Sea Level Rise: How far, how fast?

A paper appeared in Science this week about sea level rise in the last interglacial (about 129-116,000 years ago). It has sparked the usual predictable headlines as it points out that during that period, sea level rose by about 6-9 metres but that that the ocean temperature as far as it can be reconstructed, is about what we see now, that is about 0.5C warmer than the preindustrial.

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Guardian reports on latest study

In a sense this isn’t that “new” – we’ve known about higher sea levels during the last interglacial for ages and that the global mean temperature was roughly 2C above the pre-industrial global mean. This is in fact one of the reasons for the Paris target (though some scientists speculate that it’s also pretty much already out of reach).

However, the sea surface temperature stuff makes it extra interesting as the ocean is a pretty big source of uncertainty in global climate models and mot models do not manage to reproduce modern day ocean temperatures all that well.

It should also be said that the last interglacial is only a good analogue for 2C world up to a point – it was warm because of enhanced solar input, not because of greenhouse gases as this plot from an Antarctic ice core, edited by the awesome Bethan Davies at the Antarctic Glaciers blog shows:

vostok_420ky_4curves_insolation_to_2004
Carbon dioxide (CO2), Methane (CH4) with reconstructed temperature from the Vostok Ice Core, taken in Eastern Antarctica. Enhanced with modern methane, CO2 and temperature measurements by Bethan Davies. Note that the “modern” value of CO2 here is from 2004. In 2017 it is currently measuring 403 ppm.

It’s also interesting to speculate where the water came from – the Greenland ice sheet was much smaller than today but it was still there and now “only” contains 7m of sea level rise today. So the complete disappearance of Greenland cannot explain the rise in global sea level. The small glaciers and ice caps of the world can’t contribute more than half a metre or so either. Therefore it has to be Antarctica contributing the most – East or West is the question and it really is a very very longstanding question.

The progress in the international polar year (IPY) in mapping the bedrock of Antarctic in the BEDMAP2 brought quite a few surprises, including the discovery of several very deep marine basins in the East that could potentially contribute a lot of water to sea level.

More recently, channels under the floating ice shelves of west Antartica, along with various modelling studies have proposed that the west could be much more unstable than thought. Actually this has been a very very longstanding problem in Antarctic science since at least the late 1970s when John Mercer first proposed the marine ice sheet instability hypothesis.

In any case, events in both Denmark and the UK have brought this problem home more sharply.

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The silent storm surge – coastal flooding in Copenhagen on the 5th January – the water in the harbour is not normally this high! Source: Brian Dehli, shared by DR 

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The “silent storm surge” in January 2017 around the coast of Denmark was  a hundred year event in many places, but as Aslak Grinsted points out, sea level rise makes a hundred year event a 20 year event with only a small rise.

Sea level will not rise equally everywhere, the fingerprint of Greenland ice sheet loss is felt largely in the Pacific, Antarctic ice melt will be felt in Europe. It matters where the water comes from. A point not generally appreciated.

So this new paper is also important, but it only underlines that we need to be able to make much much better estimates of how fast and how far the ice sheets will retreat, which is the justification for much of my own scientific research.

Finally, I think it’s probably necessary to point out that sea level is already rising. This was asked by a listener to Inside science, one of my favourite BBC radio 4 programmes/podcasts. I was a little surprised that an apparently scientifically literate and interested member of the public was not aware that we can measure sea level rise pretty well – in fact to an extent, the global warming signal is more easily detected in the ocean than in the global temperature record. This is because the ocean expands as it warms and there is ocean pretty much everywhere, whereas temperature observations are patchy and mostly on land. Clearly, scientists like myself are *still* not doing a very good job of communicating our science more widely. So here is the global mean sea level record to date, it’s updated pretty regularly here and on average, sea level is rising at about 3mm per year or 3cm per decade.

 

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Sea level variation measured by satellite since 1993 from NASA

When we look at tidal gauges,sea level rose about 20cm in the 2oth century

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Sea level rise in the 20th century measured by tide gauges, plot by NASA, data from CSIRO

The big uncertainties we have on whether or not this will accelerate in years to come is largely down to missing processes in ice sheet models that we don’t yet understand or model well – mostly calving by glaciers and ice shelves. I promised Steve Bloom a blog post on that at some point – I have a paper to finish and new simulations to run, but hopefully I’ll get round to that next.

UPDATE: I was made aware this morning of a new report from the European Environment Agency about climate change impacts and adaptation in Europe. In the report they state (correctly) that while the IPCC 5th Assessment Report suggested that in the 21st century the likely sea level rise will be on the order of half a metre, some national and expert assessments (I took part in a couple of these) had suggested an upper bound of 1.5 – 2m this century, for high emissions scenarios.

This is a big difference and would be pretty challenging to adapt to in low-lying countries like the Netherlands and Denmark, not to mention big coastal cities like London or Hamburg. It’s laso important to emphasise that it doesn’t jsut stop at the end of the century, in fact our simulations of the retreat of Greenland ice sheet suggest it’s only just getting going at the end of this century and the next century the rate of ice loss will really start to accelerate.

All of which is to say, there’s really a very good reason to act now to reduce our emissions. The EEA has also produced this very nice map of observed sea level rise in Europe over the last two decades based on  Copernicus environmental data.

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Check out how much the sea level is rising where you live… Source: European Environment Agency, data from Copernicus Marine Environment Monitoring Service

With the prospect of American federal funding for environmental observations being reduced or strongly constrained in the future, it’s really important we start to identify and support the European datasets which are the only other sources of environmental monitoring out there right now.

 

 

The Unicorn and The Lamprey

On the 16th may 1619 two ships, the Unicorn and the Lamprey, set sail from Copenhagen searching for the fabled North West Passage. On board there were 65 men, led by their captain, the Danish explorer Jens Munk. A year and a half later, the Lamprey limped back into Bergen (Norway) with just 3 men, including Munk, on board.

Almost all of the other crew members had died of scurvy in Hudson Bay .

The story of this terrible voyage, their sailing round Iceland, Greenland, Baffin Bay and into Hudson Bay is outlined in this wonderful atmospheric podcast from DR.

jens_munk_map_1624
A Map, hand-drawn by Jens Munk in 1624 of the area between Cape Farewell and Hudson Bay, seen from the north; (Source: Tromsø University library)

The UK has similarly many tales of Arctic and Antarctic suffering, listening to the podcast I was put in mind of Coleridge’s famous “Rime of the Ancient Mariner”, but we rarely hear of the similar stories from other nations, a clear benefit of learning other languages is being able to access these archives and stories*.

The podcast contains a wonderful description by a Greenland pilot of the sea ice and how tricky navigating it can be along with interviews and inputs from many others. If you are at all familiar with Danish – I really recommend the series.

However, the description by a nutritionist of the terrible effects of scurvy had me wondering. I learn (via Dutch family and confirmed by the OED) that the name of the disease, caused of course by a lack of vitamin C in the diet, is probably from the Dutch Scheurbuik – rip belly – an eloquent description of one of the notable later stages of the disease.

Rip here is less a description of enhanced musculature and much more a description of what it feels like when your internal organs start to bleed and your muscles and bones are weak from lack of nutrition.

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Photo of chest cage with pectus excavatum and scorbutic rosaries – from this paper

Upon looking it up (Thankyou Wikipedia), I learn that the causes of scurvy had been repeatedly identified, forgotten and mistaken since at least the middle Ages. There is an estimate that around 2 million sailors died as a result of scurvy between 1500 and 1800.

2 Million almost entirely preventable deaths and 2 million men who died in appalling agony.

And this happened in spite of what appears to be the first recorded medical trial by James Lind in the 1750s, it still took the Royal Navy 40 years to start giving out fresh citrus fruits as a standard on their ships. Vitamin C itself was only finally recognised and extracted in 1932.

This story is an outrage in many ways, but a clear example also of how science and medicine, properly conducted, can help to improve and save lives. It is also a clear warning to conduct thoughtful experiments with care and to listen to those warnings when they have been issued.

It might also be a recommendation that learning foreign languages is not only fun and useful but can be it’s own reward.

*I should also mention here that the rather awesome Danish Arctic Institute are currently producing a very well written series on Danish exploration in the Arctic in English, based on their own very comprehensive podcast series. These are published online in the Arctic Journal. Both the series of historical accounts and the newspaper in general are absolute top recommends for those interested in the subject of the Arctic environmentally , socially and politically.

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The lure of the poles (Svalbard in Spring, the coldest time of year)

Extract from “Rime of the Ancient Mariner” by Samuel Taylor Coleridge

…With sloping masts and dipping prow,
As who pursued with yell and blow
Still treads the shadow of his foe,
And forward bends his head,
The ship drove fast, loud roared the blast,
And southward aye we fled.
And now there came both mist and snow,
And it grew wondrous cold:
And ice, mast-high, came floating by,
As green as emerald.
And through the drifts the snowy clifts
Did send a dismal sheen:
Nor shapes of men nor beasts we ken—
The ice was all between.
The ice was here, the ice was there,
The ice was all around:
It cracked and growled, and roared and howled,
Like noises in a swound! …”

 

A Sea-Ice Free Arctic in 2016?

UPDATE: The Arctic Sea ice Outlook I mention in the post below has just been published for 2016. We will follow this up in September when the final results will be known, but here are the 30 entries using a rage of different techniques including sophisticated computer models, statistical estimates and what is kindly called “Heuristics” but which may be characterised as an educated guess by people who have been studying this field for a while…

sio_2016_fig_1_30june

Professor Wadhams has not contributed an estimate this year but it can easily be seen that none of the estimates reach as low as the putative 1 million square kilometres. Nonetheless the view of 27 expert climate scientists put forward by Kay, Bailey and Holland (pdf), not to mention the very sophisticated RASM model (one of the most sophisticated in this area, run by the US Naval Postgraduate school), put the September extent at a very low 3-4 million km2, in the same range as the record low of 2012.

It will be interesting to see how low it does go. The latest results from the polar portal show that Arctic sea ice is currently still on the record low 2012 line but a careful look shows also that the 2012 and 2013 curves diverge around mid to late June. The year 2013 is pretty representative of a “new normal” over the last 4 years or so, it is therefore difficult to tell based on simply extrapolating along the curves which path 2016 is likely to follow.

SICE_curve_extent_LA_EN_20160711.png
The area covered by at least 15% sea ice in the Arctic from 1981 to present, the black and red curve shows the year 2016 and is updated daily on the Polar Portal 

The current weather plots on the Polar Portal (based on weather forecasts produced by the European Centre for medium Range Weather Forecasting, probably the best numerical weather model in the world) show no unusual temperatures in the Arctic Ocean right now, though parts of Arctic Canada and Siberia certainly look warm.

Wthr_Anom_SM_EN_20160711.png

We’ll have to wait and see until September…

Original post below from June, 14th, 2016. 

The Polar Portal has become part of our daily life at DMI where I work in the last few years, it combines detailed observations and models from the Greenland ice sheet, the Arctic sea ice and, soon hopefully, permafrost. I am particularly involved in the Greenland pages where we daily calculate the amount of snowfall and snow melt which gives us a surface mass budget and which we sum up over the year to work out what it means for the health of the Greenland ice sheet. This year has been especially interesting with an extraordinarily early start to melting driven by warm Arctic temperatures. Many records in Greenland have been broken in April, May and June. Spectacularly, last week Nuuk set a new temperature record for June that managed to last only 24 hours, before it was broken again.

This is the new reality in the Arctic. And it is also having an effect on sea ice. The Arctic sea ice extent has long been used as a bellwether of climate change with much effort exerted by both activists and sceptics in trying to prove or dismiss claims about climate and its effects on sea ice.

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Crossing the sea ice in front of Paulabreen, a surge type glacier with a calving front in Svalbard

I trained as a glaciologist originally,  but even then I came across sea ice and was first of all unnerved by it, crossing on scooters to visit glaciers in Svalbard, and then fascinated by it. Recently I have been working pretty closely with my colleagues in DMI who are sea ice scientists and I have learnt quite a lot. We even published a paper together in the journal Polarforschung earlier this year. Not only that, I am now part of a big ERC Synergy project known as ice2ice  with scientists at four institutions in Bergen and Copenhagen working on the complex connections between sea ice, ocean, atmosphere and ice sheet in the Arctic. More on that another time, but suffice to say it’s fascinating work and I know a hell of a lot more about sea ice than I did even three years ago.

So when this news story crossed my email this evening courtesy a BBC researcher and journalist I knew pretty well straight away what it was about. Basically the scientist Professor Peter Wadhams had made some statements about the extent of Arctic sea ice which might be considered somewhat eyecatching.

Capture-wadhams

Professor Wadhams is a well-known scientist who did some incredibly valuable and indeed ground-breaking early work on sea ice. More recently he has also done some very valuable work reconstructing thickness based on submarine observations during the Cold War (see below on why this is important). I well remember seeing him talk about this as a young graduate student, he is an excellent speaker and gave a very interesting and compelling talk. In the last few years he has made several statements that have been widely reported and perhaps misinterpreted, with regard to the future fortunes of the Arctic sea ice.

Now, I need and want to be clear about this. Most of the global climate models we use are not very good at reproducing the observed historical sea ice extent. They have improved significantly in the last few years but still struggle to reproduce the actual observed decline in sea ice area from satellites. And there are actually very good reasons why this should be. There are some very good stand alone sea ice models which have done a very good job and the key difference between these models is our clue. Sea ice models are generally partly forced with actual observations, or climate reanalyses which assimilate observations, so the atmosphere and the ocean are close to reality. Basically sea ice responds to weather, and if you have a more accurate weather driving your sea ice model you will get a better fit to the observations.

So, is Professor Wadhams correct? Will the sea ice “disappear” this year.

Well, it is pretty clear that given the changes we have already observed in the Arctic, as well as what we know about Arctic amplification and the general direction that anthropogenic emissions are heading in, that unless something changes pretty soon, we will likely see an end to a significant cover of sea ice in the Arctic at some point in the next few decades. But was does that actually mean?

Reading his actual comments in the article he appears to define 1 million km2 as “no sea ice” and that partly reflects how we define sea ice extent. Since most of the data sets use a cut-off figure (typically 15%) to define when a grid square or pixel is or is not a sea ice point. This is known as sea ice concentration and is really something of a hangover from the days when sea ice was observed from ships and an attempt was made to estimate how much sea ice in the area  was around the vessel.

There are however lots of things that can affect sea ice extent, including winds and currents and melt ponds. The latter also affects how different algorithms assess the area that is or is not covered by sea ice. As there are a number of different sensors in use and a number of different algorithms processing that data, it is not entirely surprising that there actually a number of different estimates (I will use OSISAF) for how much of the Arctic is covered in sea ice. And this number will vary in years with more winds for example, or stronger ocean currents, sea ice will disperse faster. It is quite likely that much of the variability in sea ice area in recent years is at least partly attributable to different winds, as well as, for example in 2012, big storms that have arrived at just the right moment (or wrong one depending on how you look at it), to break up the sea ice into smaller, more easily transportable pieces.

As an aside, a better measure for how much Arctic sea ice there is actually present is sea ice volume. Unfortunately this is very difficult to measure, especially outside the winter freeze up season, though a research group at the UCL, centre for Polar Observation and Monitoring have developed a way to do so. Here for example is the most recent plot, which as you can see has not been updated since May 2016 due to the presence of melt ponds on the surface of the sea ice which the Cryosat radar cannot penetrate.

So 1 million km2 is probably a reasonable cut off for assuming an “ice-free” Arctic in the sense that it indicates that there will still be some sea ice drifting around (it always forms surprisingly quickly when the winter begins) in summer, even if it is dispersed.

Over the last 40 or so years (we have good observations going back to 1979, it gets patchy after that), in September, when the area covered by sea ice is at it’s lowest, that extent has been between about 7 and 9 million km2, more recently that has dropped and 2012, the lowest on record had an extent of about 4 million km2, which you can see on the latest polarportal sea ice chart below.

SICE_mod

I well remember 2012, we had a large melt event over Greenland that year also, but it was still quite a long way from the 1 million km2 quoted by Professor Wadhams. Again, let me be clear, we are pretty sure that at some point on a time scale of a few years to a few decades, the Arctic will become “ice-free” in the summer time. We can predict this, even if we don’t know exactly when, since, as I hope is clear now, sea ice conditions are very dependent on the weather. The weather this year so far, at least this Spring has been very warm and congenial to sea ice melt. The big dive shown on the graph above is no mystery when considering some of the temperature anomalies in the Arctic, as shown also on the Polar Portal.

Nevertheless, the recent plots seem to show that the 2 metre air temperature in the Arctic is returning to close to normal and there is little reason to suppose that will change significantly anytime soon.

meanT_2016

Having said that, weather forecasting has improved massively in the last few decades, a true quiet revolution, but we still do not know how the weather will pan out over the whole of this melt season. I am sure that at some point Professor Wadhams will be proved correct, but we do not know when and it is even possible or rather likely that we will have a few years where we switch back and forth between ice free and not ice free conditions. So, the answer to the question I pose above is probably no. But don’t bet on it remaining so for too long.

UPDATE: I recalled this morning on my way in to work that I had somehow failed to mention the Sea Ice Prediction network. This group of people under the auspices of ARCUS, gather predictions on y´the end-of-season sea ice extent ever year. The call for predictions for the 2016 season is now open. Many different research groups as well as one or two enthusiastic amateurs will post their predictions over the next few weeks. It is an interesting exercise, as you can see based on last year’s report (see also figure below), it is not the first time that Profgessor Wadhams has predicted a 1 million km2 extent in September, and his is the lowest (and least accurate) in the rankings.

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Downloaded from the ARCUS SIPN website

So keep an eye out on this and if you think you can do better, consider submitting a prediction yourself… 

Endnote: There has been quite an absence of posts from this blog recently. I have been too busy with work, family, travel and more recently the EU Referendum (for which I have been threatening a post for quite some time and may yet get around to before polling day). However, a question about Arctic sea ice has been flickering on the edges of my consciousness for a while now so this was a quick (EDIT: not so quick!) blogpost to try and address it when I should actually be writing something else…

 

Bless the rains down in Africa #DACEA3

An ultra-quick post today. I have been spending a lot of time lately writing a grant proposal (and occasionally tweeting about it  on the #DACEA3 hashtag).  Finally it’s in and after a celebratory beer or two at the famous Mikeller last week I have managed to get around to a very brief summary of what it’s all about… 

Around 17,000 years ago, Lake Victoria more or less completely dried out. I still find this absolutely staggering. In fact, the lake has dried out and reformed at least 3 times since it first formed about 400,000 years ago.

Lake Victoria is the largest lake in Africa and indeed the tropics, containing 2.75 cubic kilometres of water (though compared to the 2,850,000 cubic kilometres of water in the Greenland ice sheet that seems small, which merely goes to prove how much of our fresh water is locked up in the ice sheets), making it the 9th largest lake by volume in the world.

Gratuitous wildlife shot: A raft of hippos chilling out in the river. Photo: Pim Bussink
Gratuitous wildlife shot: A raft of hippos chilling out in the river.
Photo credit: Ruth Mottram

Clearly, the disappearance and later reappearance of the lake, and others in the region speaks to monumental shifts in the climate. The East African Rift Valley lakes are largely fed by the East African rains, long and short, delivered by the shifting position of the Intertropical Convergence Zone as the Earth’s seasons change bringing those life-giving rains.

This grant proposal started as idle speculation around the coffee machine (in the grand old scientific tradition) about how this was climatically possible and could it happen again? My colleague (and talented PI on the proposal) Peter Thejll had been reading a book about John Hanning Speke and Richard Burton (not that one) and their famous search for the source of the Nile and has some personal African connections, which prompted the conversation and it seemed obvious to try and find out what happens to the local circulation to allow the lake to dry out. A quick google search revealed an old friend, Dr. Sarah Davies at Aberystwyth University was researching this topic actively and it all fell into place.

Now, I can guess what you’re thinking – this is usually a glaciology or Arctic Climate blog, where on earth has all this Africa stuff come from? Well what happens in the Arctic does not necessarily stay in the Arctic.

There are a number of hypotheses as to the drivers of these changes in African rainfall, among which is the interesting observation that the periods of greater aridity correlate remarkably well with Heinrich events in the North Atlantic.

Heinrich events were first identified as layers of sediment most likely transported into the North Atlantic Ocean by icebergs, known as ice rafted debris – IRD. The southerly position of many of these layers thousands of kilometres from any ice sheets either at the present day or in the past suggests a truly extraordinary amount of icebergs and cold fresh water were discharged over a relatively short period of time, from a large ice sheet. The source of these sediments is most likely the gigantic Laurentide ice sheet of North America, but there is also some evidence of smaller contributions from the British and Fenno- Scandian ice sheets (which may or may not have been joined together across the North Sea depending on how you interpret the evidence). The physics behind this is that as the enormous amount of cold fresh water was discharged into the North Atlantic, the temperature and salinity changes were sufficient to push, or keep the ITCZ far to the south, preventing the rains one East Africa.

On the other hand, other research has linked the failure of the rains to El Niño and related phenomena such as the Indian Ocean dipole and the Walker circulation. Still other scientists have noted that these drying periods seem to correlate with orbital changes in the earth which would affect the seasonality, that is the annual cycle of seasons. It is known as orbital forcing as the Earth’s seasons are driven by changes in our orbit around the sun (have a look at the excellent Orbit documentary from the BBC for a very easy to follow and beautifully filmed introduction to the importance of our orbit around the sun if you’re not familiar with Milankovitch cycles etc).

Milankovitch cycles shown from ocean cores and an Antarctic ice core at the bottom compared with the theoretical cycles. Image: By Incredio (Own work) [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
Milankovitch cycles shown from ocean cores and an Antarctic ice core at the bottom compared with the theoretical cycles.
Image: By Incredio (Own work) [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
All of these hypotheses can be supported by correlations with palaeo evidence,  but to really disentangle the connections between different regions of the world and how they affect each other’s weather and climate, we need to use a climate model. Luckily, at DMI we have the perfect tool to hand, a global climate model including ice sheets, EC-Earth. Furthermore we also have a high resolution regional climate model, HIRHAM5, my usual tool of choice. Our friends Morten Dahl Larsen and Martin Drews at the Danish Technical University are experts in using hydrology models so the answer is obvious.

We want to use these model tools and an extensive archive of observations, helpfully curated by our project partners Sarah Davies and Henry Lamb at Aberystwyth University to test all these different ideas. As an extra spinoff from the project, the Aberystwyth group have been intensively involved with the collection and analysis of a new lake sediment core from Chew Bahir in Ethiopia, so it’s going to be pretty exciting seeing if we can get the models to replicate  these kind of records.

There is of course an extra urgency to this project. It’s not just a somewhat obscure academic question. A recent paper showed that the long rains have significantly reduced over the last decade, and about 300 million* people live in this region and rely on these rains for drinking water, hydroelectric power and agricultural production. During this period we have also seen rapid changes in the Arctic. Of course the two trends may not be connected, or may be linked via a common third factor which is why the physics of climate are so important to understand.

UPDATE 2: I had no time originally in the writing of this to add a little about our other project associate. One of the best things about doing science are the very smart and friendly people  you meet along the way. Social media has really helped here to keep in touch as it is a nomadic lifestyle. By sheer chance I noticed a familiar name in a tweet that seemed to have some direct relevance to the proposal as we were writing it.

Hycristal

John Marsham was an old friend from my student days at Edinburgh University who I had slightly lost touch with. Thanks to the efforts of facebook we were soon back in touch and he is one of the Investigators on the HyCRISTAL project, part of the hugely important Future Climates for Africa Project, funded by the Department for International Development (DFiD) and the Natural Environment Research Council (NERC) in the UK. DACIA has some really obvious parallels with this project, though where we would like to concentrate on past climates, they will be focusing on present day and future climates. We hope therefore to send our PhD student to collaborate with the HyCRISTAL and FCFA projects where our insights from palaeomodelling palaeodata can make a real difference to the way future climate change is adapted to in East Africa. It will be very nice to collaborate with John’s group at Leeds as well as the Aberystwyth group, now we just have to hope we get the money to do it..

Or, to put it another way, “bless the rains down in Africa” ** (As an aside, for years I had always heard this as “I miss the rains down in Africa”, assuming it was about someone from Africa who missed being there).

UPDATE 1: Having viewed the original pop video again, I am rather troubled by the casual racism, sexism and naked orientalism on display (yes it was the 70s but still…) so I think I prefer to post instead this particularly witty deconstruction courtesy of @spaceforpootling

*(based on a back of the envelope calculation based on population statistics from Wikipedia if you know the correct number do let me know).

**(Apologies if you now have cheesy 1970’s pop music going round your head all day… 🙂 )

Conversion Factors

The official end of the hydrological year in Greenland (1st September to 31st August) means I am rather busy writing reports to give an overview of where the ice sheet is this year and what happened. I will try to write a quick blogpost about this in the next week or so (in case you’re curious here’s a quick plot to show the entire annual SMB, see also: http://polarportal.dk/en/groenlands-indlandsis/nbsp/isens-overflade/)

Daily and accumulated surface mass budget of the Greenland ice sheet, 31st August, 2015, last day of the hydrological year
Daily and accumulated surface mass budget of the Greenland ice sheet, 31st August, 2015
Anyway, as I find I am constantly switching between Gigatonnes (or indeed Gigatons), cubic kilometres and sea level equivalent, here is a quick and handy guide to converting different units of mass, for my own use as much as anyone else.

1 gigatonne is 1 billion metric tonnes  (or 1 milliard if you like the old British style, that is one thousand million).

However, on the Polar Portal we usually reckon everything in water equivalent. This is to save having to distinguish between snow (with a density between ~100 kg/m3 when freshly fallen and ~350 kg/m3 m when settled after a few days), firn (snow that has survived a full annual cycle with a density up to ~800 kg/m3) and glacier ice (anything from ~850 kg/m3 to 900+). Water has a density (at 4C) of 1000 kg/m3

1 gigatonne of ice will still weigh 1 gigatonne when it is melted but the volume will be lower since ice expands when it freezes.

1 metric tonne of water is 1 cubic metre and 1 billion metric tonnes is 1 km3 (a cubic kilometre of water)

A cubic kilometre of ice does not however weight 1 gigatonne but about 10% less because of the density difference.

100 gigatonnes of water is roughly 0.28mm of sea level rise (on average, note there are big regional differences in how sea level smooths itself out).

Finally, 1 mm sea level rise is 360 Gt of ice (roughly the number of days in a year) 

EDIT: – thanks to ice sheet modeler Frank Pattyn and ice core specialist Tas van Ommen on Twitter for pointing out I’d missed this last handy conversion. Interestingly and probably entirely coincidentally this is very close to the amount of mass lost by the Greenland ice sheet reported by Helm et al., 2014 for the the period January 2011 – January 2014 (pdf here) of 375 +/-24 km3 per year.

Over the last 10 years or so, Greenland has lost on average around 250 Gigatonnes of ice a year (Shepherd et al., 2012), contributing a bit less than a millimetre to global sea level every year with some big interannual variability. This year looks like it will be a comparable number but we will have to wait for the GRACE satellite results in a couple of months to fill in the dynamic component of the mass budget and come up with our final number.

Of course, gigatonnes and cubic kilometres are rather hard to visualise so we have skeptical science to thank for this post that tries. And as aside, Chris Mooney wrote a nice piece in the Washington Post on the difficulties of visualising how much ice is being lost which contains the immortal  line “Antarctica is clearly losing billions of African elephants worth of ice each year”.

A question of observation?

It’s been a while since I lasted posted anything, not for want of ideas but mainly lack of time. I shall try to catch up over the next few weeks. For now I was inspired to write an ultra-quick post about a very trivial question that came up at work today. I think it really captures how observational meteorology works (or should work).

Today, a colleague, John Cappelen, (also known as Mr. Greenland observational data), happened to mention in passing that on the 15th July this year, the weather station at Summit on the Greenland ice sheet had transmitted back to us in Copenhagen, a temperature observation of 2.5°C. This was during one of the highest melt periods this summer.

Automatic weather station operating at Summit, June 2015
The automatic weather station doing it’s thing at Summit, June 2015. Photo: DMI

Bearing in mind that Summit Camp is at roughly 3,216m, this is a pretty high measured temperature. In fact it would be rather noteworthy, especially as it occurred on one of the highest melt days of the summer. Temperatures above 0°C at Summit are not unknown and the record, during the famous summer of 2012 when around 95% of the ice sheet surface experienced melt, the water sweeping away a bridge on the Watson River near Kangerlussuaq, was 3.6°C.

Now, my colleague is a very experienced and careful scientist. He had checked the observations and the temperatures before and after this measurement were well below zero, so, my colleague asked, was there any reason to believe this measurement or can we assume an instrument failure of some kind?

My office mate in the Arctic and Climate Research section and I obligingly had a quick look at our Polar Portal Greenland ice sheet surface plots (see below) and at the melt extent plots that are updated daily on the DMI website. We had to conclude there was no evidence of melt that high on the ice sheet and there was also no reason to believe that a sudden sharp warming had occurred at Summit on this day based on DMI’s own weather forecast. We then turned to check the weather plots, also on the polar portal and based on data from the European Centre for Medium Range Weather Forecasting (the ECMWF – probably the best weather forecast modellers in the world).

Again, the anomaly plots showed rather cold conditions prevailing over the ice sheet during this period, though at the same time very high melt and low surface mass balance from the ice sheet due to the clear skies.

Graphs showing area of the Greenland ice sheet experiencing melt conditions, compared with the average (dark grey line) and range of past summers (1990-2012), for more detail see the DMI website
Graphs showing area of the Greenland ice sheet experiencing melt conditions, compared with the average (dark grey line) and range of past summers (1990-2012), for more detail see the DMI website
Temperature record from Summit Camp for the last month.
Temperature record from Summit Camp for the last month.

Fortunately, due to the American Summit Camp we have access to a back-up dataset very close to this location and after a quick web search John Cappelen was able to confirm that indeed this measurement was an error as the nearby station has not seen anything like that during the period in question (see right).

This kind of thing happens all the time and is therefore not at all newsworthy or interesting enough to write a publication about. However, when a recent record high temperature in the UK can lead to 2 critical articles in the Daily Telegraph and a particularly vigorous exchange on twitter for Met Office scientist Mark McCarthy, as well as this corrective piece on the Carbon Brief blog, perhaps we should be more vocal about just how careful and critical we as scientists are about observations, including the ones we decide to discard as well as the ones we keep.

Surface mass balance of the Greenland ice sheet on the 15th July 2015. Intense melting around the margins led to very negative SMB (the red colours) during this period.
Surface mass balance of the Greenland ice sheet on the 15th July 2015. Intense melting around the margins led to very negative SMB (the red colours) during this period.

Addendum: I was alerted by this tweet from Gareth Jones, also a Met Office scientist, to some slightly strange cherry picking in the blogosphere of climate records from a couple of DMI stations in Greenland. These have apparently been used to claim no climatic warming trend in Greenland over the 20th Century (I’m not going to link to it).

Screenshot of tweet

Anyone who is really interested in the observational data could try checking these reports by Mr Greenland observations himself instead, here is a quick summary: 

Mean annual temperature in Copenhagen, Torshavn (Faeroes) and selected DMI weather stations in Greenland from 1873 - 2014. Figure from DMI
Mean annual temperature in Copenhagen, Torshavn (Faeroes) and selected DMI weather stations in Greenland from 1873 – 2014. Figure from DMI

Calling all students…

I’m off to the UK next week for a workshop at Sheffield University where we will discuss the Surface Mass Balance of the Greenland Ice Sheet. The ISMASS workshop includes all the main modelling groups and observation groups who are involved in assessing surface mass balance in Greenland. I will be representing DMI’s Greenland SMB work there (not an easy task condensing it down to a 20 minute talk!).

In the course of preparing my presentation I have been making plots and figures and really investigating some exciting results. Sadly, I very rarely get the chance to spend time on this these days and I am keen to recruit students to assist in this work. Should any potentially interested students want to discuss this at Sheffield do let me know.

At the risk of spoilers in my presentation, here for example is one showing how different ways of characterising the surface snow pack affects our estimates for surface mass balance, and how the effects of the specific changes can be very different in different years.

Surface mass balance map plots of Greenland
Surface mass balance for the hydrological year (Sep -Aug) ending in 2012 and 2013 calculated using HIRHAM5 with 2 different surface schemes. The maps on the right show the difference between the 2.

As I mentioned I rarely get enough time to analyse the output from our runs and I would be very happy to hear from any students who are interested in doing a project on our simulations. We have lots of MSc and Bachelors projects already listed on our website at DMI but we are always happy to hear new ideas from students on related topics. I have terabytes of data from simulations I would like to be properly analysed and this could be very interesting given we are talking about Greenland and the Arctic in the present day and in the future. It’s a really nice opportunity to work with some cutting edge research. I am also happy to hear from students who would like to do a summer project and for the right candidate I would be able to look into a paid “studentmedarbejderhjælper” position for a few months, especially if you are already a trained computer science candidate….

If you are an undergraduate looking into an MSc, I urge you to consider Denmark. EU citizens usually qualify for generous support grants (rare these days!) as we have a shortage of candidates wanting to study in the sciences in Copenhagen. The research and teaching are world class and done in English at MSc level. The possibilities for projects in Greenland are literally endless.

If you want any more details or to talk about any of the possibilities, do get in touch!