Retreat?

So now it's time to say goodbye! After having spent the last couple of months, while writing this blog, looking at what we are dealing with in terms of sea level rise, and the some of the ways we are trying to cope with the problem. I've really been struck by the extent of the problem that we are faced with. While it will effect some coastlines alot more than others, this is definitely a global problem.


I've looked at a range of possibilities from man made to natural that can help protect our coastlines, but the over-riding impression that I get is that long run these cannot help us. I've started to wonder whether it is ever really possible to mitigate for sea level rise in the long term or whether we are just putting off the inevitable For some areas I think that in the long run (and I don't even mean that far into the future) retreat is the only option. Future projections include considerable sea level rise, and I for one am not sure I would feel comfortable living behind a sea wall or dyke and below sea level.


Already some communities are facing up to the possibilities of relocating. Many island nations in the Pacific have already been forced to abandon some of the lower lying islands, and communities in Alaska are considering moving inland after increased storms and coastal erosion (Kelman, 2008). Of these Tuvalu is perhaps the most at risk, but they are also the only ones with a plan - when Tuvalu is finally submerged the Tuvaluans can have a home in New Zealand. This is not the case for all the island nations, others such as Vanuatu, and the Cook Islands have no such safety net in place. This brings up the really interesting question - whose responsibility is it to re home these communities? And then there are also all the communities that won't be washed away but rising sea level could destroy their way of live, and their livelihood. These will be the majority - how can we look after these people too? Relocating comes with problems too - one village in Alaska voted to move inland after higher sea levels and storms were threatening to destroy their houses. But having announced their intentions they were no longer able to secure funding for important infrastructure such as hospitals and schools (GRIST). It's really important that these people don't become refugees in another country, and that it is possible to find a new home where they can keep their sense of community rather than being spread about and swallowed up by another country.

I've also been thinking - to what extent is this a problem of our own making? We have released unprecedented volumes of greenhouse gases into the atmosphere causing warming, and ice melting and thermal expansion. But we have also situated many of our largest population centres next to the coast making them unnecessarily vulnerable to any small changes in sea level whether it be from natural variability, or from anthropogenic causes. Never before in human history have our numbers been so many, our populations been so static, and it been so difficult for us to relocate to higher ground.

Credits: Nick Lyon

 

So what would Canute do? This is the thing about King Canute, he never actually thought he could hold back the sea. He only sat on the beach and told the sea to stay back as a way of proving to his courtiers that he was not all powerful, and could not control nature. Perhaps this lesson from a thousand years ago is worth remembering as we move forward. We cannot control nature, and perhaps it might be better to use our resources to look after ourselves rather than holding back the sea.

Future Sea Level Rise

I've spent the last couple of months looking into the various reasons for sea level rise, so I think it's high time (or high tide?) I put all this together and consider how the sea level could change in the future.

This looks like a worst case scenario for King Canute (Credit: Henry Tapper)

Scientists have varying degrees of confidence in each component of sea level rise, and for a long time all the different causes could not explain the extent of sea level rise we were observing - until groundwater was included that its. Even as it is, the melting of ice sheets have the highest potential to increase sea level - perhaps global mean sea level rise of up to 70 metres, but ice sheet dynamics aren't fully understood yet (Rahmstorf, 2007), so as you can imagine it is very very difficult to predict how sea level is going to change in the future. So let's start with the IPCC AR5. They state:

"It is very likely that the rate of global mean sea level rise during the 21st century will exceed the rate observed during 1971-2010 for all Representative Concentration Pathway (RCP) scenarios due to increases in ocean warming and loss of mass from glaciers and ice sheets."

 

This is very important because even the RCP2.6 scenario, in which emissions stabilise and then start to decrease before 2100 (van Vuuren et al. 2011), involves increased sea level rise. You can see this more clearly on the graph below. So even if we do everything we can in terms of reducing our emissions, models agree that we will experience continued sea level rise of at least 30 cm by 2100.

Projected sea level from the RCP2.6 and RCP8.5 scenarios relative to 1986-2005 from the CMIP ensemble (IPCC)

I think in terms on the way a warming climate will impact on our lives, increasing sea level is arguably the biggest problem that we are going to have to deal with. You can start to get an idea of the extent small sea level rises will effect us here. Just taking a look at some of the countries I've already looked at and a 1 metre global sea level rise, it's really quite scary the extent of the problem. So forward planning, mitigation, and adaption are essential. But how reliable can any projection of sea level rise be?


Rahmstorf et al. (2007) compared model projections from the 2001 IPCC report with subsequent observations of sea level. The models, which began in 1990 and didn't incorporate any sea level observations after this date, consistently underestimated global mean sea level rise between 1990 and 2006, despite accurately predicting other aspects of the climate system such as carbon dioxide. Church et al. (2013) suggests that this could be natural variabilty, such as the 60-year sea level oscillation proposed by Chambers et al (2012), not recreated by process based models, rather than a systematic misrepresentation of the climate system. Models have advanced considerably since, alongside out understanding of the climate system, and computer power allowing inclusion of more processes, but it's still worth remembering that all models are wrong - they can only be an indication of future change. With any projection we should always bear their limitations in mind.


So we are pretty sure global mean sea level is going to rise, but it's a bit hard to comprehend the effect this will have on us. I think it's really important to look at the impacts on a more personal regional level. We don't expect sea level to rise uniformly all over the world - things like salt content, ocean dynamics, and post glacial isostatic rebound mean that sea level changes differently all over the world. Some countries will experience accelerating sea level rise, while others will experience falling sea level despite the trend in global sea level. It's all about being prepared, if this is something you are interested in, definitely check out Yuchao's blog on the future economic impacts of sea level rise.

Windmills

I think it's about time to take a brief look at how the Netherlands are coping with rising sea level. The Netherlands are famous for their low topography, half the country is less than one metre above sea level, and the highest point in the Netherlands is actually in the Carribean! Because of this the Dutch are the experts of mitigating against sea level rise, I mean I've been reading the IPCC's Assessment Report 5 ALOT recently, and I can't help but notice that 10/57 of the contributing authors for the sea level change chapter are from the Netherlands. Everyone knows about that little Dutch boy with his finger in the dam, but somehow I don’t think that's the secret of  the Netherlands' mastery of keeping the sea at bay.



Source: Flexitreks



The Netherlands is really interesting because not only do they have many types of coastal defence, but they have also been actively reclaiming land from the sea for hundreds of years. I want to use this post to look at windmills, for some reason I was convinced that these were an intrinsic part of the tulip growing process, but actually windmills have been instrumental in allowing the Dutch to live below sea level. This is definitely something that more countries and cities will have to start considering in the future.


It all started hundreds of years ago when the Dutch started draining marshland to make more farmland. They built canals and ditches to drain the land into rivers, but the land started to subside until it was the same sort of level as the rivers, and didn't drain any more and very liable to flooding (van Schoubroeck, 2010). Obviously this was a bit of a problem a so they made dykes to protect their land from the rivers, more ditches to divert the water, and windmills to pump the water out of the new farmland, to somewhere less annoying, usually a storage lake. The windmills either used a scooping wheel or an Archimedes screw to get the water to higher levels. This was the start of the typical polder system. I have found a little diagram here:



How windmills get water to go uphill (Source: iamexpat)



A really good example is Kinderdijk, a village below sea level that built 19 windmills, together with a combination of ditches and sluices, in the 18th century to pump water out of the land into a storage basin higher up. Who knew all you needed was a bit of wind and an Archimedes screw to live below sea level?


Sadly, the original wind-powered water pumps are no longer in use - but their steam then electrical pumping station descendants are still essential all over the Netherlands. In fact the Dutch pumps are so good, that when the Somerset levels flooded last year the Environment Agency borrowed 13 to try and reduce the water level (BBC News).

 

I love windmills, and I think that it's fantastic that it's possible to live several metres below sea level with a bit of ingenuity and wind power. I'm sure that something along these lines could be used elsewhere - I mean they have certainly proved that they work.

Halosteric Sea Level Rise

Sort of scraping the barrel here - the salt barrel that is (are salt barrels even a thing?), time for salinity induced sea level rise aka halosteric rise. This together with thermosteric rise that we discussed way back in October is the steric component of sea level. Basically meaning that heat and salt effect how much space the water in the oceans actually takes up.

Just in case you weren't sure ... here is some salt (PlanetScience)

But how on Earth does salt effect sea level?


It's the same as thermal expansion - it's all to do with changing density of the oceans. The saltier the oceans the more desnse they are and the less space they take up. But as we are seeing at the moment the oceans are getting fresher as ice melts around the world and effectively dilutes the oceans. So they are less dense, and this means they take up a little more space, so sea level is higher (Antonov et al, 2002).


But it's actually quite a bit more complicated than this...


Because the oceans are so so so big, they can pretty much be assumed to have constant salinity even over hundreds of years (Antonov et al, 2002). This means that halosteric sea level rise is nowhere near as important for global sea level as thermosteric rise. Ishii et al (2006) have estimatic that halosteric sea level rise accounts for only 0.04 mm a year between 1955 and 2003, compared to the 0.31 mm a year they attributed to thermal expansion for the same period. 


Infact, the interesting thing about halosteric rise is that only ~1% of the halosteric expansion actually adds to the global sea level. This is because when freshwater is added to the oceans, while this does decrease the salinity and density, this is counteracted by the freshwater getting denser because it has mixed with the salty oceans. This basically means that you cannot really look at the two processes separately - the sea level is mainly rising because water has been added to the oceans, rather than the salt concentration decreasing (IPCC AR4). As you can see in this graph from NOAA, the halosteric component of sea level rise is minimal:

A very small upwards trend is observed in the halosteric component of sea level change (NOAA)

But don't disregard it so quickly, salt is still a really important control on regional sea level - the oceans aren't the same saltiness everywhere. As salt content is redistributed through the various ocean basins there can be an major impact on local sea level. At the moment the salt content of the Pacific Ocean is decreasing, while the Atlantic is getting saltier. Modelling suggests that in the Atlantic the high salt content is strongly counteracting the effects of thermal expansion, so sea level is not rising as fast as it could be (Durack et al, 2014).


So less salt on your chips are more in the ocean? Might help the decrease the sea level! On second thoughts - probably there isn't enough salt in the world to account for all the other causes of rising sea level that I have previously discussed.

Artificial Reefs

Now for another type of hybrid coastal defence - artificial reefs. Artificial reefs can be made out of anything from concrete, to old tyres, to intentionally sunken boats (NOAA). And they have loads of exciting and useful benefits ranging from providing new habitats to marine life, supporting beach nourishment programmes, reducing wave energy reaching the shore, or even creating great waves for surfers (Corbett et al, 2007). Interestingly, artificial reefs were first created as a way stock enhancement in an area to improve fishing (Chuang et al, 2008).

I had to use this one because Nemo (or his dad) is in the bottom right corner (NileConstructionInc)

Ferrario et al (2014) looked at lots of studies on the efficacy of reefs in terms of protecting the coast. They found that the reef crests dissipated on average 86% of wave energy heading for the shore, and that the reef flats, which are closer to the shore could deal with 65% of the remaining energy. In total, a complete reef system can dissipate on average 97% of the wave energy that would otherwise reach shore. On top of this research has shown that in areas such as Tuvalu where the natural coral reef has been mined, become degraded, or cannot keep up with rising sea level the amount spent on new artificial barriers increases (Ferrario et al, 2014).


Clearly natural reefs are really important, artificial reefs are attempt to try and replicate these benefits. While they don't actively protect the coast from higher sea level, by minimising wave energy reaching the beach they reduce coastal erosion and protect active sea level defences such as sea walls. In this respect they can be quite successful, and are likely to become increasingly common as the oceans become stormier with a warming climate (Moschella et al, 2005).


However, as with all things they have to be thought through carefully - in Florida an attempt to build an artificial reef from tyres as a way of increasing biodiversity has turned into an environmental nightmare. The tyres quickly became loose and currents moved them around so they build up against the natural reef system causing death and destruction for marine life (USAToday).



How not to make an artificial reef - Osbourne reef today (Source:ProjectBaseline)



So essentially - I think artificial reefs are great but natural reefs are way way better. In any case, forgetting about their other benefits, in terms of sea level rise they are short term responses for protecting other defences and reducing erosion. I do also wonder a bit if some artificial reefs are a form of pollution - whether it be concrete, tyres, or old boats, none of these things are meant to be on the sea floor - and their presence can't be great for marine biodiversity in the long term - new coral reefs will never be able to grow underneath, and apart from anything else we are really going to confuse the geologists of the future...

Post-glacial adjustment

Happy Christmas Eve!!! I've got an unwanted sequel, or perhaps the Christmas special, of my melting ice mini-series - post glacial isostatic adjustment - or - what happens when all the ice leaves.



(BGS)

The isostasy concept is really simple, imagine that the lithosphere, and all the tectonic plates are floating on the asthenosphere. The plates would really like to be in isostatic equilibrium with the asthenosphere and are always making little adjustments to make sure this is the case (Karner et al, 1984). So when a 4km thick ice sheet grows over North America, the lithosphere responds by sinking a little deeper into the asthenosphere. Sea level in these areas will appear to rise. Adjacent land not covered by the ice sheet also responds by rising slightly higher - this is the peripheral forebulge (Fjeldskaar, 1994). Sea level here will appear to fall.


Click here for a great interactive demonstration of isostasy from Cornell University. I just hope that what happens when you increase the block density to 3.4, is not representative of the real world...


But then, when the ice melts, the land beneath it is still depressed - it has to start making isostatic adjustments again to put it back into equilibrium with the asthenosphere - this can be a really slow process over thousands of years. At the same time, any land that was pushed up during glaciation starts to subside (Fjeldskaar, 1994). It isn't just ice formation and melting that can upset the isostatic equilibrium, volcanoes, erosion, and mountain building can also do it too (Watts, 2001). 


Post-glacial isostatic adjustment is particularly interesting for modern sea level changes because we are still experiencing isostatic adjustments from the last ice age all over the world. In fact it's hard to go anywhere that isn't still responding to the effects of the ice sheets. Locations which were furthest from the ice - generally in the tropics - are called farfield sites - these are also some of the best places to go if you are trying to reconstruct sea level for the past (Bassett et al, 2005). A good excuse to do research in a pleasant climate I think.  


Isostatic adjustment affects only affects regional sea level - by directly raising the land, or causing it to subside. Take the UK for example - 18000 years ago during the Last Glacial Maximim (LGM), Scotland and all of England apart from the south-west, was covered by the British Isles Ice sheet (Massey et al, 2008). The increased weight of the ice caused the land to sink down, with the exception of the south-west which rose up. But now the ice has melted Scotland and the North of England are rising out of the asthenosphere and gaining elevation - they are experiencing a relative sea level fall. Meanwhile, the south is experiencing subsidence and a relative sea level rise throughout the Holocene (Massey et al, 2008, Shennan et al, 2000).


So essentially, post glacial isostatic adjustment is like the secret effects of melting all the ice. It's something that we really can't control, and will still be affecting sea level long after the ice all melts. 

Sinking Cities

And now to Venice; City of love, rotting egg odours, and serious sea level problems:

 

I don't know - these guys look pretty happy - maybe it's like this there all the time? (The Journal, 2012)

 

In terms of water level, Venice's problems are twofold; firstly sea level is rising, and secondly the city is sinking. During the 20th century relative sea level rose by 25 cm, 13 cm of this was due to sea level rise, and the remaining 12 cm was due to subsidence of the city (Carbogin et al, 2010). This combination means that ordinary high tides, and storm surges are increasingly likely to cause flooding of the city which has an average elevation of just 90 cm above sea level (Carbogin et al, 2010).

 

 

But the big question is what are the Venetians planning on doing about it?

 



The main plan of action is the MOSE project - named after Moses who parted the sea to let the Israelites escape Eygpt (do you think there is a reason that there are no tidal barriers named after King Canute?).

The Mose Project, once completed, will be a complicated mile long barrier system across the entrances to the Venice Lagoon. When needed it can be raised to protect Venice from high tides and storms, when not needed it will be completely concealed beneath the water. I've found a little video of how the barrier will work:

But the project is not going smooothly;  construction which started in 2003 was due to finish in 2014, but the deadline has just been extended to 2016 (TheVeniceTimes, 2014). On top of this concerns have been raised about the impacts on the lagoon's ecosystem, and that raising the sam will prevent sewage from leaving the city, making it smell even worse - apparently Venice doesn't have a sewage system, it all goes into the canals (Venepedia) - not sure what the people in the first picture are thinking! And then to top it all off, the Mayor was arrested for corruption linked to the project and put under house arrest (TheVeniceTimes, 2014). But, forgetting about all that for now (!!!), the barrier will be complemented by raising pavements and walls in low lying areas, and by building a lock to at the entrance of the lagoon to allow boats to leave even when the barriers are up. 

Sounds interesting, and especially so because Venice is a very high profile city. I do really like that the barrier will be hidden when not in use - although having said that I do think that our Thames Flood Barrier is really quite beautiful. The current plans will just hold back the sea when necessary, but as we saw with the Thames Flood Barrier, all barriers have a life span. Without a way to raise the city up out of the water, there are going to be serious issues in the future - it will be interesting to see what happens.