Antarctica: ice gain or loss?

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 20^th 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.

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Sea level versus temperature

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

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

(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.

 

 

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.

 

 

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.

 

“Delta-dictator”

(Nederlandse versie hier)

 

Update to the previous post on the Dutch Deltacommission being accused of exaggeration:

 

Via a Dutch newsreport the term “Delta dictator” has been widely circulated, but the source of the term was unclear to me until now. It appears to come from a report (in Dutch only) written by a consultancy firm, which provides an overview of opinions that arose during workshops held with stakeholders. As such, the contents don’t necessarily reflect the opinions of the Deltacommission, as is indeed mentioned in the foreword. 

 

The much discussed term was apparently brought up during a stakeholder meeting with young professionals, in the context of effective action in case of emergencies. I don’t find the term appropriate at all, but the importance of decisive acting when disaster strikes, and preparing for that with organizational measures, is evident.

 

Elsewhere in the report it is mentioned that charismatic persons can help in creating support for measures.

 

Compare this with the RTL newsflash, which said (my translation): “The communication advice to the commission suggests that a charismatic leader, a so called ‘Delta dictator’, should quickly put the plans in motion.” This is a misrepresentation of what was said at a round table meeting and then calling it “communication advice”. Hmm. 

 

Does the Deltacommission exaggerate?

(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

(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.