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...

Big change for small Arctic creatures

Sometimes it’s important to study the obvious. Among almost all, except some right-wing Americans, it is known that we are experiencing a period of rapid climate change. And with climate change come biological changes. But it matters to find out what exactly is happening; to get an idea of the scale of the phenomenon, and of how various aspects of it interact. So even though everybody can guess that the Barents Sea is warming, and that that must influence whatever small critters are living at its bottom, I spent two years trying to figure out what the difference really was between, roughly, 20^th and 21^st century foraminifera in that basin.

Some of the foraminifera species we studied

How did that all come about? In the 90s, Morten Hald, Per Ivar Steinsund and Sergei Korsun had made an effort, collecting all available information on which foraminifera species lived where. And in the years 2005 and 2006, a vessel of the Norwegian Institute of Marine Research had gone and collected samples in the same area. My job was to analyse these samples for foraminifera, and compare my findings to the database of foraminifera assemblages that had been compiled. The paper is published in Global and Planetary Change, as 'Changes in distribution of calcareous benthic foraminifera in the central Barents Sea between the periods 1965-1992 and 2005-2006', by M.H. Saher, D. Klitgaard Kristensen, M. Hald, O. Pavlova, and L. Lindal Jørgensen, to be found here. 

We found changes indeed. At my sample sites, temperature had on average increased by almost 1°C. All the cold water species had lower abundance in the 21^st century than in the 20^th. Evident? Maybe. But yet another small piece in the whole picture of how exactly various species respond to climate change.

Our main goal had been finding out which species had changed in abundance and how, but just out of curiosity I also wanted to calculate how big the total change had been for each sample. I found an appropriate method for quantifying change, calculate a value for each sample, and had some mapping software produce a map of the results. When it came out I was surprised to see a clear pattern. A band of highest amplitude of change followed the general outline of the maximum ice edge in spring. I hadn’t expected that!

The main figure in the article (but changed in style as to not breach copyright), with the yellow circles indicating the amount of change between the 20th and 21st Century as found in the foraminifera assemblages.

The maximum ice edge is a place where many things come together. In the Barents Sea, inflowing Atlantic water mixes with polar water. Where these waters meet tends to be where the ice edge is. Atlantic water is too warm to freeze over. And the water covered in ice doesn’t receive much sunlight, so there isn’t much life that can eat the nutrients in the water; at the ide edge, the wind can stir them from underneath the ice. That makes the ice edge a very productive region. And now we found it is not only where life is abundant, but also where it changes in abundance.

So what are the implications of this? With the cold foraminifera species vanishing from the Barents Sea, they may soon vanish altogether. They can’t move much north from there. Will anybody miss them? They are not charismatic macrofauna, after all. But it would be a few more species gone, which are fascinating, if only you are willing to find out.

And if there was already such change happening in the pre-2007 years, it would have been very interesting to see what 2007, the epicyear of low sea ice, would have shown. That year is famous for its summer ice minimum; not its spring ice maximum. But that was low too. 

The extent of sea ice in March 2007; March is normally the month with the largest extent. The area has dropped steadily over the years (left) with a big dent in 2005-2007. After 2007 it has been rather stable. Data: Fetterer, F., K. Knowles, W. Meier, and M. Savoie. 2002, updated 2009. Sea Ice Index. Boulder, CO: National Snow and Ice Data Center. Digital media.

The research vessel did set out to collect samples in 2007. Unfortunately, the weather was too rough, and no samples were collected. The next year, the ship didn’t even go. So there is still much to find out at the bottom of the Barents Sea. I hope some time in the future, someone will go back there, collect samples, and then write a comparison with both the 20^th century database and this work. I am quite keen to see what’s going on there in the post-2007 years…

The Greenland story

It was all over the news, all over the world: 97% of the surface of the Greenland Ice Sheet melting! Quite a lot of sources, including 350.org,  de Volkskrant and het NOS journal got carried away, and announced, either on twitter or on national TV, that 97% of all Greenland ice was gone. They should have spent a second to ponder this: for instance, the offices of de Volkskrant would be flooded if that had indeed been the case. But 97% of the surface experiencing melt is spectacular enough in itself. Generally, no more than 50% undergoes melting in summer. 97% is really rare.

Some twitter sources also mentioned this event had been predicted. In an article, that attracted quite some attention, Jason Box of Ohio State University, and co-workers, stated they expected melt over 100% of the surface to occur in the near future. So what did they base that on, and were they really that precise?

The map showing decreased reflectivity over almost the whole of the ice cap, which gets the bulk of the attention.

Box et al. studied the reflectivity of the Greenland ice. Reflectivity, or albedo, is one of these things that stabilises ice sheets; it reflects sunlight back so effectively that the radiation can hardly make a start at melting any ice before it finds itself reflected back into space. But if high temperatures manage to get the melting process going, this lowers the reflectivity, and then your ice and snow are in peril. This self-reinforcing process, also known as positive feedback, might well herald your ice cap’s decline. What’s even worse is dirt blown on top of the snow; this may start melt at lower temperatures.

So what did Box and his fellow scholars do? They basically measured reflectivity and melt from a satellite, calibrated these results with observations from weather stations on the surface, and ran a climate model in order to get an idea of the sensitivity of the reflectivity to temperature. And what is so new about this research? Satellites have been measuring the albedo of Greenland for many years, and ground-truthing with weather stations has been done since early days too. But the results of Box et al. go all the way to the year 2011, bringing this research up to date. And their combination of observations and modelling could potentially give new insights in how the process works.

Observations of reflectivity

So what did Box and colleagues find? The reflectivity of the Greenland Ice Sheet is at a low point; 8% lower in 2011 than it was in 2000. And this is not an incident; they have observed a significant trend, though admittedly a short-term one. They further found a 26% increase in melt between 2000 and 2011. And to give you an idea of how much that is: if that rate would remain constant at 2011 level, the ice cap would be lost in roughly 6000 years. And the sensitivity of the reflectivity to temperature? That’s where it gets confusing. Over large areas of the ice sheet, reflectivity only goes up with higher temperatures. This can be explained by warm air bringing in more snowfall. But strangely enough, snowfall doesn’t always correlate with higher reflectivity in their data. And when you look at the sensitivity of the reflectivity to temperature, or in other words; by how much the albedo goes up or down with every degree temperature change, it becomes clear that their data is only statistically robust in the regions that are melting already.

The authors warn that they think summer melting will occur over the entire ice sheet in another decade, if the coming years will be like 2010 and 2011. But that is a big “if”. Box emphasizes only the decreased reflectivity in his own blog post, without being too specific about the lack of straightforward relation with actual melting. The big take home message of this paper might be that the processes governing ice melt are not yet sufficiently understood. And we want to understand it, if only to get an idea what we should do with our coastal defences. The amount of melt in 2011 measured already translates to more than a millimetre of globally averaged sea level rise. And that does not sound like much in itself, but it does when you realise it was only 1.7 mm/year on average for the 20^th Century in total; that includes for instance Antarctica, mountain glaciers, and thermal expansion.

So did they predict the ~100% melt?

Well. In a way they did. But what they really predicted was a shift to net melt over the area that nowadays experiences net snow accumulation, averaged over the whole summer. They did not mention short periods of 100% surface melt. However, you can’t get to net summer melt without, well, melting large areas of the surface once in a while. So people who say “they predicted this!” are exaggerating. But Box and colleagues are right in saying that this event greatly supports their conclusions. Given the uncertainties in their data, this was more a lucky guess than rock-hard data, yet I hope it will attract attention to the danger of Greenland melt. It’s not as if we who are alive today will ever see an ice-free Greenland, but we may well see a Greenland Ice Cap that raises average sea level by 2mm per year or more, and that is something we need to prepare for. Those who love Amsterdam, London, New York or one of these other iconic cities near sea level might wish Box luck in keeping up the good work…