Sea level project re-aligned

We’re midway. Are we on schedule? For 1.5 years, researchers all over the UK have been trying to get a grip on interglacial sea level changes, within the iGlass project (official link). We’d like to know if sea level during periods of low polar ice cover fluctuates in general, and if so, how much then. Sea level is currently rising; how much faster can we expect it to go? Geological data suggests the fastest rates of sea level occur when there is loads of ice; quite as one would expect. But just the fact we don’t expect rates of several metres per century doesn’t mean we can sit back and relax. So we do not. And the time had come to see how far we had come. 

All researchers involved in the project from the various institutes gathered in Southampton, where our coordinator was based. And in a meeting room with a view on the very sea, we brought each other up to date. There is one team trying to constrain interglacial sea level changes using stable isotopes in foraminifera from the Red Sea (how they manage that is a complicated story – that merits a blog post in itself). A team in Oxford was trying to use dripstone formations for this purpose (same there!). We had been coring around in Norfolk and Cambridgeshire, looking for marine microfossils in sediments, to do our bit. And all that adventure was complemented by the sturdy attempts of several teams of data gatherers and modellers: the former would compile all information already available in literature so as to not have to do double work. And the latter would try to understand the distribution of land ice at the time intervals for which we had sea level data. They would also try to get a grip on the depression and subsequent bouncing back up of the Earth’s crust due to fluctuations in that ice; you can use that to detect where large masses of ice have appeared and disappeared (related to the process described here), and thus point to a specific ice mass as a culprit if you find a big sea level change in your data. 

 

The view from our meeting room

So what, other than just being kept in the loop, is the use of getting together? Well, science isn’t a linear process. A project never works out exactly as it was set out in the beginning. And one needs to adapt to such changes. 

We were faced with several changes relating to the people involved: five of us would move to a different institute than we started out in at the beginning of the project. Two go abroad; out of reach of our UKfunding agency, so we needed replacements. One of us would even leave science altogether. 

More detailed issues that needed to be discussed were for instance which interglacial periods (there are many; for practical reasons, we limit ourselves to the last five) we will focus on, and which time intervals within these periods. The previous interglacial (~125.000 years ago) is a favourable one, for the simple reason it is the most recent, so it’s best documented in the sedimentary archive. Three interglacials back (~400.000 years ago) is a special one too; the Sun and the Earth were configured in practically the same way as they are now (quite unlike in the previous interglacial), and it was a very long one; we decided prioritise these two. And the time slices the modellers will target are the ones for which the data gatherers have found the most information. 

 The home base of the project: the National Oceanography Centre in Southampton

Another issue to be discussed was that we, the micropalaeontological team, had only budgeted for fieldwork in the UK, but we had found out about much more promising sediments in the USA. Should we shift some money around so we could chase these up? It was decided we would. An exciting prospect opened up! 

Two days of presentations, discussions, and a nice dinner at the end of the first day later, we all dispersed again. We were all singing from the same hymn sheet again, and the music on it has been brought up to date with where our data has taken us!

Finally quantified

“Sea-level rise finally quantified”. That was a header on the BBC news website this week; it looked nicely definitive. Now we know how much sea level rise there is! Or do we only know that BBC news headers are short?

The header of the actual article was a bit more precise: “Sea-level rise from polar ice melt finally quantified”. It still sounded definitive. And it also sounded like it had never done before; finally, that lack of knowledge has been resolved. Is that true? Or do we now only know that 53 characters is still too little to say anything scientific?

The article gets to the point quickly; the first sentence is “Melting of polar ice sheets has added 11mm to global sea levels over the past two decades, according to the most definitive assessment so far.” A sentence only slightly bigger than a large tweet, but it carries the essence of the journal article in Science it discusses. There had been estimates of how much polar ice sheets contribute to global sea level before, but this time researchers from 26 institutes had pooled their knowledge, and together produced a result that was much more reliable than all individual efforts. What they come up with, though, still has a large uncertainty: the figure presented is 11.2 +/- 3.8 mm.

 A beautiful picture of the Antarctic Ice Sheet, found on Wikipedia, and taken by my dear old colleague Stephen Hudson.

Why is quantifying the mass balance of ice caps so difficult? There are two main methods of measuring this: one is satellite altimetry, and the other one satellite gravimetry. Satellite altimetry simply measures the top of the ice sheet and calculates its mass from that. And there are two main challenges associated with that: the first concerns the top of the ice sheet. Are you measuring the top of the ice? Or perhaps the top of several metres deep a pack of snow? It matters a lot for the mass you will calculate from your height measurements. 

The other problem concerns the bottom of the ice: you can’t measure that with your altimeter. You have to estimate that from other data, which might be somewhat imprecise. So if last year your measurement of the top of the ice at some point on an ice sheet was 200m above sea level, and this year it is 201m, does that mean there is 1m of ice more? Or has 5m of ice melted off, the whole continent bounced up half a metre as a result, and 5.5 m of fresh snow fallen on top? It’s an extreme example but it does illustrate the difficulties involved.

Satellite gravimetry measures the gravitational pull of the ice sheet concerned, so it needs not distinguish between snow and ice. But it sure needs to distinguish between ice and rock. This can in practice only be done with modelling, and that produces some of the uncertainty that is hard to get rid of. 

Another difference between altimetry and gravimetry, which can be used to one’s advantage, is that altimetry is localised, while gravimetry gives by definition a regional figure. If you use the one to verify the other, the accuracy of your estimates increases. 

 

A picture of Greenland, also from Wikipedia

The authors of this Science paper combined not only these different approaches, but also pooled the data from all the institutes they represent. That way they acquired much longer time series, and thus higher accuracy. If you have overlap in time and space you can calibrate the various data sets with each other. Their figure of 11.2 +/- 3.8 mm sea level equivalent mass loss over the period 1992-2011 can be considered the best available. 

In the Science paper, numbers are also given for the individual ice sheets, and for various time periods. These results show that East Antarctica is mainly gaining mass; that makes sense, as warmer seas tend to produce more snowfall over the continent. All the other ice masses (West Antarctica, the Antarctic Peninsula, and Greenland) are consistently losing mass. And all of them are accelerating; both Greenland and the Antarctic Peninsula display a four-fold increase in annual mass loss between 1992-2000 and 2000-2011. West Antarctica melt doubles between these periods. The mass that East Antarctica gains in the latter period is outweighed (by a factor of almost 2.5) by the loss of the rest of the continent. 

The authors of this work do not discuss the future, but one can hardly resist mentally extrapolating the graphs shown. A rise of 11 mm in 20 years may in itself not be much, but half of that has happened in the last 5 years. And this is only the polar ice caps; there is of course also the influence of factors like thermal expansion and melting low latitude glaciers. It would be nice if a similar effort was made to reconcile all records of the remaining components of sea level rise, and bring the uncertainty of these to a minimum as well. The real pressing question, on what the future will hold, can only be begun to be reliably answered when we know what is going on right now…

Fingerprinting the ocean

Which is more likely to cause flooding in Europe; the Greenland ice cap or the west Antarctic ice sheet? They are both currently melting. And we can measure how fast, but it’s only been a recent development we have satellites that can resolve this, so it’s hard to draw conclusions on future melt rates from that. We might want to look at the past. And glaciologists have ways to find clues on how big ice sheets have been in times gone by (like they show here), but that information is often patchy. Sea level itself provides clues too. There are ways of telling where water that runs into the oceans has come from.

If you have an ice sheet, and a part of that ice sheet melts, several processes take place. The ice sheet becomes lighter and smaller, causing the Earth’s crust to bounce back up like a lilo, and the ice also lessens its gravitational pull on the sea water around it. The whole sea would in effect flow away from the shrinking ice sheet. So strangely enough, the most sea level rise you would find would be on the other side of the globe. Near the ice sheet, relative sea level would only fall.

Modelled results of what happens if 1mm sea level equivalent melts from the Greenland Ice Sheet: the resultant sea level change ranges from <0mm (blue) to>1.2mm (dark orange). From: Mitrovica, Tamisiea, Davis and Milne, Nature 409, 2001

So what if the Greenland ice sheet melts? That would be ~6m overall sea level rise, so that would be felt everywhere, but the southern hemisphere would be hit hardest (apart from the northern hemisphere having many more big cities in low-lying coastal areas). For Europeans, it’s the west Antarctic ice sheet that’s the main threat.

So how can that feature be used? If you want to know where past sea level rises originate from, you need to make reconstructions at a wide range of latitudes. The spatial pattern of where the rise is highest to where it even may be negative will tell you where the water involved came from. Simply speaking, the hemisphere where you find the smallest rise is the culprit. This process is called "fingerprinting"; this term has a rather chemical ring to it, but sea level scientists use it in a more spatial way. And if you can then find out under what circumstances it is which ice sheet that reacts, you may get an idea of what will happen in our future. And that is information of which it is quite imaginable it will be ignored by the relevant authorities, but at least everyone with access to scientific literature will have an idea of where not to buy a house...

How high was sea level in the last interglacial?

Let me start with a disclaimer: I will not give a definitive answer to the above question. That being out of the way I can now freely discuss the difficulties of trying to answer a question like that.

It is, in a way, an interesting question: the last interglacial was only ~125.000 years ago, so the continents were all practically in the same location as they are now, and the oceans were equally deep. We know from ice cores and fossil plants and such things how high CO2 levels and CH4 levels in the atmosphere were. We know from ice cores, microfossils, and many other sources how warm it approximately was. So you could say that if you know how high sea level was back then, you know how high sea level is at these CO2 levels and these temperatures, and that might be useful. Additionally; if you can reproduce that in a climate model, you have reason to believe your model resolves sea level well, and may be able to predict it too. Unfortunately, it's not that simple. 

Picture of the 1953 flood

One thing is that we had already passed the CO2 levels of the last interglacial when the Mauna Loa observatory started measuring in the fifties. We may not yet have reached reached globally averaged temperatures comparable to those of 125.000 years ago, but it's hard to pinpoint that; every place on Earth has its own temperature history, and it's not easy to compare two periods that are so similar. And ice does not melt instantaneous. Just suppose we reach, this very day, the very temperatures we had in the last interglacial, the ice caps won’t have had time to adjust to that. And we don’t give them any; temperatures keep rising. We can only get an equilibrium when the situation remains stable for a while. That won’t happen in our lifetime!

So apart from the issues associated with a comparison, we still have this sea level issue to deal with. If you talk about “sea level” in a given time, without further specification, it almost automatically means “globally averaged sea level”. And the problem with that is that you can perhaps measure it today; satellites scan the entire Earth surface, and you can calculate the average of all their data. And sea level famously isn’t level; temperature differences, the rotation of the Earth, wind, gravitational pull of things such as ice sheets and so on, all make the sea surface rather bumpy. So you can’t measure past global average sea levels (further back than the satellite era), as it’s not possible to make a sea level reconstruction for every location on Earth covered by sea. So what can we measure? Local sea level, evidently.

There are many ways of reconstructing local sea level (such as this, this, this, this, and this), and every method, of course, has its own caveats. They also might not represent the same aspect of sea level; some might e.g. reflect low tide, while others are more representative for mean sea level. And tide ranges don't stay constant over time.

And if you have produced a local sea level reconstruction, you're not done; you also have to take into account that not only sea level in itself, but also the Earth’s crust might have gone up and down. Locations affected by earthquakes, volcanism, or (occasional) ice cover are prone to do a certain amount of moving vertically, and fast enough to pose a problem over the time scales discussed here. You can, of course, only use stable regions, but that leaves you with quite a small number of data points. An iconic paper by Robert Kopp and co-workers in Nature tried to extract a global average from a limited amount of data from the Last Interglacial. They, for instance, had no data points along the eastern and western shores of the Pacific; the only data from that ocean came from islands in the middle, and from its polar boundaries. They come up with an average of ~7m higher, but given the data scarcity it is hardly surprising they give rather large uncertainties. And these 7m, being a global average, of course are only a mathematical reality; at any specific location the value may be drastically different. 

The difficulty with reconstructing sea level also makes its prediction difficult.We only have good data coverage over the last few decades, and only moderate coverage over the last few centuries. If your sea level model manages to hindcast the patterns observed in that time interval, that doesn't necessarily mean it can resolve anything novel happening, like, say, the collapse of the West Antarctic Ice sheet. And that's exactly the sort of things we would like to get a handle on.

So if you headlines in the newspaper, on Twitter or wherever, that say something along the line of “sea level in period X (say, the last Interglacial) was #m higher/lower than today”, do realise it could mean all sorts of things. If one person for instance claims sea level was likely to be 8m higher than today in the Last Interglacial, and you read somewhere else it was 8m lower, it doesn’t mean one of them has to be wrong, as much as climate skeptics would like that to be the case. In this case it concerns one global average, and a local record from the Netherlands. And it's many, very many of such results, which may seem contradictory, that altogether will paint a comprehensive picture of past sea level. And in the long run, this will hopefully give us a solid grasp on future sea levels.

North Carolina and the sea - an update

Is North Carolina asking citizens to think for themselves? Regulations, put in place with the best intentions, often turn against their purpose. Think of regulations that limit the height of the ladders window cleaners are allowed to use in their work; the idea behind them is increasing the safety of those who clean windows. But it results in window cleaners having to resort to scaffolding or boom lifts more often; this makes their services much more expensive. With the result that people will try to clean their own windows. Without the experience that comes with the profession. This law is reported to only have resulted in an increase in window cleaning accidents… And boom lifts aren’t fail-proof either; I found seven cases of death and 19 of injury due to boom lift in the Netherlands since 2003.

Source: Aubrey Dale, Creative Commons

So well-meant laws often have a reverse effect. But could it work the other way? As I wrote in a blog post in early June; North Carolina was considering making it unlawful to base coastal protection policies on up-to-date sea level change research. This law didn’t make it; the House of Representatives have voted for its revision. The new version, which will have to get an OK from the governor before it becomes a law, contains the clause that the Coastal Resources Commission shall direct its Science Panel to actually study scientific literature, and come up with recommendations only after that. They have until December 2015 to write the report. So the threat of North Carolina making it unlawful to be informed about sea level change is gone. On the long term, that is. The new version also contains a clause that says, and I quote, “The Coastal Resources Commission and the Division of Coastal Management of the Department of Environment and Natural Resources shall not define rates of sea-level change for regulatory purposes prior to July 1, 2016.”

An example of coastal erosion from California

Science (the magazine) considers this bad news; those who wish to know if it’s wise to build a new house somewhere on the coast get no guidelines from their state for the next four years. But it also means they are allowed to think for themselves. If policy makers are forced to give out meaningless guidelines, it may well go unnoticed by the consumer of these guidelines that these are completely detached from reality. But when the state publicly announces silence on the topic, people will be quite aware they have to find out for themselves. And not everybody will be pleased that they pay their tax, but still have to spend time on making their own guidelines; and some people will undoubtedly find incomplete or inaccurate information in their search, but altogether I think it’s a much better situation than one in which it is downright illegal to do the sensible thing. So this revised law is a watered-down version of a law aimed at keeping the people ignorant; this version may serve to make them better informed that they were before!

The solution to sea level rise

Imagine the sort of country in which a threat is dealt with by the highest authorities by making it illegal to consider this threat. Let me guess; you are now picturing some Orwellian world. No need for such exoticism, I’m afraid! Just picture North Carolina. And I’m not joking.

North Carolina has a rather long coastline, with extensive coastal plains behind it. At many locations, the coastline  is eroding (look here for an interactive map). You would think a state like that is most interested in the latest developments in sea level research. You would want to know what the chances are of what areas being flooded or eroded away, right? If you have a coastal settlement, or are considering to build one, you would want to know how expensive it will be to defend it against the sea. If you may want to buy a house, you would want to know if it will be toppling into the sea within a few decades. But no.

North Carolina and its topography

What North Carolina wants is to ignore the problem and hope that that will make it go away. A bill has been proposed (Replacement House Bill 819; read it here yourself; it's hilarious!) which makes it illegal to try to accurately predict sea level rise. It also makes it illegal to adopt policies, rules or guidelines based on educated sea level predictions. You have to hand it to the Americans: they do think outside the box sometimes. So far out you are surprised any adult would be willing to admit to such thoughts.

So what will they base their policies on? If this bill will be passed, it will only be legal to make sea level predictions by linearly extrapolating sea level change since 1900. So that’s sorted then! The sea will surely be so polite as to take heed of such legislation. Those who want to live near the sea are best off to do so in this enlightened state; in other states, the sea might very well do what she pleases!

 So far the sea has not refrained from jeopardising property: coastal erosion at Cape Hatteras, NC. Picture: Gary Braasch, taken from the BBC website.

If this bill is passed it may set a precedent; I can see more problems being tackled. I can see a law in predicting economic growth (or shrinking). Only extrapolations from the last month could be allowed! That will sort the economy out.

But jest aside; evidently a bill like that can be proposed. And if you can propose a bill that makes it illegal for authorities to inform civilians about sea level rise, and protect them against it, then what else can you propose? There are many other things one would hope local and regional authorities protect their citizens from; can all of that be made illegal at the whim of someone with a vested interest and a big mouth? I sure hope the answer is no...



How to make a sea level record

“So how do you know how high sea level has been in the past?” I get that question a lot. It’s a good question; if you’re not a sea-level scientist (because that’s cheating) you might wonder how something as elusive and evanescent as sea level could be traced back in time. One may think of marine features above the reach of modern tides such as raised beaches, or coral reefs, or signs of wave erosion, such as wave notches. And as signs of sea levels having been lower one can think of drowned ruins. But these are all isolated indicators. What if you want to know what happened over a long period of time?

A fossil coral reef in a forest, which is clearly a terrestrial environment; a clear indicator of relative sea level having been higher in the past. Picture taken in Windley Key Fossil Reef Geological State Park

 

When geologists and geographers want to have a continuous record of something they often stick a corer into the ground.  If you pick the place where you do that right, you get a core comprised of sediment that has been deposited over the time period you are interested in, and contains the information you need. So just suppose you want a sea level record that goes back 500 years, how would you go about that? You need to find a place near current sea level where sediment has been deposited in a calm environment for quite a while, and where there are things in the sediment that record the position of the sea. Think about it: not many places will do. Many coasts are erosive. Many are energetic. Many just contain sand, with dead shells, that have washed ashore. So what did the scientific community come up with? The salt marsh.

 A salt marsh

A salt marsh is an environment that gets flooded periodically by tides, and where salt-tolerant plants grow, trapping sediment. If sea level rises slowly, they can build up; if it drops slowly, they can build out, provided there is space for that. If sea level changes too fast the marsh will become either a fresh-water environment, or drown.

On the surface of salt marshes not only plants grow; they tend to host a plethora of micro-organisms too. Among these are foraminifera and diatoms. Foraminifera are small animals that build a shell, either of calcite or of particles they find. Diatoms are algae that build a kind of shell from silica. And there are countless many species in either group. And all of these species have their own preference for how often they are in the water and how often they are dry, and how salt the water is they live in, and many more of these factors. In other words: they are picky about where they live with respect to sea level. And the good thing is: they take that pickiness with them into their graves. The organisms themselves decay, but under favourable circumstances their shells are preserved. Ready for a passing geographer to interrogate…

A sediment core from a salt marsh

What we do is roughly as follows: we take samples from the surface of the marsh over a transect from high to low. We also take a core, trying to get the location right so that the entire period of interest is preserved in its sediments. Using surveying equipment we find out at what elevation these are all taken from.

Later, in the lab, we determine which species of the microorganisms of our choice we find in the surface samples, and their absolute and relative abundance. That tells us at what elevation the various species live. Then we determine the species assemblages in samples taken from the sediment core. Given that we have established at what elevation interval these species live we then know at what elevation these samples must have been deposited. And if we, for instance, take a core and find an assemblage typical for an elevation of 30 cm above sea level at half a meter down in the core, we know that when these sediments were deposited sea level was 80 cm below the current land surface. And if you do that for every centimetre of the core, you get a rather detailed record. If you then manage date the core with good precision you have your sea level reconstruction!

This is what you would then see through your microscope. A salt marsh foraminifer is encircled.

We tend to look for salt marshes with a tide gauge nearby; these record sea level in real time. The oldest records go back hundreds of years; if our reconstructions match these records we have faith in them, and assume they also describe the period from before the start of the tide gauge record as reliable.

So after all that work, what are the questions we want to answer with this kind of research? There are many. One is, for instance, if we find accelerations in sea level rise of which the timing is consistent with the industrial revolution as a possible cause. Another one is how fast such accelerations in the past have been; it might be good to know how fast such processes can take place. Another one is if we can attribute specific sea level changes to specific ice caps; if so, we get some idea of what to look out for. If the West Antarctic Ice Sheet has collapsed in the past, and we can figure out how fast that went, we know what to get ready for. It may happen again! And how we can distinguish between the various ice caps (there are many other sources of sea level change, of course, but in the long run the ice caps are the biggest players) is material for another entry…

The author on a body of ice. Not an ice cap, admittedly.

iGlass

iGlass

“Don’t get bogged down in the marsh work!” Whether that was a witty comment or not I’ll leave to the readers, but it sure was the concluding remark of the iGlass kick-off meeting. Some may think iGlass is one of the newer Apple products; it seems there once was an April Fool’s day spoof claiming Apple had indeed produced some sort of cyber-glasses. And there's more of such to be found; no idea if they're real, but there seems to be an iGlasses app that distorts images, and there are iGlasses for the blind, with obstacle detection... But in this context it’s a big scientific project, funded by the Natural Environment Research Council (NERC), aiming at studying interglacial sea level.

So why is interglacial sea level worth so much of your tax money? We are currently living in an interglacial, sea levels are rising, and billions of people live pretty close to sea level. In order to not let the lives of countless many of these get disrupted we need to know how fast sea level can rise under circumstances like those of today. And where can one find information on such things? In past interglacials. Earth has experienced a switching between cold (glacial) and warm (interglacial) states for roughly a million years now; an interglacial occurs approximately once every 100.000 years. Sediments from these periods have survived, and they contain a wealth of information on the environment in these periods, available for those who are willing to interrogate them thoroughly. And that information will provide a good idea of how fast sea level can rise by how much in times like ours.

Policy makers need such input to base their adaptation policies on. If you know within what range sea level changes will be, you might know whether you can get by with strengthening your defences, or whether it’s time to start building wholly new ones. Or perhaps plan a retreat, if defence isn’t economically feasible.

So why the reference to marsh work? This project seeks its data in four fields: sea level information contained in dripstone cave sediments (speleothems), corals, Red Sea sediments, and tidal marsh microfossils. And yours truly is part of the team that will scour the lands for interglacial salt marsh sediments to which we can apply our usual method of sea-level reconstruction. What is that? Explaining that will take another blog entry. We have already found salt marsh sediments with micrfossils we can prod. That, too, is material for another blog post. The project will run for four years, three of which involve me; the stream of blog entries from this project will be considerable. Welcome to iGlass!