Guest post: Arctic sea ice thinning.

Exciting guest post on a newly published paper by Angelika H. H. Renner.

I’ve met Angelika on a cruise in the Antarctic Circumpolar Current a long time ago where we worked on an instrument together and created an advent calendar to keep up everybody’s morale during the second month of the cruise before flying home on christmas eve, and we’ve since gone white(ish) water kayaking, hiking in the norwegian mountains, visited each other’s institutes, helped each other out in research and teaching crises (mainly Geli helping me out, to be honest ;-), and we are planning an exciting project together. Angelika and coauthors recently published the paperEvidence of Arctic sea ice thinning from direct observations“. In today’s post, Angelika writes about how the observations that went into the paper were obtained, and I am excited to share her story – and her amazing photos – with all of you.

There’s been so much liquid water on Mirjam’s blog lately, I was happy to take her invitation for a guest blog to bring back some of the most amazing, interesting, and beautiful variation of sea water: sea ice!

Sea ice comes in various shapes, from very flat, smooth, and thin sheets of newly formed ice to huge ridges several tens of meters thick. Assessing the thickness of the sea ice cover in the Arctic remains one of the biggest challenges in sea ice research. Luckily, methods become more refined, and numbers derived from satellite measurements become more accurate and reliable, but they don’t cover a long enough period yet to say much about long-term changes.

My first proper science cruise in 2005 went to Fram Strait, the region between Greenland and Svalbard. I learned how to measure sea ice thickness the hard way: drilling holes. And more holes. And even more holes. Or the slightly-less-hard way: carry an instrument around that uses electromagnetic induction to measure ice thickness (since sea ice is much less salty than sea water and therefore much less conductive). This instrument is called ”EM31” and we kept joking that the number comes from its weight in kilograms…. So, using drills and the EM31 we measured on as many ice floes as we could and given that the cruise went all the way across Fram Strait, that gave as quite a few datapoints covering quite a large area.

These measurements have been done by the sea ice group at the Norwegian Polar Institute every summer since 2003, and in some years also in spring. It takes dedication to build such a time series! When we could, we also used an airborne version of the EM31, the EM-bird, to do surveys over larger areas. Now, finally, the results of all these measurement have been processed, and analysed – and what do we see? The sea ice in Fram Strait is thinning a lot. Depending which measure you use (nothing about sea ice thickness is straight forward…), the ice has thinned by more than 50% over the 10 years from 2003 to 2012!

It’s one thing to know that it has thinned, but it’s a lot more interesting to find out why. Fram Strait is a special place: Most of the sea ice that is formed somewhere in the Arctic Ocean (and doesn’t melt there again) leaves the Arctic through Fram Strait. It is a very dynamic region with strong currents and winds, which results in a lot of deformed ice regardless of its age. The extent of the ice cover here is not necessarily linked to the development of the ice in the Arctic Basin – most prominent example was the heavy ice year in Fram Strait 2007 whereas this was up to then the year with the lowest Arctic-wide ice extent in the satellite era.

We looked in more detail at where the ice came from and found that this, too, does not correlate with our thickness time series. While the source region of the ice varied from year to year, it was continuously thinning – in our opinion a sign that the thinning occurs Arctic-wide.

A lot of effort went into this paper and the dataset behind it, and I was very very lucky that I got the opportunity to participate in several of the cruises, do the data analysis and write the paper. It’s even more satisfying to see your work published when you know how much work drilling all those holes was……

Why melting sea ice does not contribute to sea level rise.

Simple experiment on why the impact of glaciers and sea ice on sea level, respectively, are not the same.

It could be so simple: An ice cube sinks into water until the mass it replaces is equal to its own mass.

The mug is as full with water as it gets. But even if I stared out of the window at the mountain and the snow until this swimming piece of ice had completely melted – the water level in the mug would not have changed.

Since the mass of said ice cube is not changing when it melts, under the assumption that the difference in volume due to the temperature difference of the melt water and the water in which the ice cube swims is negligible (reasonable assumption in most cases) that means that a swimming ice cube can’t change the water level in a cup and a swimming ice berg can’t change sea level. Things are different for glaciers or other ice that is sitting on land rather than freely swimming.

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I should have thought about how I would transport the plate on which the mug with the ice is sitting back to the kitchen once the ice has melted. In other words: Yes, the mug will spill over.

This is a very easy demonstration and while it is intuitive that in the second case a mug that was completely filled with water when the ice was first added will spill over once the ice melts, the first case seems to be very difficult. Most students are not quite sure what they are expecting to see, and even if they are, they don’t really know why.

My typical drawing to explain this topic. The potato is supposed to be an ice berg floating in water.

I have always been teaching this by drawing the water level and the ice berg on the board, and then by marking the volume of the whole ice berg and the part of it that is under water, and trying to stress how the mass of the ice berg is the same as that of the water replaced by the part of the ice berg that is under water (because the molecules are more densely packed in liquid water and yada yada) — there must surely be a better way to explain this? Any ideas out there?

Experiments the Isafjördur way. Can you spot the two mugs and the ice in the middle of the window sill? Floating ice on the left, a “glacier” resting on forks above the water level on the right.