Melting Ice - Part 1 - Glaciers

Now for how changes in the volume of ice affect sea level – of all the causes of sea level rise this is perhaps the most tangible – it’s really easy to imagine how melting ice literally adds to the amount of water in the oceans. And there is just so much ice – think of Antarctica, Greenland, the Arctic, glaciers, permafrost, that iceberg that sank the Titanic...

 

But, is all ice equal when it comes to impacting on sea level? And how important is melting ice to current sea level rise? This is quite a big topic so I’m going to split it over a few posts. First up: glaciers and icecaps - except for the ones in Antarctica and around Greenland. Glaciers form on land so reduction in glacier mass is really important to sea level because they are effectively storing water on land.



 Ice mass loss of glaciers (excluding those on the periphery of ice sheets), Greenland, and Antarctica, and the equivalent sea level rise (IPCC)

 

The most important glaciers in terms of their impact on sea level are the ones at high latitudes because the poles are warming more than low latitudes, and modelling forecasts suggest most warming here in the future. However, their very locations mean that these are some of the fewest measured glaciers (Arendt et al, 2002). Measuring ice mass loss is difficult even in amenable locations, and measurements are often sparse, or concentrated on specific smaller glaciers which leads to bias in sea level estimates (Jacob et al, 2012). The IPCC, in Assessment Report 5 states:

‘The average rate of ice loss from glaciers around the world, excluding those on the periphery of ice sheets was very likely 226 Gt/yr over the period 1971 to 2009, and very likely 275 Gt/yr over the period 1993 to 2009'

A large component of the recent reduction in glacier volume is from the loss of marine-terminating glaciers which become increasingly unstable when their bases are below sea level (Meier et al, 2007). Instability promotes iceberg calving, thinning, and retreat. Seems like a bit of a vicious circle here - sea level rise, more unstable glaciers, more sea level rise, etc etc etc.

 

 

So it seems that the rate of glacier reduction around the world is increasing. Lemke et al (2007) calculated that melting all the glaciers and ice caps (excluding those near Greenland and Antarctic) would cause a 15-37cm rise in sea level. This is important because these glaciers can change volume relatively quickly in comparison to the ice sheets of Greenland and Antarctica (Oerlemans, 1992). Some glaciers have already disappeared and glaciers are retreating in most areas (Frezzotti and Orombelli, 2014), but are all the glaciers likely to melt?

 

Well, it's difficult to say, and not helped by the unequal distribution of glacial measurements. The IPCC (one day it will be the Independent Police Complaints Commission and then you'll all be surprised!) only have 'medium confidence in the use of glacier models to make global projections'. They have modelled glacier decline using several possible representative concentration pathways (RCPs) for greenhouse gas emissions until 2100. Projections range from a 15 - 55% decrease for RCP2.6, to 35- 85% reduction in glaciers if the RCP8.5 is used. Worryingly the RCP2.6 represents the lowest greenhouse gas emissions of all the RCPs over the next 100 years, and modelling still suggests it would equate to a huge loss of land ice.

 

Sounds a bit doom and gloom, and I haven't read anything on this topic to cheer me up, so here is a nice picture a glacier - better look at it while we still can.



Nice picture of the Perito Moreno Glacier, Argentina (nicewallpaper). Interestingly if you type 'nice picture of a glacier' into Google you get icecream.

 

 

Lovely weather for ducks...

When speaking to one of my less environmentally inclined friends about my blog, he declared that he would only read it if there were ‘cute pictures of ducklings or something’. Far be it from me to put someone off my blog for lack of some cute animals, so here they are:

From Breffni O’Rorke’s private collection of duckling photographs...apologies for the ugly duckling on the right – I’m sure that soon it will be a beautiful swan.

 

And how exactly can I link ducklings to sea level change? I was flummoxed too until I came across this article in The Telegraph. Yes, it’s a few years old but nevertheless a link between ducks and sea level rise in Bangladesh.

 

In 2009 the UK government pledged £70 million pounds to help Bangladesh cope with climate change and sea level rise. Part of this money was to be spent on encouraging people to keep more aquatically inclined poultry, so it’s ducks in and chickens out. Apparently ducks can even help with rice farming by fertilizing the land, and acting as a natural fertilizer – they may even reduce methane emissions from the the paddy fields (ClimateChangeNews) – sounds amazing – why did anyone ever keep chickens?

A little more research into ducks and Bangladesh ,and turns out it’s not even conventional ducks. Being able to swim just isn’t enough in the cut throat world of sea level rise – the normal ducks were getting ill as groundwater becomes increasingly saline with rising sea levels. So saline happy Superduck - aka the Campbell duck – to the rescue (ChristianAid). I’m imagining a duck with a mask, cape and plough so I’m not going to post a picture of the real Campbell duck because you’ll only be disappointed.


So I've been thinking about this (and trying not to let cute duckings clog my mind), and short-term it does seem like a wonderful idea - as least people won't lose everything when it does flood. I also think ducks would be a great way of coping with sea level rise or just more general flooding, if it was temporary, and seasonal. But it isn't, especially in Bangladesh which, being essentially a floodplain, doesn't have any topographic advantages. Ultimately, while ducks are a nice stop-gap they don't deal with the bigger picture. The groundwater shouldn't be so saline that more conventional poultry can't survive because humans won't survive either. And long- term as the sea levels rise and flooding becomes more common, or permanent, where will all the people and their ducks go? Even ducks like land sometimes.
 

Thermal Expansion of the Oceans

Time to start investigating why sea level changes. I’m going to begin with thermal expansion of water - the biggest cause of observed global mean sea level change (GMSL) since 1971 (IPCC).

 

 

The proportion of sea level rise attributable to thermal expansion or contraction of sea water is called the thermosteric change. According to the recent IPCC report, between 1971 and 2010 thermal expansion caused on average 0.8 mm/yr rise in GMSL, but satellites observations between 1993 and 2010 show a 1.1 mm/yr rise in GMSL as a result of thermal expansion. So observations over the last fifty years suggest that the rate of thermosteric rise is increasing.

So, how does this work?

It’s all to do with the heat content of the oceans. Ocean water changes density with temperature and salinity – colder, more saline water is denser than warmer, fresher water. As the heat content of the oceans increases, the density decreases, and the water takes up more space causing a rise in sea level (e.g. Johnson and Wijffels, 2011). So it is important to know how much heat the oceans are accumulating in order to work out how this can affect sea level.

 Figure 1: Comparing changes in global ocean heat content for the top 700 metres, and the top 2000 metres (NOAA).

Figure 1 shows that heat content is increasing, but also that the oceans aren't warming uniformly - there is a higher rate of heat content rise in the top 700 metres. Heat content of the top 2000 metres has increased by 24.0 x 10²² J from 1955 - 2010, and of this 16.7 x 10²² J is in the top 700 metres alone (Levitus et al, 2012). This is because the ocean is warmed at the surface where it interacts with the atmosphere and absorbs heat from the sun.




Figure 2: Ocean heat content change as a function
of depth 1955 - 2010 (Levitus et al, 2012)

 

Unsurprisingly heat content change in the oceans also varies with location. Figure 2 shows the Pacific Ocean  has the highest  heat content change for surface waters in the last 50 years. But deeper than 200 metres the Atlantic Ocean has the highest change in heat content of all the oceans. There has also been warming in the deepest parts of the oceans - of the waters below 3000 metres depth, the Southern Ocean has shown the highest heat content change (IPCC).


So it is clear the oceans are warming but how exactly does this translate into sea level rise?


Well, it all depends on the original temperature of the water, and the pressure it is under. Colder water at lower pressures will expand less than warmer water under higher pressure when warmed by the same amount.


So pressure and temperature...


Thermosteric sea level change is affected on decadal timescales by pressure-changing climatic phenomenons such as El Nino, the Southern Oscillation, and the North Atlantic Oscillation (Ishii et al, 2006). Atmospheric temperature affects thermal expansion in the oceans on seasonal and longer term timescales (Wigley et al, 1987). This is what we are observing right now with greenhouse gases increasing atmospheric temperature.


All this means that, while thermal expansion is the biggest contributor to sea level rise, it does not have an even contribution across the world (see Figure 3). This map actually surprised me - I was expecting highest thermosteric sea level rise around the equator where the water is warmer. In fact, while there is is some rise near the equator above Australia, other equatorial regions are experiencing thermosteric sea level fall and the main areas of thermosteric sea level rise are actually in the north Pacific. I'm not actually sure why this is - I will have to do some more reading and get back to you - in the meantime if you have any ideas (or indeed the answer!) let me know. 

Figure 3: Estimated thermosteric sea level change in the top 700 metres, 1993 - 2003 (Church et al, 2008)

 

 

Floating Houses

So what’s the most obvious way to stop sea level affecting you?....Build a house that floats. Who cares what happens to the sea level as long as you’ve got a good anchor. Failing that you can always use a rise in sea level as an opportunity to find some new neighbours, or position yourself closer to Tesco. Decided Waitrose is more your style? No worries, what with the changeable climate it won’t be long before you can float off again and maybe this time be deposited in a really desirable location.

I knew Rosie and Jim were on to something. Turns out someone else realised too...

Ominous clouds over the floating houses in Ijberg, Amsterdam. Check out Marlies Rohmer's website for more information (and photos of the houses being towed through the canals of Amsterdam to get them to their final position)

Architects in the Netherlands have designed and built houses that can bob up and down with changing sea level (OK, you got me – these houses are chained down so sailing off into the sunset with your house isn’t technically possible...yet). The houses, on the outskirts of Amsterdam, float using the Archimedes principle of displacing more water than their weight so even though their hulls are made of concrete they still float. To make sure that the houses are perfectly balanced, the future owners have to decide in advance where they are going to position their furniture so that the builders can alter the weight of the walls accordingly – personally, I’d worry about moving furniture around and unbalancing the house.

So how is Canute doing in his floating house, (or castle)? Well providing castles aren’t top heavy, and he doesn’t do anything drastic with the furniture he'll be sitting pretty. These houses are not even the first of their kind – turns out there are floating settlements all over the world - for instance Pori, Finland, and Aberdeen in Hong Kong. I do wonder what sort of effect floating houses have on biodiversity by blocking out light and increased noise and pollution. These particular houses are built on a lake – if this area became tidal with future sea level rises the designs might have to be modified to cope with the waves. Long term if these problems can be allayed then floating homes do seem like a good way of coping with sea level rise -  as long as you don't get sea sick! 

Would you live in a floating house?

Don't know the answer yet? Watch this short interview with one of the floating home residents or visit the Marlies Rohmer website to find out more:

 

A whistle stop tour of changing sea level

First,  before I start looking at the hows, and the whys, and the whodunnits I want to put everything into context with a brief overview of how global mean sea level has changed since the oceans first formed ~4 billion years ago (Nutman, 2006).

Hallam (1984) reconstructed sea level from the Neoproterozoic using sequence stratigraphy and marine epicontinental cover. It is worth bearing in mind that with increasing age, and the inevitable erosion of data, the error margins are larger, but nevertheless it does show something interesting: global mean sea level has varied hugely over the last 590 million years (Fig. 1). Crucially the reconstruction suggests that sea level has been higher than today for most of geological history, only coming close to present day levels around the Permian - Triassic boundary and then more recently during the Cenozoic.



Figure 1: Sea level change for the last 590 million years as inferred from seismic sequence stratigraphy. Graph adapted from  Lambeck and Chappell (2001) using Hallam's data.

So what’s all the fuss about? Fluctuating sea level is not a new thing; in fact, it’s pretty much routine, happened loads of times before, maybe this subject is getting a little old? Maybe I should change my blog topic...

 

But let's zoom in on more recent history (still geological history mind!). Fleming et al (1998) reconstructed global mean sea level (GMSL) since the Last Glacial Maximum (LGM) using data from thirteen locations with an emphasis on farfield sites (Fig. 2). Farfield sites are important because they were furthest from the ice sheet coverage during the LGM so are not affected by isostatic rebound. Because of this reconstructions based on data from these sites are thought give a better indication of GMSL during the Holocene (Milne et al, 2005). Figure 2 shows sea level rising by ~120 metres after the last ice age and this appears to be followed by a period of relatively stability from 7ka to the present day. 

So there is relatively constant sea level for thousands of years as our ancestors are populating the world, and choosing where to bring up baby Ugg. And if the sea level's not going to change why wouldn't you build your hut with a sea view?
 

 

Figure 2: Sea level reconstructions for (a) the period following the LGM,and (b) for the Holocene (Fleming et al, 1998).

Zoom in again, and this time with the most accurate data yet. Sea level has been measured using tide gauges from around the world, and since 1993, satellite altimetry can be used too. Figure 3 compares several different reconstructions of eustatic sea level from tide gauge data since 1880. While there are differences in the individual reconstructions, all of them show that the period of relative stable sea level is over and that sea level is rising.



Figure 3: Reconstructed eustatic sea level using various methods and tide gauge data (IPCC)

 

On top of this rising sea level the IPCC has stated that:

'It is very likely that there will be a significant increase in the occurrence of future sea level extremes in some regions by 2100, with a likely increase in the early 21st century'

A slight problem then for the millions of us who are living near the coast, or in some cases already living below sea level.

Finally, I want to leave you with one more image. Figure 4 shows the global changes in sea level as measured by satellite altimetry from 1993 - 2008. Clearly, the rate of sea level change is variable across the globe. I'm excited to find out how the countries in a whole new kind of red are coping with the sea level rises and what plans (if any) they have for the future.



Figure 4: Sea level changes from 1993 - 2008 as measured by satellite altimetry from the Laboratory for Satellite Altimetry

Ahoy there!

So we can’t all be like the late great King Canute and take our thrones to the edge of the sea and forbid the waves to come any closer. His failure to take control of the ocean hasn’t prevented us trying to understand the oceans and from coming up with more imaginative (albeit more complicated) ways to force back the tides.

I want to use this blog to explore two inter-related topics:

1. The reasons behind sea level change. Whether it be through thermal expansion of sea water, changes in ice, tectonics, or isostasy, sea level has been changing ever since there was a sea level to change. I will investigate how these processes interact and affect sea level on global and regional scales.

2. How humans have reacted and adapted to mitigate the effects of the encroaching or retreating tides. At first glance, solutions range from the highly technical, such as predictive dams in the Netherlands, to the simple, let's all move house or indeed just sitting in the surf and pretending nothing is happening. I am going to examine the effectiveness of these solutions and consider how they might work in the longer term.

 

But I’ll leave that for next time, for now I will just leave you with Canute (currently dry) on the beach.



Canute likes to be beside the seaside....