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”.

Let it snow…

I have found myself shovelling a lot of snow this winter. As with last winter, it has been cold and snowy across northern Europe so far, which has led to the usual questioning of climate change by the usual suspects. There is some very good work examining this on the real climate blog and Marcus Brigstocke did his usual amusing best on the Now Show towards the end of last year, so I’m not going to write about the difference between weather and climate, or about how regional and global average temperatures differ. Rather, the time spent shovelling snow and wandering around the city streets camera in hand to take photos, really brought home how many of the snow processes that are subjects of active research in remote or mountainous areas are currently on display in our cities.

For instance, today in the local park I noticed that there is preferential melt occurring around the trees. The dark tree trunks absorb and emit more radiation that then melts snow around the trees faster than it melts in the open areas of the lawn. This is an important consideration in the planting of forests in snowy areas, since the presence of vast forests can significantly alter the albedo of the earth’s surface, that is how much radiation is reflected back in to space. Planting trees in the tundra to combat climate change may have the unintended effect of actually enhancing warming through changes of this kind.

Picture of glacier table - a boulder balanced on a thin stack of ice
Glacier table in Switzerland (Glaciers Online)

The process can also be seen to spectacular effect on glaciers, where rocks and boulders shield the ice below them from melting but enhance it around them, leading to the formation of so-called glacier tables, such as this one in Switzerland (from glaciers online).

More seriously, the heavy snow on rooves around the city is currently posing an avalanche hazard rarely encountered outside the mountains. The effect of sunshine on heavy snow, which is resting on a slope of a critical angle, can be extremely dangerous to the unwary. As are the large numbers of icicles which have developed. These are not just a sign of poorly insulated buildings (where the heat leaking out has caused the snow to melt and then quickly refreeze in the low temperatures we’ve had). Icicles falling from buildings show the same mechanics as seracs falling from the steep parts of glaciers known as ice falls. In this case, the ice builds up to such a degree that the sheer weight of it eventually causes fracture when a critical threshold is reached. Pedestrians are learning to walk on the outside of pavements and to look up frequently at the overhanging cornices of snow and ice.

But back to the snow shovelling. I have not done so much digging since fieldwork last winter in Svalbard, where we set up some experiments to study the properties of snow and how this affects the melt, or conversely the growth, of glaciers. Specifically, we were studying the effects of liquid water from snow melt or rain on the snow pack and the glacier surface. Liquid water filters into the snow, or else runs off bare glacier ice if there is no snow and will typically freeze, forming ice lenses in the snow pack, or large areas of what is known as superimposed ice on the glacier surface.   As you can imagine, there was a lot of snow shovelling, especially as the high winds on the glacier kept filling in the trenches we dug to work in.

Now this probably sounds like a fairly esoteric set of experiments, but the purpose is actually quite serious, since we need to know how much melt water refreezes to work out how much the glaciers and large ice sheets of the world are melting and how sea level rise is likely to progress in the future in a warming world.

Identifying the melt area of a glacier or ice sheet is a relatively straightforward task using satellite imagery, but identifying how much of that melt runs off or refreezes is impossible at present, so we generally use a model, based on observations and experiments like these, to make an approximation. We also need to factor in the effect of latent heat, (heat that is released when liquid water becomes solid ice) since this can warm up the snow pack significantly. In Greenland for instance, it is likely that the effects of higher temperatures over the last 20 years or so have been buffered somewhat by the snow pack and refreezing processes. However, as temperatures continue to increase, melt will probably accelerate partly because the saturated snow pack cannot absorb additional melt water but also because it has a higher temperature from the release of latent heat and thus requires less additional energy to melt.

Last winter I tested out some of the techniques we used in Svalbard, in a pile of snow in my back garden. I am also aware of at least one study into permafrost, where patterned ground usually found in Arctic climates was created in a back garden in St Andrews, so it’s even possible to do valid experimental work during the winter time when conditions are right. However, the climate of glaciated regions is generally unlike that of the cities of Europe so there will still be a need to go to places like Svalbard to do experiments quantifying these kind of processes. Nevertheless, I still find this kind of weather inspiring and I’m hoping to get more insights as the winter progresses.