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.

Groundwater Use

I'm sort of amazed that I haven't run out of things that are contributing towards sea level rise. Here is another thing that I would never really have thought would contribute to sea level rise on a global scale- groundwater removal. Turns out I'm not the only one who's been slow on the uptake - of all the contributors to sea level rise - the addition of groundwater is one of the last to have been quantified, and it's not even that insignificant (Mascarelli, 2012).


Groundwater has some really important economic benefits and it's extraction has exploded in the last fifty years. But the extent of groundwater use has lowered the water table in some places, increasing the cost of extraction and causing subsidence (which also adds to sea level), and reducing water quality (Konikow and Kennedy, 2005). It's all a question of scale really - there are billions of us who need to drink, and there are millions of square kilometres of crops that need watering, and there are all the industries that need water too. And in some areas there just isn't enough rainfall, so we pump water out from the ground, and when we've finished with the water we don't put it back underground, more often than not it runs out to sea and starts to add to sea level (Konikow, 2011). But how much of a problem is this in terms of sea level?


Thing is, it is really difficult to model groundwater depletion and recharge. It's even difficult to know how much there is in the first place. In some places it has been accumulating for thousands of years, or accumulated when there was a wetter climate, so actually it can be considered a non-renewable resource - rather than fossil fuels, we're talking fossil water (NOAA).


It's older than you think (Hydrology)

Pokhrel et al (2012) modelled global land based water storage and how it was used with a particular emphasis on human activities such as reservoirs, and agriculture. Reservoirs are interesting because they actually  lead to fall in sea level because so much water is stored on land. Their model suggested that overall, changes in groundwater had been contributing a whopping 0.77mm a year between 1961 and 2003. This accounts for 42% of sea level rise. Wada et al (2012) also modelled the contributions of groundwater to sea level rise. They used three general circulation models, with 50 km grid spacings, which suggested that groundwater contributed 0.035 mm a year to sea level in 1900 and that this rose to 0.57 mm a year by the year 2000. Which ever way you look at it, this is alot lower than the calculations by Pokhrel at al, but both examples do show that groundwater is hugely important in terms of sea level.

 Wada et al (2012) also went on to model groundwater contributions for the future. Their model included socio-economic projections for each country or region, so they could try and include how water use in various areas would change with future developments. They found that by 2050 groundwater would be adding 0.82 mm a year to the global sea level rise.

Wada et al (2012) projection of groundwater contribution to global sea level until 2100 using two general circulation models and three emission scenarios. The B1 emssion scenario is the only one which starts to suggest that groundwater contributions to sea level could start to decrease in ~2050. This scenario involves increased awareness about the environment from the population, as opposed to the  A1 in which economic
development continues unabated, or the A2 scenario in which there is no unified response from the global population (IPCC).

 
So I think there are two issues here, first that the current rate of groundwater use is unsustainable and may ultimately mean that wells run dry or that any water that does come out of them is undrinkable. And secondly that groundwater withdrawal is contributing to sea level rise.  I think the important point for both issues is one of sustainability - we are already into the mindset of sustainable forests, but is it possible to extend this to sustainable water supplies? Looking at the projections for sea level rise as a result of groundwater alone it is obvious that we are going to have to do something really drastic to combat groundwater use, or to stop it from getting into the oceans. The B1 scenario , the only one to show an eventual decrease in groundwater use, is an extreme case, and one that frankly I don't think we will able to replicate.


 

Beach Nourishment

During the post on what NASA was doing to protect their property from rising sea level I briefly touched on beach nourishment as a possibility that they were trying out to protect themselves from sea level rise. I want to have a look at this in a little more detail because, as I mentioned last week, it is a hybrid defence solution that makes a compromise between cost and effectiveness. It is a commonly used method of trying to mitigate against sea level rise, storm surges, and coastal erosion (Trembanis et al, 1999).

Beach nourishment in action (GreenfieldGeography)

Beach nourishment is a really simple concept - more sand or pebbles are added to the beach - either from other beaches further along the coast, or material dredged from offshore. It is a form of soft protection as opposed to hard protection methods like building walls or more man-made barriers (Hanson et al, 2002).  Soft protection can be advantageous because as well as reducing erosion, maintaining beach profiles and protecting inland areas from flooding(Hanson et al, 2002), it has the added benefit of also being more aesthetically pleasing, and is an attempt to fit in better with the local environment (Peterson et al, 2005). However, it's not all good news, even with the best intentions in mind, beach nourishment can have a negative impact on the local environment by burying habitats, reefs, and wildlife (Peterson et al, 2005).

'Please don't bury me' beach nourishment can be very bad news for sea turtles (ABCNews)

But most importantly for this blog - how effective is beach nourishment at protecting coastal communities from the effects of sea level rise?


As we saw last week in the Resilience to Extreme Weather report, beach and dune nourishment is the most effective of the non-engineered solutions, but it is also very expensive solution. And it's not a one off cost - beach nourishment is very far from permanent - from the moment the beach is fed the sand is being eroded again. It is important to look at the rate of erosion from the beach first to see if the repeated cost of replacing sand or pebbles is worth it in the long run. Because of this, nourishment, especially on long beaches, is usually combined with more permanent harder protection, such as groynes to try and reduce constant erosion (SNH).


I think it's becoming very obvious that there is no final solution to sea level rise - even the most engineered solutions which take into account future sea level rise projections still have a shelf life. To me, beach nourishment is a bit like trying to evade the truth - by extending the beach you can imagine that sea level rise is not happening  - you still live by the sea and your view has not been obscured by a dam or sea wall. I think that there are also some serious issues with ecosystem disruption at both ends of the process. First when removing material from another beach or from offshore, and then secondly when burying the ecosystem on the beach where you are performing nourishment. Unlike sea level rise, covering all these organisms with sand or pebbles doesn't give them time to adapt or move on.

 

Ocean Dynamics and Sea Level

So we all know about the thermohaline circulation redistributing heat and salt around the World, it's easy to imagine how it can have an important role in climate through heat transport, but perhaps a little more difficult to imagine how changes in the ocean currents can affect sea level. Struggle no longer!


Until three months ago I didn't even know that changes in ocean dynamics could affect sea level, but now I do, so I'm going to tell you all about it. I'm going to talk about the Gulf Stream, part of the Atlantic Meridional Overturning Circulation, firstly because I think as ocean currents go, it's quite famous, and secondly it is causing serious problems on the American east coast.

Satellite altimetry image of the Gulf Stream (NOAA)

The Gulf Stream flows along the American east coast until Cape Hatteras where it changes direction and starts moving in the towards Europe where it has a warming influence - European countries are a lot warmer than land at equivalent latitudes in North America (Bryden et al, 2005). The Gulf Stream flows close to the surface, this means that its surface velocity controls the sea surface gradient on either side. This means that sea level on the American Atlantic Coast can is up to 1.5 metres lower than you might expect without the effects of the Gulf stream (Atkinson et al.,2013).


But guess what? The speed of the Gulf Steam is highly variable, and its distance from the coast also changes. Both of these have a direct influence on sea level - the faster the Gulf Stream the lower the sea level, and increased distance from the coast also contributes to lower sea level (Ezer et al, 2013). At the moment sea level along the coast is rising up to three times faster than global mean sea level, and this has partly been attributed to changing ocean dynamics  - the weakening Gulf Stream (Ezer et al, 2013).


So why is the Gulf Stream weakening? The main reason is thought to be climate change. As we know the warming climate is encouraging ice to melt. The North Atlantic is an important site of deep water formation, and increasing melting of glaciers and ice sheets are adding fresh water to the North Atlantic and making the water less dense, and therefore harder to sink (Leverman et al, 2005). This is causing a slowdown of the Gulf Stream, decreasing the sea surface gradient, and therefore causing a relative sea level rise along the coast. Research by Bryden et al (2005) at 25 degrees north, suggests that the Gulf Stream has slowed by as much as 30% between 1957 and 2004. This sounds really high, but it is important to remember that they do not have any data prior to 1957, and for such a short time period it is difficult to say whether this could just be part of natural variability. However, models of Gulf Stream strength and sea level along the US east coast with additions of freshwater in the North Atlantic have backed up their findings (Leverman et al, 2005).


But the Earth is continuing to warm, and as we have looked at previously, forecasts do suggest a reduction in land ice in the Arctic Circle. So continual ice melting will have a secondary effect on sea level - in addition to adding water to the oceans it can also change the ocean currents and therefore the sea level. Everything is connected. For the American east coast - it is all connected in a really bad way.

And this is what the Gulf Stream looked like 200 years ago, so as you can see it really has undergone some serious changes...! (NOAA)



Effectiveness and Cost of Coastal Defence

I went to a great lecture on Wednesday at the Royal Society. They have just released a new report all about resilience to extreme weather, including coastal flooding.


They had a really interesting concept that I had not heard before of 'resilience deficit' and suggested that actually many areas are already in this situation of spending money to recover from extreme weather events, rather than spending the money on mitigating or adapting to the extreme weather. This means that in the long run we are spending more that necessary to replace damaged infrastructure, rather than protecting it in the first place.


In terms of coastal flooding they divided the various defensive options up into three categories; engineered, eco-system based, and hybrid, and then produced this graph based on cost and effectiveness.


Source: Resilience to Extreme Weather, The Royal Society

So all the grey circles are engineered defences, the orange ones are hybrid, and the green ones are ecosystem based. The thickness of the circle is about how much evidence there is to support the efficacy of the defence, and the positive signs inside circles indicate that there is another benefit from the defence solution. One of the speakers made the interesting point that people feel disproportionately safe behind engineered defences. In fact while these forms of hard protection are the most effective, when they fail, they fail suddenly. In comparison, the softer forms of protection such as mangroves and beach nourishment, fail more slowly, giving more warning and so more time to come up with another solution.


I think that this is a really great graph because it shows really clearly that while at the moment engineered defences are the most effective, some of the eco-system and hybrid based approaches are not far behind, and in all cases are cheaper. I think it's really interesting that maintainence of mangroves is the cheapest option but is the 5th most effective out of eleven, and has other positve consequences, unlike dykes, levees, and coastal barrages which have no other additional benefits.


To me it also looks like more research needs to go into how effective ecobased solutions are - the green circles are the thinnest of all the ones on the graph. I also think that I definitely need to look more into hybrid solutions for coastal defense - they offer a compromise in terms of cost and effectiveness, but also can come with additional benefits. So I think I will be looking into beach nourishment and artificial reefs in the next couple of weeks. 

Tectonics

I've spent the last few weeks looking at how sea level could change from changing the amount of water in the oceans by melting ice. Now I want to look into something completely different - tectonics - how can the movement of tectonic plates affect sea level? In terms of sea level change in our lifetimes this is one of the less important factors, but it's still important to take it into account.


Long term geological patterns in sea level change are thought to be mainly the result of changing spreading rates at mid-oceanic ridges. When mean spreading rates are faster more new crust is created, newer crust is warmer and more buoyant so the topography of the ocean basins in higher. This makes sea level appear higher at the coast (Moucha et al, 2008). Or indeed vice versa.



So here you can see the relative ages of the sea floor moving away from the mid-oceanic ridges (in the middle of the red), unfortunately it doesn't come with a key but red is younger and the oldest ocean crust is dark blue (NOC). So you can see that there is alot more younger crust in the Pacific Ocean than the Atlantic. According to one of my old lecturers mid-oceanic ridges are where all the sexy stuff happens, I'm yet to be convinced.

However, it has been argued by various groups that this theory is too simplistic. It doesn't take into account subduction zones which can also change the height of the oceanic lithosphere and therefore sea level. In fact, faster subduction rates at the same time as fast spreading rates, and the subduction of older, colder, oceanic crust may have a counteractive effect on sea level. Subduction of old oceanic crust can cause subsidence of the overlying lithosphere, and therefore a relative sea level rise. (Husson and Conrad, 2006).


Husson and Conrad (2006) used a dynamic model to determine the effects of changing velocity of tectonic plates on sea level. They concluded that short term changes in plate velocity had little effect on the volume of ocean basins - only up to 22 metres change in relative sea level (still sounds like quite a lot to me). Whereas for longer term changes in plate velocity, the amount of subducted crust decreases. The model suggested larger variations in ocean basin volume, and sea level changes of up to 78 metres.


But this isn't all, the age of the sea floor (and therefore it's buoyancy) is also related to the layout of the continents. When the continents are joined together in a supercontinent formation, such as Pangaea, sea levels are generally low because the sea floor is older (Miller et al., 2005). Supercontinent formations  also promote ice on land, and also mean more mountain building as the continents collide also resulting in lower sea levels (Miller et al., 2005). Ice sheets are also more likely to form when there is land at the poles - it doesn't matter whether this is in a supercontinent or dispersed formation - Antarctica is a really good example of this - it's the coldest place on the planet. Ice sheet formation locks up water on land, and so lowers the sea level.


Pretty serious stuff, however before you all go out and invest in a boat it is worth remembering that this all on geological time-scales. The IPCC put the current effect of tectonics on contemporary sea level rise at only 0.1 mm a year. Earthquakes are the only way that tectonics can cause rapid sea level change. They do this by causing rapid vertical land movements and tsunamis which can cause sudden sea level rise, but even then they can't effect the global mean sea level (Melini et al, 2004).