Holiday recommendations – blog break summer 2018

Holiday recommendations – blog break summer 2018

Even dedicated workaholics such as the editors of your EGU GD Blog Team sometimes deserve a break! Let me clarify that by saying ‘an intentional break’ (because uploading every Wednesday is hard!). We will be ‘on holiday’ during August, so there won’t be any new blog posts then. But don’t worry: we will be back stronger than ever in September and we already have a lot of very good blog posts in the pipeline for you. To start the holidays properly and to get you in the holiday spirit as well, the EGU GD Blog Team shares their geodynamical holiday recommendations with you. Enjoy & relax!

Iris van Zelst – Edinburgh

Hutton’s Section with a very young me (in 2012) for scale

Go. To. Edinburgh. Seriously: Edinburgh is the place to be for anyone who has an affinity with the Earth sciences. In this beautiful, historic city, James Hutton – the founder of modern geology, who originated the idea of uniformitarianism – lived and died. Everywhere in the city you can find little reminders indicating this iconic scientist lived there. You could, for example, visit his grave, and hike to his geological section on Edinburgh’s Salisbury Crags. There are also little plaques spread around the city that mark significant James Hutton places and events. The city itself is also steeped in a mix of geology and history: Edinburgh Castle, situated on the impressive volcanic Castle Rock, boasts an 1100-year-old history and towers over the city. Directly across from the castle, connected by the charming Royal Mile is Holyrood Palace, where you can soak up even more history – Mary Queen of Scots lived here for a while. Nearby, there is Holyrood Park where you can find the group of hills that hosts Hutton’s Section and a 350 million year old volcano named Arthur’s Seat. Climb it when the weather is nice and you will have the most amazing view of Edinburgh. The whole park is perfect for day hikes and picknicks.
Even if you (or your travel buddy) are not that into Earth Sciences (or history), Edinburgh has plenty of other attractions. It is the perfect place for book and literature lovers with the large International Book Festival every August and a very rich literary history with iconic writers such as Walter Scott (Ivanhoe), Robert Louis Stevenson (Strange Case of Dr Jekyll and Mr Hyde; Treasure Island), Arthur Conan Doyle (Sherlock Holmes), and – more recently – J. K. Rowling (Harry Potter). Theatre fans will also love Edinburgh, particularly during August when it hosts the Edinburgh Festival Fringe – the largest arts festival in the world.
I totally should’ve booked a trip to Edinburgh this year… Learn from my mistakes and enjoy it in my stead!

The view of Edinburgh when you’re standing on top of Arthur’s Seat: a more than 300 million year old volcano. Pretty epic.
Picture by me in 2012 (also: proof that the weather can be good in Scotland!)

Luca Dal Zilio – Aeolian Islands

My recommendation? I vote for the Aeolian Islands! Smouldering volcanoes, bubbling mud baths and steaming fumaroles make these tiny islands north of Sicily a truly hot destination. This is the best place to practice the joys of “dolce far niente“: eat, sleep, and play. The Aeolian Arc is a volcanic structure, about 200 km long, located on the internal margin of the Calabrian-Peloritan Arc. The arc is formed by seven subaerial volcanic edifices (Alicudi, Filicudi, Salina, Lipari, Vulcano, Panarea, and Stromboli) and by several volcanic seamounts which roughly surround the Marsili Basin. The subduction-related volcanic activity showed the same eastward migration going from the Oligo-Miocene Sardinian Arc to the Pliocene Anchise-Ponza Arc and, at last, to the Pleistocene Aeolian Arc. My favourite island, Stromboli, is one of the few volcanoes on earth displaying continuous eruptive activity over a period longer than a few years or decades. I like Stromboli because it conforms perfectly to one’s childhood idea of a volcano, with its symmetrical, smoking silhouette rising from the sea. Most of this activity is of a very moderate size, consisting of brief and small bursts of glowing lava fragments to heights of rarely more than 150 m above the vents. Occasionally, there are periods of stronger, more continuous activity, with fountaining lasting several hours, violent ejection of blocks and large bombs, and, still more rarely, lava outflow. I can’t quite explain what made it so special to me. It may be because Stromboli itself is an island, and all the time during the hike I enjoyed splendid sea views (with a beer in my hand). It may be the all encompassing experience, where I could see, hear and literally feel the lava explosions. It was simply fantastic.

Credit: Flickr

Anne Glerum – Montenegro

In case you don’t make it to Montenegro/Serbia this summer, it’s fun in winter too. And yes, it’s fun in spring too – there’s snow, mountains and a younger me on a tiny sled. Photo courtesy of Cyriel de Grijs

My geo-holiday-destination: Montenegro!
A summer without beach-time is not a summer to me (already got one beach-day in this year, phew). Being Dutch, a proper holiday also requires some proper mountains – or hills at least. And no trip is complete without cultural and culinary highlights to explore.
Montenegro is a country that ticks all the boxes. Situated along the Adriatic Sea it hosts a score of picture-perfect beaches; quiet or taken over by the jet-set, intimate coves or long stretches of white sand, take your pick.
Further inland, you reach the Dinarides orogenic chain, the product of 150 My of contractional tectonics and later collapse during the Miocene. Traversing the chain into neighboring Serbia will lead you past complete ophiolite sequences, syn-orognic magma intrusions and major detachment zones of the extensional orogenic collapse.
Visit the centuries old fortified coastal cities of Budva or Kotor or one of the many churches and frescoed monasteries spread around the countryside. For more bodily sustenance, enjoy the fresh fish dishes, rich meats or the regional cheeses and yoghurts. Seasonal fruits are eaten for dessert or, even better, turned into wine and rakija. Ehm, why I am not going there again this year – this time in summer?

Not-so-sunny spring view from St. John’s fortress onto Kotor along the Bay of Kotor. Photo courtesy of Cyriel de Grijs

Diogo Lourenço – CIDER Summer School

This year, my favourite geodynamical destination is CIDER 2018! It’s far from holidays… but it’s really cool! For the last three weeks (one week to go), we have been intensely learning about heterogeneity in the Earth, and trying to understand it in an interdisciplinary perspective with contributions from geochemistry, geodynamics, and seismology. Quite an intense schedule and a lot of information to process, but I think we are all learning a lot, and hopefully in the future we will use more constraints coming from other fields into our own work. Oh, and did I mention that it is happening in Santa Barbara? Great Californian weather, beautiful coastal landscapes, barbecues by the beach, and swimming in the ocean, all sprinkled with scientific discussions! Quite the geodynamical destination, no?

Just had to cross the street from the KITP building where the conference is happening to take this photo…

Grace Shephard – Svalbard

Geoscientists are no strangers to travelling to exotic places and many of us take the opportunity to turn a work-related trip into potential holiday scouting. My suggested destination is most probably the northernmost point you can quite easily travel to on this planet – Svalbard.
Svalbard is an Arctic archipelago located around between 74-81°N latitude. It is sometimes confused with Spitsbergen, which is actually the name of the largest island where the main settlements, including Longyearbyen and Barentsburg, are situated. The islands are part of Norwegian sovereignty, though with some interesting taxation and military restrictions (the Svalbard Treaty of 1920 makes for some pretty interesting reading). Svalbard is host to a stream of tourists and scientific researchers year-round, and this week I will travel back to Longyearbyen as a lecturer for an Arctic tectonics, volcanism and geodynamics course at the University Centre in Svalbard (UNIS).
Geologically speaking, Svalbard makes for a very interesting destination. It offers a diverse range of rock ages and types; having experienced orogenic deformation events, widespread magmatism, and extensive sedimentary and glacial processes.
If you’re after a more usual tourist package amongst the draw cards are of course iconic polar bears (though please keep your distance), stumpy reindeer, arctic foxes, whales, birds and special flora. There are many glaciers – in fact around 60% of Svalbard is covered in ice – as well as fjords and mountains, former coal mining settlements… the list goes on. You are even spoilt for choice between midnight sun or midday darkness, depending on the time of year, so prioritise your activities wisely. Plus, did I mention those miles and miles of unvegetated, uninterrupted rock exposures to keep any geology enthusiast happy?… if you’re lucky you might come across some incredible fossil sites.

Itinerary recommendation, tried and tested: Whale watching and fjord cruising to a Russian mining ghost town (Pyramiden) followed by an important sampling of the world’s northernmost brewery.

50 years of plate tectonics: then, now, and beyond

50 years of plate tectonics: then, now, and beyond

Even if we cannot attend all conferences ourselves, your EGU GD Blog Team has reporters that make sure all significant geodynamics events are covered. Today, Marie Bocher, postdoc at the Seismology and Wave Physics group of ETH Zürich, touches upon a recent symposium in Paris that covered one of the most important milestones of geodynamics.

On the 25th and 26th of June, the Parisian Collège de France was celebrating the anniversary of the plate tectonics revolution with a symposium entitled 50 years of plate tectonics: then, now and beyond. For this occasion, the organizers Eric Calais, Anny Cazenave, Claude Jaupart, Serge Lallemand, and Barbara Romanowicz had put together a very impressive list of presenters, starting with Xavier Le Pichon, Jason Morgan, and Dan McKenzie during the first morning!

The very impressive program of the 50 years plate tectonics symposium

Needless to say, it was a blast, and a great occasion to focus on the big picture and reflect on the evolution of Earth sciences within the last 50 years.

Watch it online!

But don’t panic if you missed it: all the presentations are available online now on the Collège de France website. So relax, brew yourself a cup of coffee, and enjoy the symposium from the comfort of your own home 🙂

Xavier Le Pichon
Image courtesy of Martina Ulvrova

Important panel
Image courtesy of Martina Ulvrova

Dietmar Müller
Image courtesy of Marie Bocher

To serve Geoscientists

To serve Geoscientists

The Geodynamics 101 series serves to showcase the diversity of research topics and methods in the geodynamics community in an understandable manner. We welcome all researchers – PhD students to professors – to introduce their area of expertise in a lighthearted, entertaining manner and touch upon some of the outstanding questions and problems related to their fields. For our latest ‘Geodynamics 101’ post, Fabio Crameri, postdoctoral researcher at the Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Norway, joins us again. Continuing from his earlier post on the harmful use of the rainbow colour map, Fabio shares his thoughts on some of the expressions and phrases used in the community that propagate confusion, and how the new “Ocean-Plate Tectonics” concept offers relief for at least some on them. 

Blog author Fabio Crameri – in a shirt that translates from Tamasheq as “deserts” or “empty spaces.” You can expect no empty spaces in your lunchtime conversations after reading this post.

Do you, after reading the title, still wonder what this blog post is all about?
I’ll give you a hint, it’s about the Earth. No, wait, it’s about Earth, or perhaps earth, or isn’t it? And maybe it is a little bit about Moon, I mean the Moon. But also, it is about the Venus, I mean Venus.

It is not confusing, it is just well mixing.

You’ve got it; it is about confusion in the Geosciences. Confusion caused by symbols, letters, words and phrases through misuse, ambiguity or over-interpretation. So, after all, this is a blog post about geo-semantics rather than about culinary excursions.

The Geodynamics community is a diverse group of people with different backgrounds, native languages and customs. This is an attractive breeding ground for semantic related problems, particularly when you throw in some inherent peculiarities of the English language in which we largely operate.

Some use the symbol “a” for years, for years and years.

In line with a widely used standard definition (Holden et al., 2011) – but against the common convention of the Geosciences – the author of this blog post was using the unit of time “a”, or arguably just its symbol, for “years” (and I mean calendar years, neither financial years nor dog years), for years and years. A distinction between discrete points in time and the duration of time is at the heart of this confusion, and indeed has plagued a sub-selection of discussions, working groups and interpretations of the International System of Units (SI; e.g., Christie-Blick, 2012).

Figure 1. An ambiguously phrased situation near the recent end of the Cretaceous.

The symbol “a” for “annus” [year] (“Ma” being the symbol for 106 years, or “mega-annus”) in the Geosciences is most commonly used for a specific time or date in the past as measured from now. For example, “At 65 Ma (which is 65 Myr ago), the dinosaur looked up the sky.” (see Figure 1). On the other hand, “yr” for “year(s)” is commonly used for a duration of time, as in “The Cretaceous period ran for 79 Myr (from approximately 145-66 Ma).”. Other mutations within the convention of time in the Geosciences include “My”, “Myrs”, “Mya” or “m.y.” for “Millions of years”. Thus, the time unit and symbols for multiples of a “year” are likely amongst the most ambiguous expressions in the Earth Sciences, likely because, in contrast to the “second”, a universally applied scientific definition for the “annus” still remains elusive (Thompson and Taylor, 2008).

Such quibbling over semantics may seem petty.

Amongst other examples to cause geodynamic misunderstandings (e.g., Figure 2) might be the misuse of the phrase “stagnant slabs”? Are slabs ever really stagnant? Or are they just being deflected, slowing down, interrupting their downward motion, not directly entering the lower mantle at the same speed and trajectory as before?

Figure 2. One ambiguously phrased geodynamic explanation.

From the literature, you might be forgiven for having the false impression that slabs either fully stagnate around the upper-mantle transition zone or directly and effortlessly penetrate it; they likely do neither of the two (as explained in e.g., in an earlier Geodynamics101 post here).

When these slabs sink, and not temporally stagnate, they induce flow in the surrounding mantle. “Slab suction” is the downward suction induced by the nearby mantle that is set in motion through its dynamic coupling with the slab [e.g., Conrad and Lithgow-Bertelloni 2002]. Or isn’t it? “Slab suction” is also contrarily used as an upward directed force on the slab itself that is induced by the upper plate and might foster low-dipping shallow-depth slab portions in the uppermost upper mantle (unambiguously speaking of which: see again Figure 2).

The downward directed version of “slab suction” can induce “dynamic topography”. Estimates of the maximum amplitude of “dynamic topography” on Earth range from only a few hundred meters up to a few kilometres (see e.g., Molnar et al., 2015 and references therein). Such unusually large ranges of estimates are, as a general rule, a quite solid indicator for an underlying ambiguous definition, or in this case, rather a mix-up of multiple different definitions for the term “dynamic topography”. 

If you’re not confused, you did not pay attention.

As I keep talking about geodynamics, I hope we are all on the same page about subduction, one of the key players: Let’s assume planet XY has one single active subduction zone. Another subduction zone initiates on the opposite side of the same planet. Did “subduction” start once or twice on that planet?

It started once on that planet. Because “subduction” describes a process and not a physical feature; it is nonetheless easily mistaken for a physical feature.

And what about “plate tectonics”, the 50 yr old overarching concept that fascinates us, and for so many of us has become the foundation of our professional lives. Let’s approach this by considering the big question: When did “plate tectonics” start? Serious opinions in the plate tectonics community range from around 850 Ma (Hamilton 2011) all the way back to 4.3 Ga (Hopkins et al., 2008). – Remember what unusually large estimate ranges often indicate? – It is not surprising that the only commonly accepted specific answer everyone seems to agree on currently is that it depends on the very definition of plate tectonics.

So, what is the definition of “plate tectonics”? According to its original formulation, “plate tectonics” is the horizontal relative movement of several discrete and mostly-rigid surface-plate segments (Hess, 1962; see the corresponding visual representation in Figure 3). A generous interpretation of the original formulation might additionally define the plate-interface nature, but that is all.

Figure 3. As long as it is not overinterpreted, there is nothing wrong with the original definition of plate tectonics that solely describes the horizontal motion of several discrete surface plates: It does not discriminate the oceanic from the continental plate, does not consider the important framework of mantle convection, and does not specify the underlying key driver of the surface motion.

Considering the knowledge we have gained about the moving surface plates and their underlying causes and consequences during the past 50 yr, this is an extremely broad definition: As of today, we know that (A) the surface plates with their relative motion are an integral part of whole mantle convection (Turcotte and Oxburgh, 1972), that (B) Earth’s surface has a characteristic bimodal nature due to the partitioning into long-lived continental plates and short-lived oceanic plates (e.g., Wilson, 1966), and that (C) the latter are mainly driven by their very own subducted portions (i.e., all or parts of their slabs; Forsyth and Uyeda, 1975; Conrad and Lithgow-Bertelloni, 2002).

A clear, unambiguous and up-to-date definition for such a crucially important, wide-reaching concept is imperative. It is therefore not surprising that less ambiguous re-definitions have been suggested recently. To avoid propagating confusion, the introduction of alternative phases of plate tectonics that describe the various different possible modes of mantle convection during Earth’s evolution have been cast into the arena (e.g., Sobolev 2016). These include “plate-tectonics phase 1”, in short “PT1”, describing regional, plume-induced plate tectonics (e.g., until 3.0 Ga), “PT2” describing episodic, global plate tectonics (e.g., between 2.5-1.0 Ga), and finally “PT3” describing stable, global plate tectonics (e.g., 1.0-0.0 Ga). Other efforts result in different naming conventions, such as “modern plate tectonics”. However, apart from the fact that “modern” is a time dependent term, “modern plate tectonics” might be a somewhat unfortunate expression, as other planets like Venus might have undergone different, modern styles of plate tectonics than present-day Earth.

Stern and Gerya (2017) then actually suggests an entire update to the definition of “plate tectonics”:

“A theory of global tectonics powered by subduction in which the lithosphere is divided into a mosaic of strong lithospheric plates, which move on and sink into weaker ductile asthenosphere. Three types of localised plate boundaries form the interconnected global network: new oceanic plate material is created by seafloor spreading at mid-ocean ridges, old oceanic lithosphere sinks at subduction zones, and two plates slide past each other along transform faults. The negative buoyancy of old dense oceanic lithosphere, which sinks in subduction zones, mostly powers plate movements.”

Unfortunately, such a re-definition of the same old phrase makes it impossible to know which version of the definition (i.e., the original or the updated one) an author of a subsequent study should be applying and referring to.

In an effort to prevent all of the above problems, we recently introduced an entirely new concept; one that can coexist in harmony with the original definition; one that fully captures the dynamics of the oceanic plate according to our current knowledge. The concept is called “Ocean-Plate Tectonics” or, if you really like the term, “OPT”.

“Ocean-Plate Tectonics is a mode of mantle convection characterised by the autonomous relative movement of multiple discrete, mostly rigid, portions of oceanic plates at the surface, driven and maintained principally by subducted parts of these same plates that are sinking gravitationally back into Earth’s interior and deforming the mantle interior in the process.” – Crameri et al. (2018).

“Ocean-Plate Tectonics” captures not only the relative horizontal surface motion of plates, but crucially also accounts for (A) the importance of the whole mantle framework, (B) the bimodal nature of Earth’s surface plates, and (C) the underlying engine of the surface-plate motion (see Figure 4).

Figure 4. “Ocean-Plate Tectonics”, the unambiguous up-to-date definition describing the dynamics of the oceanic plate that crucially incorporates the bimodal nature of Earth’s surface, the convecting-mantle framework, and the key driver of surface-plate motion (after Crameri et al., 2018).

“Ocean-Plate Tectonics” is here to serve Geoscientists.

The concept of “Ocean-Plate Tectonics” is intended to bring together the extremely diverse research communities, but also the general public, to meet on common, fruitful ground in order to discuss and further develop our understanding of the fascinating dynamics involved in Earth’s plate-mantle system; the unambiguous “Ocean-Plate Tectonics” is here to serve us.


Christie-Blick, N., (2011), Geological Time Conventions and Symbols, GSA Today, 22(2), 28-29, doi: 10.1130/G132GW.1

Conrad, C. P., and C. Lithgow-Bertelloni (2002), How mantle slabs drive plate tectonics, Science, 298 (5591), 207–209, doi:10.1126/science.1074161.

Crameri, F., C.P. Conrad, L. Montési, and C.R. Lithgow-Bertelloni (2018), The life of an oceanic plate, Tectonophysics, (in press), doi:10.1016/j.tecto.2018.03.016 .

Forsyth, D., and S. Uyeda (1975), On the relative importance of the driving forces of plate motion*, Geophysical Journal of the Royal Astronomical Society, 43(1), 163–200, doi:10.1111/j.1365-246X.1975.tb00631.x.

Hamilton, W.B. (2011), Plate tectonics began in Neoproterozoic time, and plumes from deep mantle have never operated, Lithos, 123, 1–20, doi:10.1016/j.lithos.2010.12.007.

Hess, H.H. (1962), History of ocean basins, Petrologic studies, 4, 599–620.

Holden N.E., M.L. Bonardi, P. De Bièvre, P.R. Renne and I.M. Villa (2011), IUPAC-IUGS common definition and convention on the use of the year as a derived unit of time (IUPAC Recommendations 2011, Pure Appl. Chem., Vol. 83, No. 5, pp. 1159–1162, 2011. doi:10.1351/PAC-REC-09-01-22

Hopkins M., T.M. Harrison, C.E. Manning (2008), Low heat flow inferred from >4 Gyr zircons suggests Hadean plate boundary interactions, Nature, 456, 493–96, doi:10.1038/nature07465.

Molnar, P., P. C. England, and C. H. Jones (2015), Mantle dynamics, isostasy, and the support of high terrain. J. Geophys. Res. Solid Earth, 120, 1932–1957. doi: 10.1002/2014JB011724.

Sobolev, S.V. (2016), Plate Tectonics Initiation as Running Hurdles, Workshop on the Origin and Evolution of Plate Tectonics abstract, Ascona, Switzerland,

Stern, R.J. and T.V. Gerya (2017), Subduction initiation in nature and models: A review, Tectonophysics, doi:10.1016/j.tecto.2017.10.014

Thompson, A., and B.N. Taylor (2008), Guide for the Use of the International System of Units (SI) NIST Special Publication 811, 2008 Edition (version 3.2). [Online] Available: [2018, 05 02]. National Institute of Standards and Technology, Gaithersburg, MD.

Turcotte, D. L., and E. Oxburgh (1972), Mantle convection and the new global tectonics, Annual Review of Fluid Mechanics, 4 (1), 33–66.

Wilson, T. (1966), Did the Atlantic close and then re-open?, Nature, 211(5050), 676–681, doi:

How good were the old forecasts of sea level rise?

How good were the old forecasts of sea level rise?

Professor Clint Conrad

The Geodynamics 101 series serves to showcase the diversity of research topics and methods in the geodynamics community in an understandable manner. We welcome all researchers – PhD students to Professors – to introduce their area of expertise in a lighthearted, entertaining manner and touch upon some of the outstanding questions and problems related to their fields. Our latest entry for the series is by Clinton P. Conrad, Professor of Geodynamics at the Centre for Earth Evolution and Dynamics (CEED), University of Oslo. Clint’s post reflects on the predictions of sea level rise since the first Intergovernmental Panel on Climate Change (IPCC) report in 1990 and the near three decades of observations and IPCC projections that have been made since then. Do you want to talk about your research area? Contact us!

This past week I flew over the North Atlantic with a direct flight to California from Europe. From the plane we had a beautiful view of glaciers on the western edge of the Greenland ice sheet, where the ice seems to be disintegrating into the ocean. We’ve been hearing lately that the ice sheets are slowly disintegrating – is this what that looks like? Using my mobile phone’s camera, I took a photo of the glacier that happened to be visible from my seat and compared it to images of the same glacier saved in Google Earth (Figure 1). This is an interesting exercise if you like looking at glaciers, but I can’t tell about the overall dynamics of the ice sheet this way.

Figure 1. A glacier on the west coast of Greenland on September 2, 2017 (left) taken with my iPhone. From my plane’s in-flight entertainment system, it seems that this glacier is between the villages of Upernavik and Niaqornat. For comparison, the image on the right is a screenshot of the same glacier from Google Maps.

Actually, we’ve been worried about ice sheet melting – and the sea level rise with it – for decades. I re-realized this during this past summer, as I finally started unpacking the boxes that we shipped to Oslo one year ago from Hawaii. Some of these boxes probably didn’t need to be unpacked, like the one labeled “High School Junk”, but it turns out there is interesting stuff in there! Here was my diploma, a baseball glove, some varsity letters, and a pile of old schoolwork – most of which I have no recollection of creating. But I did remember one of the items – a report on global warming that I wrote for Social Science class in 1989. In particular, I remember being fascinated by the prediction that human activity would eventually cause enough sea level rise to flood land areas around the world. For years, I have been personally crediting that particular high school report as being my first real introduction to the geosciences – but until this past summer I had never revisited that report to see what I actually wrote at the time. Now here it is – twelve yellowed pages of dot-matrix type, with side perforations still remaining from the printer feed strips that I tore off 28 years ago.

My report is entitled “Global Warming – What Must Government Do?” and now I can see that it is mostly a rehashing of reporting from a bunch of newspaper articles written in 1989. It was a bit disappointing that my younger self wasn’t more creative or inspirational, but the content of the report – really the content of the newspaper articles from 1989 – is fascinating because much of the material could have been written today. There is discussion of how the warmest years in recorded history have happened only recently, that climate skeptics were unwilling to attribute recent changes to human activity, and that a few obstinate countries (then, it was Japan, the USSR, and the USA) were standing in the way of international agreements to curb CO2 emissions. Another statement is also familiar: that “oceans could rise from 1.5 to 6.5 feet”. For those of you not familiar with that measurement system, that is about 0.5 to 2.0 meters! I know that recent predictions are not quite as dire as 2 m of rise (at least in the 2100 timeframe), although sea level acceleration has been getting more attention lately. Did people in 1989 consider 2 m of sea level rise a possibility? I checked the cited New York Times article from 1989, and indeed it seems that I dutifully reported the estimate correctly. The article says that 1.5 to 6.5 feet of sea level rise is expected “to occur gradually over the next century affecting coastal areas where a billion people, a quarter of the world’s population, now live”.

Figure 2. Projections of sea level in 2100 (relative to 1990 sea level) for the five IPCC reports between 1990 and 2013, plotted as a function of IPCC report date. Shown are the minimum and maximum projections (range of red bars), and the mean of estimates (black circles).

I have contributed a little to sea level research in the intervening years, and am somewhat familiar with the current predictions. I know that the most recent (2013) report of the Intergovernmental Panel on Climate Change (IPCC) predicts up to about a meter of sea level rise by 2100, which was a large increase over the 2007 report that predicted up to about 0.6 meters. Thus, meter-scale sea level rise predictions seemed like a relatively recent development, and yet here was a prediction just as large from nearly 30 years ago. What did the IPCC have to say about sea level at the time?

I plotted the sea level projections of the five reports that the IPCC has released between 1990 and 2013 (Figure 2). Indeed, the 1990 report predicted slightly higher sea level for the year 2100 (31-110 cm higher) than did the most recent report from 2013 (28-98 cm higher). In fact, the IPCC projections for 2100 sea level declined from 1990 through 2007, until they increased again in the most recent report in 2013 (Figure 2). Why is this? Well, we have nearly 3 decades of observations that could help us to answer this question!


Figure 3. Sea level projection from the IPCC’s first assessment report (1990), showing that report’s low, best, and high estimates (blue lines) and predicted rates in mm/yr. Also shown is the University of Colorado sea level time series (red line), which is based on satellite altimetry observations from 1992-2016 and records a sea level rise rate of 3.4 ± 0.4 mm/yr.

First, let’s evaluate the initial predictions of the first IPCC report from 1990. Since 27 years have passed since the publication of that report, we can actually compare a sizeable fraction of those 1990 predictions to actual sea level observations. Left, I have plotted (Figure 3) the 1990 report’s sea level projection from 1990-2100 (Fig. 9.6 of that report) along with actual sea level observations made using satellite altimetry between 1992 and 2016, which have been nicely compiled by the University of Colorado’s Sea Level Research Group. The comparison shows (Figure 3) that the actual sea level change for the past 24 years has fallen slightly below the “best” estimate of the 1990 report, and well above the “low” estimate.

In retrospect, the 1990 predictions of future sea level change seem rather bold, because the 1990 IPCC report also concludes that “the average rate of rise over the last 100 years has been 1.0-2.0 mm/yr” and that “there is no firm evidence of accelerations in sea level rise during this century”. Yet, the 1990 report’s projection of 2.0-7.3 mm/yr of average sea level rise from 1990-2030 (Figure 2), represents a prediction that sea level rise would accelerate almost immediately – and this acceleration actually happened! Indeed, three recent studies (Hay et al., 2015; Dangendorf et al., 2017; Chen et al., 2017) have confirmed sea level acceleration after about 1990.

Thus, the IPCC’s 1990 sea level projection did a remarkably good job for the first three decades of its prediction timetable, and the next 8 decades don’t seem so unreasonable as a result. What did the 1990 report do right? Here the 1990 IPCC report helps us again, by breaking down its projection into contributions from four factors: thermal expansion of the seawater due to warming, the melting of mountain glaciers, and changes in the mass of the great ice sheets in Greenland and Antarctica. The 1990 report makes predictions for the changes in sea level caused by these factors for a 45-year timeframe of 1985-2030, and I have plotted these predictions as a rate (in mm/yr) in Figure 4. Thermal expansion and deglaciation in mountainous areas were predicted to be the largest contributors. Greenland was predicted to contribute only slightly, and Antarctica was predicted to gain ice, resulting in a drop in sea level.

Figure 4. Comparison of projections and observations of the various factors contributing to global mean sea level rise (GMSL, in mm/yr). Red bars show predictions that were made in 1990 (table 9.10 of the 1990 IPCC report) for the 45-year period 1985-2030 (range is given by red bars and best estimate is shown with a dark line). Blue bars show the actual contribution from each factor for the 17-year period 1993-2010, as detailed in table 13.1 of the 2013 IPCC report. Note both the sum of observed contributions and the direct observation of sea level change from satellite altimetry (bottom two blue bars) are consistent with recent analyses of tide gauge data (Hay et al., 2015; Dangendorf et al., 2017), within uncertainty.

Now 27 years later, we have actual observations of the world’s oceans, glaciers, and ice sheets that we can use to evaluate the predictions of 1990 report. Many of these observations are based on measurements made using satellites, which can now remotely measure ocean temperatures, changes in the mass of land ice (mountain glaciers and ice sheets) and even changes in groundwater volumes, over time. The IPCC report from 2013 (the most recent report) shows these contributions in the timeframe of 1993-2010, which are 17 years during the 45-year outlook predicted by the IPCC’s 1990 report. I have plotted these observations in Figure 4, and we can see how the 1990 predictions compare so far – remembering that the prediction and observation timescales do not exactly align.

First, we see that 1990 report overpredicted the contribution from thermal expansion, and slightly overpredicted the contribution from mountain glaciers. Of course, there is still time before 2030 for these factors to increase some more toward the predictions made in 1990. However, we also see that Greenland melting has already matched the 1990 report’s prediction for 2030, and that the prediction of a sea level drop from Antarctica did not materialize – Antarctica contributed almost as much sea level rise as Greenland did by 2010 (Figure 4). Furthermore, there is another significant contributor to sea level rise – land water, which represents the transfer of liquid water from the continents into the oceans. This occurs because groundwater that is mined for human activities eventually ends up in the ocean. According to the 2013 report, land water caused more sea level rise than ice sheet melting from Antarctica.

Thus, in 2010 the predicted rates of sea level rise from two factors (thermal expansion and mountain glaciers) had not yet reached the 2030 predictions of the 1990 report, but the contributions from Greenland, Antarctica, and land water loss have already nearly met or exceeded the predictions of 1990. Indeed, recent satellite observations between 2002 and 2014 show an acceleration of melting in Antarctica (Harig et al., 2015) and especially in Greenland (Harig et al., 2016). The recognition that Antarctica and Greenland may contribute significantly more to sea level rise in the future compared to earlier estimates is reflected in the 2013 IPCC report (Figure 2).

Figure 5. A dike near the town of Putten in the Netherlands, where the recent EGU-sponsored “Nethermod” meeting was held in late August 2017. This dike is one of many in the Netherlands that protect negative-elevation land (left) from a higher water level (right).

So far, it seems that the IPCC’s 1990 sea level projection has stood the test of 27 years remarkably well (Figure 3). It is rather disheartening to realize that we are on track for the ~60 cm of sea level rise that the 1990 report predicted for the year 2100, or more if the early underestimates of ice sheet contributions prove to be more significant than any overestimates of thermal expansion (Figure 4). Looking at my own high school report from the same time, it is also disappointing that to realize that the warmest years in recorded history have again happened only recently, that climate change skeptics are still unwilling to attribute recent changes to human activity, and that there are still obstinate countries (well, one country) standing in the way of international agreements to curb CO2 emissions. On the other hand, high school students writing reports on this topic today will likely find discussions of dropping beachfront real estate prices, governmental planning for future sea level rise, and engineering techniques for managing future sea level rise (Figure 5). I hope that these students save copies of their reports in a format that they can examine decades later, because it is interesting to consider how predictions of future sea level rise have changed over time, and how society has been responding to the challenges of this geodynamic phenomenon that is operating on the timescale of a human lifetime. One day in the 2040s these students may want to scrutinize another quarter century of data against the projections of the next IPCC report, to be completed by 2022. I wonder what they will find?