have to make choices regarding the safety of their citizens. The same tragedy plays there too: The risk we run is strongly dependent on what other countries do. But let’s not use that as an excuse to then not do anything ourselves. And let’s not use scientific uncertainty as an excuse either. It does not decrease the risk; to the contrary.


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