The Last Post

Unfortunately all good things must come to an end, and with the deadline for this blog looming it is time to wrap it up!

Having come into this blog with a reasonable understanding of SLC, I think I have come to appreciate how large the impacts of SLR could be. The processes and causes of SLC are now well understood however it is clear that uncertainty about the rate and magnitude of the individual contributions to SLC mean that not enough action is being taken to mitigate against SLC (apart from some countries... Well done the Netherlands!). This piece makes an interesting point about the link between uncertainty and inaction, while there is uncertainty around SLR it is definitely happening, the uncertainty is quite how bad it will be... 

Seeing how close so many people in Bangladesh and other places around the world are to being forced to migrate as a result of SLR it is going to become an increasing pressure on decreasing land mass. I shall leave you with this graph from the IPCC projecting SLR by 2100 under varying emissions scenarios. I'm slightly more positive today about the likelihood of emissions reductions following the news that China is about to plough $361 billion into renewable energy by 2020. So hopefully China's example will be followed, emissions will begin to fall somewhat, and we will be looking at better scenarios in the next IPCC report... Although some studies, particularly those using the semi-empirical modelling approach to SLC (e.g. Rahmstorf, 2007) project it could be much more by 2100, kinematic constraints on glacier melting suggest it will be no more than 2 m by 2100. 

IPCC predictions of SLC by 2100. Source

The potential rise in sea level could be devastating and as I contended in my first post, I still believe that SLC is one of the 'quiet' agents of Climate Change and far more awareness of the potential problems it will cause is required. As the Netherlands have showed, effective mitigation is possible but before people realise how compromising SLC can be, it could already be too late. Try and be prepared just like the Statue of Liberty is (below)!

Source

Adapting to SLR: Britain and the Netherlands

Having discussed some of the areas most at risk of SLC, I think it is important to come a little bit closer to home to discuss SLC around Britain and mitigation strategies being adopted. 
This interest has been further piqued by this article from Monday suggesting massive differences in the spending for flooding across Britain with half of national spending targeting the London area Britain is at a very interesting point of SLC, due to its location and past glaciation meaning that Scotland is rising out of the sea while southern Britain is slowly sinking below the waves… The UK is considered the 12^th most at risk country in terms of population that could be directly affected by SLC with 4% of the population at risk. 

It also has many coastal communities and people living at risk of SLC. Flood adaptation and mitigation strategies are managed by the Environment Agency (EA) who have a national-scale Shoreline Management Plan (SMP) to try and provide a long term assessment and plan for managing the effects SLC may have. This attempts to move away from the previous consensus that the only method is the hold the line method of building sea walls (see below) and other coastal defences when a more holistic approach is often required. Even sea walls have been redesigned to mimic natural conditions more effectively (see below).

Blackpool's Victorian Sea Wall. Source

Blackpool's new sea wall is designed like a sand dune to dissipate wave energy more effectively than the old Victorian Wall. Source

The full SMP can be found here which shows a variety of methods are to be employed in order to mitigate the effects of SLC. The UK is divided into a series of zones for which regional SMPs are created. These are divided into four main approaches and defined by the EA:

1. No active intervention – There is no planned investment in defending against flooding or erosion, whether or not an artificial defence has existed previously.
2. Hold the (existing defence) line – An aspiration to build or maintain artificial defences so that the position of the shoreline remains. Sometimes, the type or method of defence may change to achieve this result.
3. Managed realignment – Allowing the shoreline to move naturally, but managing the process to direct it in certain areas. This is usually done in low-lying areas, but may occasionally apply to cliffs.
4. Advance the line – New defences are built on the seaward side.

Although these approaches appear fairly set in stone, Nicholls et al., 2013 suggests that adaptation pathways are a more effective way of approaching coastal change as a result of SLC. This suggest that with monitoring of SLC and coastal change, a range of strategies for each section of coastline that are flexible should be planned for allowing action to be taken depending on the reaction of the coast. This approach has been considered very controversial in some areas, with areas considered higher value often protected at the expense of less valuable areas. This has led to suggestions that there is a bias towards more affluent areas as they are considered of higher value. Therefore to placate all stakeholders in the management of SLC requires significant work and can lead to a slowing of the process by which flood defences and strategies are implemented.

Clearly mitigation of SLR is crucial, as this website shows where you can have a look at the world map with different levels of SLR in the future. For instance for a 7 m rise (albeit nearly impossible before 2100), sees large parts of Eastern England under water and London in serious trouble, which explains the clamour to build a second Thames Barrier to protect London. This is supported by the significantly increased numbers of closures in recent years of the Thames Barrier (see graph below). The danger to London remains the case for a 2 m rise (possible but very unlikely by 2100), and coupled with the increased number of closures suggests why the government commissioned a white paper called Thames Estuary 2100, recommending a Thames Barrier 2 be built to mitigate potentially catastrophic flooding in central and east London. 

Number of closures of the Thames Barrier since 1982. Source

Having said how at risk we in the UK are, at least we aren’t in the Netherlands… Even with SLR of just 1 m (perfectly plausible by 2100) more than half the country is expected to be inundated. This is because much of the Netherlands is below sea level but protected by a sophisticated series of dams, dykes and levees. Comprehensive coastal defence plans are in place for the whole of the Netherlands and are expected to hold for the next few decades, however late into the 21^st Century whether they will be able to continue to cope is very much uncertain (Monabilu et al., 2014)  Large scale adaptation and mitigation infrastructure projects such as the Sand Motor (see video) attempt to manage the coastline in a way that both maintains the natural environment while providing additional buffering against SLR. This holistic approach works in tandem with more infrastructure heavy projects which protect the areas below sea level from SLR.

It is clear that Britain and particualrly the Netherlands are at significant risk of SLR but with long-term view adaptation and mitigation plans it is possible to reduce the effects of SLR. Along with this a drop in greenhouse emissions is required so that the rate of SLR doesn't get to a point where it is out of control and mitigation will be almost impossible.

What if all the ice melted?

Despite my previous posts suggesting relative stability of the Antarctic Ice Sheets, I googled SLR on YouTube and this video is by far the most viewed with 8.5 million views. It projects what the earth would look like if all the ice melted which would raise GMSL by 65 m. Considering by 2100 GMSL rise is estimated at  0.26 m - 0.98 m by the IPCC, a 65 m rise is a long way off...

To further investigate check out this link from National Geographic which has a more in depth study. As it shows many major cities would be inundated and hundreds of millions of environmental refugees would be created, however it is possible that had all the ice melted the earth would be at this point uninhabitable due to runaway Global Warming, so don't worry too much as it won't affect us!

Contributions to Sea Level Rise: Antarctic Ice Sheet

The Antarctic Ice Sheet is by far the largest body of ice in the world, and therefore has the potential to cause massive SLR. However during the 20^th Century its contribution to SLR has been fairly limited considering its massive volume. This is due to Antarctica having not experienced significant temperature rise due to its isolation by the massive and cold Southern Ocean.

The volume of ice in Antarctica is equivalent to 60 m of SLR

Map of Antarctica, including major bases and ice shelves. Source

Current Situation

Antarctica is separated into two major ice sheets by the Transantarctic Mountains; the East and West Antarctic Ice Sheets. Overall there has been negligible thinning over the majority of the Ice Sheet in recent years explaining the limited contribution to SLR. Recent suggestions are that in terms of SLR the West Antarctic (~7 m SL equivalent) is most likely to lead to a large contribution as there has been evidence of increased ice stream velocity and ice loss in recent years. This is most often seen on floating ice shelves in West Antarctica. This is also shown by the thinning in the West Antarctic Ice sheet as shown by Chen et al., (2009).

Accelerated thinning in West Antarctica. Source

In addition catastrophic events of ice shelf collapse have been seen on the Antarctic Peninsula. The Larsen B Ice Shelf (3,250 km2) collapsed in 2002 due to warming of the Peninsula and meltwater ponding. Following that there was significant glacier speed up and thinning (Rignot et al., 2004). This suggests a possible acceleration in SLR if large parts of the many Antarctic ice shelves collapse. 

Location of major ice shelves (left) Catastrophic collapse of Larsen B Ice Shelf (right). Source

The Pine Island Glacier in West Antarctica is second in speed to Jakobshavn in Greenland in terms of speed of retreat. It drains 20% of the West Antarctic Ice Sheet, and has been estimated to possibly contribute 10 mm of GMSL rise in the next 20 years (Favier et al., 2014). Similar to Jakobshavn the cause of this rapid retreat and ice loss by discharge is attributed to melting as a result of a warmer ocean. If this picture is repeated across a number of the major outlet glaciers then the contribution to SLR could rapidly increase.

Speed of Pine Island Glacier as it reaches the sea. Source

Future

GCMs project an increase in precipitation in the Antarctic region that could actually increase SMB across the region and therefore contribute negatively to GMSL rise. Surface melt in Antarctica is minimal due to the consistently cold temperatures and is likely to remain so for the foreseeable future as a massive rise in temperatures would be required to have any effect. The worrying areas are glaciers that are grounded in the ocean, warming ocean temperatures are the biggest threat to Antarctic Ice Sheet stability, particularly in the West. It is possible on longer timescales that the East Antarctic could contribute significantly to SLR because of marine ice melting however this is very unlikely to occur before 2100 (Mengel & Leverman, 2014).

Contributions to Sea Level Rise: Thermosteric change

The IPCC suggests there are three major factors that have contributed to the observed 20^th Century rise in SLC: Thermal expansion of the ocean, ice loss from glaciers and ice sheets and changes in terrestrial water storage. Other factors (see below) also contribute but those 3 are the most important for GMSL change. This blog will focus on thermal expansion’s impact on recent SLC and also future implications.

Causes of SLC. Source

Simple physics suggests that as water is warmed, it expands due to having increased energy: Thermal Expansion (see video...)

For the altimetry record (1993-2010) thermal expansion is calculated to have contributed 1/3 of the total GMSL change, while the graph below records estimates for its contribution to GMSL rise for the past 50 years.

Thermal expansion in the upper 700m is in red, in the deep ocean is orange. Source

The world’s oceans are the key sink of anthropogenic climate warming, estimated at having absorbed 93% of the warming of the earth’s system since 1950, and although this has been beneficial in checking the levels of anthropogenic warming on the atmosphere it has had an effect on SLC by raising ocean temperatures and subsequently causing thermosteric SLR (Sabine et al., 2004). This has been mainly in the upper section of the ocean (0-2000m). Although upper ocean warming is well constrained, thanks to the ARGO float scheme of measuring steric changes in the oceans, the deep ocean warming remains poorly understood.

The Argo Float network. Source

Studies have started to unlock this such as this Johnson & Doney, 2006 who showed recent abyssal South Atlantic warming but it is unsure over the longer timescales whether this can be applied to the whole ocean, although Johnson et al., (2007) showed a similar trend in the Pacific. Both these studies used robust methods and returned good confidence intervals that this deep ocean warming is observable. However a lack of spatial coverage of sampling from both of these studies of the deep ocean makes rigorous conclusions about temperature changes hard to apply to the wider ocean. There is not as yet a sampling system similar to ARGO (above), and therefore deep ocean warming continues to be a relatively poorly understood mechanism of thermosteric SLR. The combination of deep ocean and upper ocean warming acceleration in thermal expansion has been observed during the 20^th Century and is included in climate models to increase in the future (Church et al., 2006).

Future thermosteric rise

Projected SLR as a result of thermal expansion for three separate climate scenarios. RCP 45 is considered most likely at present whereas RCP 85 is a worst case scenario. Source

The question of whether thermosteric sea level rise will continue to increase seems clear. It is highly likely to and the rate of rise is also projected to increase. The ocean should still be able to act as a sink for some of the Global Warming. So should we be worried? This is unfortunately one of the most consistent contributors to SLR as the above graph shows and thermal expansion will continue to affect SLR it is now more of a question of how much it will increase in the future..

Sea level change since 2002

Just a short post today... This interesting video from NASA shows cumulative SLC since 2002:

It is very interesting as it shows meltwater fingerprinting particularly off Greenland and Antarctica and simply shows the recent trends in SLC, showing quite how non uniform it is across the globe and also a possible acceleration in recent years. For further discussion on meltwater fingerprinting Mitrovica et al., (2011) provide a good overview.

How do we measure Sea Level Change?

In the last post I made some assertions that global mean sea level (GMSL) is rising, but making me ask myself how do we know this? Physically measuring the rate of sea level change (SLC) is not a simple process, and as such there are different methods for doing so:

Tide Gauges

In terms of directly measuring SLC there have been approximately 300 years of coverage in an initially very limited spatial area. Tide gauges were the first form of SLC measurement, and work quite simply by constantly measuring the water level at a fixed geographical point. The first tide gauge was constructed in Amsterdam in 1700, and across Europe most major ports had them by the end of the 18^th Century.

Tide gauge stations with >40 year records. Source

Current global coverage. Source

However as the maps above show, tide gauges have lacked global spatial coverage, with a considerable Northern Hemisphere bias until the last 40 years or so.

Therefore for long term SLC they lack consistency but if you choose the most accurate and consistent gauges can still find a reasonable fit for recent SLC. This new paper from Thompson et al., (2016) shows that even when using the best quality tide gauge records it remains difficult to validate the recent observed sea level trends due to systemic underestimation of melt from 20^th century ice cap loss. This is likely due to local trends at each tide gauge station for example melting of ice caps from different parts of the world produce a variable sea level trend due to differing distribution of the meltwater across the globe. 

Meltwater impact on regional SLC from Greenland (above) and Antarctica (below). Source

These local signals imprint on the global signal and therefore tide gauges records need to be selected very carefully in estimating global SLC. As you can see from some tide gauge data I collated (below), there is much noise in the data but most show an overall trend of slow rise through the 20^th Century. 

Self- collated SLC at sites from across the world for the last 100 years. Source

The attachment of tide gauges to land means vertical land motion is another complicating factor, therefore has to be accounted for when doing reconstructions. This is a complicated process requiring data from models that is being constantly updated and therefore past sea level trends may have been poorly estimated due to issues with vertical land motion. This explains the relative sea level fall at Stockholm. Fingerprinting the difference between local/regional and global sea level trends appears to be crucial in accurately working out the SLC signal. 

Satellite Altimetry

Satellite altimetry is the more recent, more accurate form of measuring SLC. It uses satellites (with the catchy names of JASON and TOPEX…) to measure the height of the sea surface compared to a reference and therefore you can look at its change over time to get GMSL change. 

How satellite altimetry works. Source

However you still need to consider ocean basin volume changes and changes in pressure that lead to differing regional levels. This technology is only approximately 25 years old so gives a shorter term record but will be most used to assess future SLC, considering the revolution it has led to in the understanding of SLC since its inception (Milne et al., 2009). To compare GMSL change for the differing instrumental eras gives an indication of the recent acceleration in GMSL, from tide gauges 1900-2012 it is 1.7mm/yr, from altimetry 1993-2010 3.2mm/yr.  Does this reflect differing accuracy or a clear acceleration is SLC in the late 20^th Century? These questions are critical to estimating future SLC and the potential magnitude of its effect. Altimetry appears limited in measuring local changes and therefore a combination of the two major methods (tide gauges for local, altimetry for global) appears to often be the best for working out recent SLC and looking towards the future.

Another way of measuring SLC is to work out the magnitude of differing contributions to SLC. Therefore the next couple of blogs will look at the major contributions to 20^th Century SLC. Steric changes of ocean water, ice melt, changes in terrestrial water storage and groundwater depletion all contibute to SLC. These will be discussed in the coming posts...

An introduction..

Hello! Over the coming months I shall be writing a blog that will hopefully inform and discover about Sea Level Change. So why am I looking at Sea Level Change and why is it important?

• Half of the world’s population live within 60km of the sea and this is growing
• Approximately 5% of the world’s population live at an elevation of under 5 metres
• The rate of sea level change is greatly affected by anthropogenic warming of the climate
• According to the IPCC global mean sea level is rising at an unprecedented rate (3.2 mm/yr)
• With current rates of warming (4°C by 2100) up to 760 million people could find the land they live on submerged.

Flooding in Venice in 2008, will this keep happening?

These are just a few of the reasons that sea level change appears to be a crucial factor of Global Environment Change. It is clear that sea level change has the potential to be one of the biggest threats to mankind. However having given in the above bullet points some doomsday like facts a greater understanding of the mechanisms that drive sea level change will be required. The estimate of 3.2mm/yr for global mean sea level rise is both informative yet on another level useless. Yes it shows that across the globe sea level is rising, but as a global average doesn’t tell you how the rate of sea level change is in fact very non-uniform. It is important therefore not to look at simply mean sea level change, but relative sea level change as in some areas sea level is in fact falling in some areas.

I am interested in learning more about sea level change as I perceive it as one of the ‘quiet’ forces of anthropogenic climate change; little by little it is having an effect. This has been shown with the very recent Hurricane Matthew where storm surging was the major cause of damage on the US East coast, exacerbated by the rise in sea levels. In addition the devastation caused by Hurricane Sandy in 2010 was greatly influenced by the rises in sea level, as flooding may have not been as extensive. However this often receives little coverage following large events such as these when in fact it is one of the crucial factors in causing damage and destruction.

Photo showing the storm surge from Hurricane Matthew in Georgia, US

During this blog I will aim to understand the mechanisms and processes that govern sea level change, its potential effects and the areas most at risk from sea level change. So stay tuned!