NATIONAL PRESS CLUB ADDRESS
Sea level in a changing climate environment
23 August 2006
Professor Kurt Lambeck FAA
President, Australian Academy of Science
Introductory remarks
I welcome this opportunity to speak at the National Press Club. For the past six years the Academy of Science has given a Press Club Address on some topic of science or science policy. The practice has been for an incoming President to talk the first time about his/her area of science and that an opportunity arises later to talk about broader science policy issues. I welcome this practice because I have been in the position for less than four months and have hardly gotten around all the issues that Science in Australia is faced with.
The Academy of Science is a private organization of some 400 of Australia's leading research scientists covering the physical and biological areas, including medical research, applied sciences and mathematics. I am delighted by the news this morning that one of our fellows, Terrence Tao, has been awarded the prestigious Fields Medal. The Academy provides advise to Government on the scientific issues of today and for tomorrow. It is the link with the international science community and it fosters science education, something we heard about two weeks ago in Minister Julie Bishop's address here. But we are also acutely aware of the importance of public awareness of science and technology issues such that our society can contribute to informed discussion on matters that affect all of us, and that are too important to leave to Government Ministers only. Climate change, energy and resources, stem cell research. These are some of the issues about which you can expect to hear more from the Academy and I look forward to working with the Press in this debate.
Objective of address
At an earlier Press Club Address this year Ken Randall made the comment to me that one of the things that the Press Club does is to enable interesting esoteric bits of science or technology to be addressed that nevertheless actually affect the human condition. Today's address is one of that class. Changes in sea-level have affected humans in the past and will no doubt have repercussions in the future. Whether such change is good or bad is perhaps a matter for debate. But the fact that I am here at all is thanks to the past rise that created the country of my forebears. It is an issue of significance for society but at the same time it is an interesting science problem that ties together strands of information to reveal much about how the Earth System works.
What I propose to do in this address is to talk a bit about the past to give you some insight into the processes that determine sea-level change. I will then come to what the position is today and the role of the Intergovernmental Panel on Climate Change (IPCC) in reaching this knowledge. I will then turn to predictions. I will not pretend that I have got the answers. What I do hope to be able to give you is some appreciation of the complexities of the science and why it is not always possible to give unequivocal answers to deceivingly simple questions, a message that is surely equally true whenever science issues impact on societally important questions.
Past human experience – from myth to science
Over the past million or so years sea levels have been oscillating over about 120-140 m in height and each time it does so large low-lying areas are inundated or exposed. These changes occur on time scales of thousands of years, in phase with climate change: when the climate gets cold, ice sheets form and expand over the high latitude continents and sea-levels fall. During the warming phase the ice sheets melt and the sea returns.
That this is has impacted on the human psyche is seen abundantly in human mythology. The Atlantis legend is of a nation flooded and displaced to fight wars with its neighbouring Athinians. The Sumerian God Enki struggled against the sea as the Persian Gulf was progressively flooded. Noah took to his ark to avoid the rising damp.
All have their origins in the distant past when the last great ice sheets were disintegrating and sea level was rising at rates that caused coastal dwellers to shift their camps at regular intervals. Whether in the Persian Gulf or in the Gulf of Carpentaria, the encroachment of the sea onto the low coastal plains in a generation would have been enough to move camp over several km. It is from this collective struggle to understand this phenomenon that, I suspect, the legends appeared around the world wherever the coast was impacted by rising sea.
Faced with rising sea level is therefore not a new human experience. What is new is that the population has expanded, has lost its mobility through material possessions, and is prevented from moving inland by political boundaries. It is our reduced flexibility to respond that has changed. If future rise is going to be significant, rather more aggressive solutions will have to be attempted to hold back the rising sea, to reduce the rise, or to learn to live with it. Decisions on the appropriate approach requires a prediction capability of the future change that in turn requires an understanding of the underlying causes. The past record provides the background against which the causal processes can be quantified and on which the framework for the discussion of future change is built and within which orderly responses can be planned. Although, I am reminded that the Gods laugh when Man plans!
Relative sea level – ocean volume and land movements
What will be the change in the future? This is intimately related to the future of climate and to the future geophysical behaviour of the planet.
To explain this latter a short digression is necessary. When we talk about or measure sea-level change we refer to the change in the water level relative to the land level. This is what is of importance to coastal dwellers. They want to know if they will get wet feet when they get out of bed in the morning? It is this relative motion that is measured with the tide gauges around Australia's coast, for example. This relative change can mean a change in the ocean volume, a change in the land level, or – as is usually the case – because of a combination of the two.
The ocean volume contribution is closely related to climate and the land stability contribution is related to geophysics: to tectonic processes that push land up or down or land subsidence as water or gas is extracted from the soil and crust. To develop predictive models of what may happen in the future, the two components need to be understood and predictable. I will not talk about the land movement part here since it is my current area of research interest so that I can assume that this part is under control.
Glacial cycles
We are presently living in what is referred to as a climate optimum or interglacial. I should place the 'optimum' in inverted commas because today's climate may not be everyone's optimum and for some parts of the world change may actually be beneficial. It is a time when the continental ice sheets of the northern hemisphere have retracted such that the only remaining significant ice is in Greenland and Antarctica. Should the former melt, sea level will rise globally by some 5-7 m and should the latter disappear then there will be an additional rise of some 70 m. Of this about 5 m would come from the relatively unstable West Antarctic ice sheet.
The geological record indicates that these interglacials occur about every 100,000 years with a duration of typically 10,000 years. They are followed by a new cyclic onset of ice sheet development, culminating in a peak glaciation which then breaks down over about 10,000 years and leads into the next interglacial. Sea level follows this glacial behaviour, becoming increasingly lower to ~ 120-140 m below present some 20,000 years ago.
Broadly, these climate oscillations are driven by celestial forces – by the changing distribution of solar radiation received at the surface as orbital and rotational configurations change. There is little that can be done about these forces. Although that has not prevented suggestions that it can be, from Jules Verne to a US vice-presidential candidate who suggested that the planet's rotation axis could be displaced by controlled nuclear explosions – fortunately the calculations were shown to have neglected some basic physics and the experiment was not tried.
Modulation of the glacial cycles
This first part, the external forcing of the Earth-system, is largely predictable. But how the earth responds to it is determined by more terrestrial processes – by CO2 levels in the atmosphere, by land reflectivity and cloud cover, and by ocean and by continent configurations. The current interglacial has persisted for about 10,000 years and, ergo, the next ice age is about to start. Those of you of a certain age will recall that this was a prevalent view some decades back, that we were heading back to the freezer.
Lessons from the past – past interglacials
This is juggling with orders of magnitude and is not a basis for making predictions about the future. But it may be worthwhile to look briefly at the last interglacial to see what can be learnt. This period lasted from about 130,000 to 120,000 years ago and was the last time when conditions globally were broadly similar to those of today. The fragmentary records indicate that each time the planet returned to an interglacial state sea level was within a few meters of its present position.
Higher levels during the last interglacial mean that at some stage there was less ice than today and the highest levels found during this and any of the past interglacials are equivalent to a little less than either the current Greenland or West-Antarctic ice volume. Thus there appears to be a certain self-regulation in the present earth-system that discourages a total destruction of the ice caps, but smaller-scale reductions can occur that may raise sea-level a few meters above today's level. The important question is the time scale on which these natural changes occur. On this the palaeo record is still silent, not because the record may not be there but because we still do not have the tools to decipher it.
The end of an interglacial?
Interglacials do come to an end and it is reasonable to assume that this should occur on a characteristic time scale of, say, 1000 years. Is there evidence in the past record that may help understand how the transition from interglacial to glacial conditions occurred? Are there any warning signs pertinent to the current condition?
My own interpretation is that the record does reveal something about the future. A curious observations is that when the last ice cycle started, the formation of the new ice in Eurasia occurred over the shallow Kara Sea and the western Siberian Plain of northern Russia. This ice advanced and retreated several times before the large ice sheets – those we are accustomed to seeing in our geography text books – formed over Scandinavia.
What does this suggest? There are two requirements to form ice sheets: cold temperatures and a supply of moist air. Low temperatures alone are not enough. Today, precipitation in Siberia is low in winter and not enough snow accumulates during the cold months to build up a reserve that will survive the next summer. Today the Arctic Ocean is largely frozen for much of the year and there is little contact between comparatively warm sea-water and cold atmosphere, hence the low precipitation. But there is evidence that the polar sea-ice has been thinning over the past 50 years and if it were to disappear the situation may change dramatically. The contact of sea and atmosphere will increase the moisture supply during the colder months, there will be snow build up, the surface albedo will increase and more of the solar radiation is reflected back into space. Is this what occurred at the end of the last interglacial? Is this what is in store for planet Earth if under global warming the Arctic Ocean ice disintegrates. Can, paradoxically, the result be the initiation of the next ice age?
Before you all rush off to learn how to build igloos, there are too many ifs and buts in the argument for this to present a viable prediction of what may happen under on-going warming, if only because sea ice also has other roles in the climate story. I use this example only because it illustrates the many interactions that occur in the planet's climate. Dramatic feed-back consequences have occurred in the past and, if not as rapid as dramatized in 'The Day after Tomorrow, they will re-occur in the future. The climate system is inherently chaotic and despite the astronomical ordering, seemingly small perturbations can have major and still largely unpredictable consequences. It is unwise to add to these perturbations if one does not understand the consequences.
What is happening today?
So much for the past – what is happening today? For the past 100 or so years we have instrumental records for monitoring sea level. These are from the tide gauges. The measurements vary from place to place according to what the land is doing and how the ocean surface adjusts to changes in ocean volume. In Sydney Harbour, for example, sea level over the past century has risen by ~10 cm. In southern England, it has risen by about 19 cm and in Stockholm it has fallen by about 50 cm in the same interval. What does this mean? All of these localities are tectonically stable so other processes must explain this variability
Land movement, driven by the last ice age, is part of the story. I promised not to talk about this and I will simply note that when the earth is stressed it deforms so as to alleviate these stresses and sea level adjusts to the new conditions. This process continues after the ice loads have disappeared because of the earth's viscosity, and this is part of the background geological signature that explains much of what is observed in Stockholm or the south of England.
El Nino forcing
Superimposed on the century-scale trends are fluctuations on sub-annual to decadal and longer time-scales. Some of these are driven by the internal dynamics of the ocean and others are driven by weather and climate. Sea-level does respond to atmospheric forcing at seasonal and longer periods and perhaps the major signal from this source is associated with El Nino in which changes in sea level of tens of centimetres can persist for sustained periods. This leads to headline-making reports of Pacific Islands subsiding but it tell us more about the intensity of El Nino than about long-term trends and when the pressure oscillation has switched, the islands pop up again; this time unaccompanied by press comment.
This is not to trivialize the significance of what is actually happening. The consequences of the flooding are the same, whether it is land that is subsiding or sea that is rising: the essential message is that long-term change will be masked by higher frequency, larger amplitude oscillations unless the record length is much greater than the characteristic time scale of the oscillations. This means that even with new and perfect observing systems it will not be possible to identify long-term trends until the records are longer than the characteristic time scales of the oscillations.
What is the present-day rate?
With such words of caution can anything wise be said about whether the ocean volumes have been expanding over the past century or so? This has been attempted within the frameworks of the Intergovernmental Panel on Climate Change (IPCC). Large numbers of tide gauge records have been analysed and the contributions from land movements have been stripped away to leave a residual that is attributed to changing ocean volumes. This indicates that for the past 100 or so years there has been a rise in sea level at an average rate of ~ 1.5 mm/year and that there has been an increase in this rate in more recent times. Thus the average rate for the past 25 years is ~ 2.1 mm/year. More recently a new tool has become available to directly map the sea surface using satellite-mounted altimeters and these results indicate even higher rates, ~3.0 mm/year. But here my earlier caution about short records is appropriate here. All of these numbers are accompanied by uncertainties of about 20%.
When was the onset of this change?
The instrumental data starts at about the time of industrialisation. Thus a legitimate question is, is this rise of 1-2 mm/year a consequence of the industrial revolution? Or is it part of the long-term geological background signal? There are very few instrumental records that go further back and we have to look to other sea-level indicators. A key, if esoteric, observation turns out to be fish tanks constructed during the Roman Empire, 2000 years ago. The channels and sluice gates of these tanks were designed to hold fish and to be effective the tidal flow of water in and out has to be tightly constrained such that the salinity levels are maintained and fish do not escape. Today, the sea has risen enough to flood the tanks and no fish will be confined to grace a dinner table. Instead, the Roman folly has an unexpected use as a sea-level indicator, for if we extrapolate back from the modern rates we find that the Roman levels are reached only some 100 years ago. It does seem that the instrumental result is a phenomenon of only the past 100 or so years and it does suggest that something happened about 100 years ago that led to an increase in ocean volume.
I use this observation to illustrate how seemingly irrelevant observations can be used to extend the records into the pre-instrumental era. Climate research abounds with examples of proxy indicators of past conditions. But while scientists welcome such indicators as the means of accessing the past, detractors will use this as an excuse to throw the baby out, by attributing unwelcome change to the introduction of the proxy data. For example, the global temperature record that shows a substantial upturn towards the end of the nineteenth century, has sometimes been criticised because, like the sea-level data, it occurs near the transition from proxy to instrumental data. But such criticism ignores the careful work that has gone into the calibration of the proxy record and I don't accept that this is a basis for rejecting the outcomes. The alternative is to live in ignorance.
What are the contributory factors to modern sea-level change?
Leaving aside the land movements, what are the principal contributions to the modern change in sea level?
Mountain glaciers
Consider mountain glaciers. There are at least some 100,000 glaciers which, if they all melted would raise sea level by about 300 mm. From the historical records most glaciers have been retreating but direct measurements of the shrinking volumes are few. Global estimates, therefore, are model-dependent and their net average contribution to sea-level rise over the last century is about 0.3 mm/year and higher, ~ 0.8 mm/year, for the past two decades.
Greenland contribution
The fate of the large polar ice sheets has been monitored carefully in a few locations, as well as from space, but the records are still too short to draw conclusions about their overall long-term evolution. Thus the recently announced results for rapid ice loss in Greenland is only representative of the past three years and cannot be extrapolated to decadal and longer time scales. For the past century Greenland's ice sheet appears to have contributed perhaps only about 0.1 mm/year to global sea level, but possibly increasing into more recent time.
Antarctica, likewise appears to have suffered little loss of ice over the past century, losses at the coast being compensated by accumulation inland.
Thermal expansion of the oceans
The single most important contribution comes from thermal expansion of the oceans. Measurements of the temperature of the water column have been made systematically in many locations for several decades and while the rates of change are not everywhere the same, the overall trend is of a warming and expansion with an average sea-level rise of about 0.4 mm/year for the past 50 years and, about 1.6 mm/year for the much better instrumented record of the past 10-15 years.
When the various contributions independently evaluated are added together then they begin to resemble the observed rates but as stressed several times, uncertainties remain large, and it is from this uncertain base that we have to extrapolate to the future.
Can we extrapolate sea level into the future?
The principal contributions to sea level are climate driven and to predict the future of sea level climate prediction cannot be avoided.
The climate system is one of the most complex scientific problems that we have. The processes involved are physical, chemical and biological and they do not act in isolation of each other but interact in ways that are only beginning to be understood. These processes then have to be converted to mathematical equations, and effective numerical computational schemes have to be devised and squeezed into computers. Assumptions have to be made about what external forcing functions operate and the outputs have to be tested against past climate records that are far from complete. Thus it would not be surprising that different efforts under the same forcing may lead to different outcomes. If this were not true I would suspect that everyone was making the same assumptions and using the same computer code.
If differences between models occur this is not a reason for dismissing the results, but it becomes the basis for understanding what needs to be done to improve upon the results. It is out of these differences that the missing physics or chemistry is identified and from which improved computational schemes emerge. This is where the Intergovernmental Panel on Climate Change and similar global efforts to understand the future for the planet's climate play an important role.
The IPCC process
The IPCC aims to do at least two things:
- to make an assessment of the current state of knowledge of the climate system and to assess the quality of predictions based on this knowledge, and
- to assess the potential impacts of climate change and to identify possible options for adaptation and mitigation.
The IPCC process is initiated by the World Meteorological Organisation and the UN Environment Program and to some this may be cause for rejection alone. But the work is done by the world's scientists who have been selected because of their expertise, with some attention given to geographic distribution and to participation from the Developing World, and with some avoidance of the same scientists participating in successive assessments. Once identified, the various panels work under a set of common guidelines, with opportunity for interaction between panels, but otherwise free from structured central pressure.
The chapters in the reports are individually refereed by the wider science community at three stages. At the end of the 2001 process over 1000 scientists commented and made inputs into the final documents. An independent judiciary is set up to ensure that all criticisms are properly answered. The scientist amongst you will recognise what a nightmare this can be and no one in their right mind will want to be a lead author twice running.
In past and current rounds Australian scientists have been well represented and, despite my nightmare comment, probably most active Australian climate researchers have been involved at one time or another since the first assessment. Thus there is no basis for arguments that the Government should ignore the science outcomes of the IPCC report and listen instead to its own scientists because these have already strongly shaped the outcomes.
The 2006 report, the fourth assessment, is nearing completion but the results are still under review and not publically available. But because of how the science has been evolving over the past five years we should be able to anticipate the broad outcomes.
I have not been involved with the current report but was a lead author for the 2001 report.
Uncertainties of predictions
There has been a growing awareness over successive IPCC iterations that the assessment of confidence one can have in the outcomes is at least as important as the predictions themselves. Have the consequences of approximating or ignoring components of the process been evaluated? What happens if, under the same economic scenario, certain modelling parameters are modified, or if a new feed-back mechanism is introduced into the equations?
These are the type of questions that have been explored in recent years and I anticipate that in the 2006 report there will be a greater convergence of predicted results, not because of a consensus conspiracy but because the science of climate modelling actually has advanced.
Economic scenarios
But if there is a convergence of model results there will be less agreement on what is the appropriate economic scenario under which the climate models for the future should be run. I am not prepared to enter into this minefield because I am aware that economics is a bit like Greek politics. Collect three economists together and there are bound to be four economic theories.
Instead, I prefer to consider the scenarios as hypotheticals: Take alternate scenarios for future CO2 and other gas emissions and assess how the climate system responds in the different climate models.
The IPCC approach has been to define a range of scenarios that range from:
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steady-as-it-goes,
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a major reduction in emission output that will require us to switch of the lights before long,
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the developing world reaching emission levels equal to those of today's developed nations without any emission reduction.
This yields a spectrum of outcomes but all point to the same trend, one of ongoing rising greenhouse-gas concentrations, and of a global-climate response to these changes. What differs from one scenario to the next are the amplitudes and time-scales of the future trends.
Criticism of the IPCC result?
Where the 2001 IPCC report probably erred was (i) not to be sufficiently critical of the various model predictions, for clearly some were based on more complete science than others, and (ii) not to be sufficiently clear in separating out uncertainties in scientific knowledge and modelling from uncertainties arising from the future scenarios. By bundling all combinations of models and scenarios together the result gave an impression of far greater uncertainty than actually is the case. Critical readers of the science part of the report will be able to sort this out and reach their own conclusions but it is unrealistic to expect readers of the summary statements to do so.
Climate predictions in 2006?
I anticipate that the 2006 report will predict that global temperatures continue to rise up to the end of this century and that they will begin to plateau out at about this time only if present rates of CO2 and other emission are significantly reduced early in this century. A scenario with strong commitment to markets, an affluent world with high energy and materials demands, will only be able to keep this increase below about 2.5-3º C if it is accompanied by a technology-aggressive approach that would have to include more efficient generation, distribution and use of energy, clean coal technology and the permanent isolation of greenhouse gases, and new energy technologies including nuclear, renewable energy, fuel cells, etc.
The sea-level prediction
How the planet responds to a global temperature increase of ~3 degrees is a matter for another discussion, and by another person, and I will only comment here on the likely consequences on sea-level rise. Globally the levels can be expected to rise by about 0.4 m by 2100, a number that is not dissimilar to that obtained in the 2001 report for a similar economic scenario. Thus the globally averaged estimates for future sea-level rise appear to be reasonably robust. The principal reservation about this number is the contribution from the large ice sheets because the predictive models do not yet include realistic responses of these ice sheets to global warming, so that the actual rise may be larger. I suspect that the 2006 outcome will be similarly limited and research into the ocean-ice interface is a priority matter.
In the 2001 report, different models led to widely different predictions for the spatial variability of the long-range change and I anticipate seeing similar dispersion in the next report. But all models indicate that this spatial variability may be large, of the order of ±50% of the average rise. Add to this the temporal variability that occurs in sea-level now, and rises of the order of a meter persisting during El Nino cycles, for example, are likely to become increasingly common in this century.
Conclusions
I am reaching the end and I have not offered you simple answers and have not even attempted to give you simple solutions. But I hope that you have gained some understanding of the complexities of the science that underpins the oceans' response to climate and why such answers and solutions are not yet possible.
The present rates of sea level rise may appear small and insignificant against the background of the daily tidal changes, and I would not counsel you to sell your beachfront property on the strength of what I have said. But the hidden time-bomb is the longer-term acceleration that is becoming evident in the more recent observational data as well as predicted by the models. Thus if you intend to leave your beachfront property for enjoyment by your grand children then decisions about reducing emissions will have to be made soon.
