Florida and SLR

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A cartoon depicting the 'positive' side of SLR in Florida. After my last post, this made me laugh!

Tropical Storms and SLR: The Perfect Storm?

In my introductory blog I mentioned how SLR was affecting the damage caused by major hurricanes including Sandy (2012) and Matthew (2016). Now as a Christmas present to my dear readers I will (finally!) address this and show how the small incremental change of SLR is having a large effect on the damage that these hurricanes are causing.

Along with the projected increase in cases of extreme (category 4 & 5) hurricanes in the future as a result of climate change (Goldenberg et al., 2001), SLR is projected to increase which means that storm surges associated with hurricanes will inundate further affecting more and more infrastructure and homes. A significant amount of the damage caused by Hurricane Matthew has been attributed to storm surging, while Hurricane Sandy saw large parts of New York under water. 

Flooded taxis in New York following Hurricane Sandy. Source

There is an element of luck to how bad a storm surge will be, if the peak surge coincides with high tide then that can raise the height of the surge by many metres. This occurred during Hurricane Sandy in New York, causing significantly more damage than had it been at low tide. The acceleration in SLR (see below) is being particularly affected by thermal expansion with a further acceleration being reported in the last decade (see below), disproportionately so on the Atlantic Coast of North America (Rietbroek et al., 2016). This is thought to be due to a combination of meltwater fingerprinting and changing ocean cycles as I have indicated in earlier posts, the regional signal of SLC is critical in some areas.

Source

Hurricane Matthew caused up to $6 billion of damage in both Haiti and America, with the state of North Carolina experiencing $1.5 billion worth of damage alone. SLR contributes significantly to this due to the rising of the base sea level, so what would have caused less flooding 100 years ago now causes significant amounts of flooding (see video below). Despite this officials in the Florida Department of Environmental Protection are not allowed to use the phrases ‘climate change’ or ‘global warming’ . Apathy to this issue will allow for SLR to continue to affect America, and despite this Florida, one of the most at risk areas doesn’t have a state-wide action plan to deal with climate change. As I showed in last week's post, The Netherlands' well-defined and wide ranging action plan significantly mitigates the effect SLR can have therefore it is very surprising that Florida don't have an action plan that could save lives and billions of dollars in the long run.

  

It is clear that hurricanes can cause significant coastal flooding which can be exacerbated by SLR. A paper by Woodruff et al., (2013) compared present day situations to early Holocene records where rapid SLR was experienced. Using past conditions can act as an analogue for the present and the future so to look at periods where similar conditions were experienced has certainly informed on the potential future tendencies They found that during the early Holocene SLR meant low-lying coastlines had low resilience to storm impacts. In addition they found that the worst storm surges are caused when winds are highest, considering storm activity is projected to increase with the strongest storms having the greatest effect (Zhang et al., 2001), it is likely that with SLR and increased activity coastal flooding could worsen. This is particularly likely in low-lying areas such as Bangladesh, where inundation of significant land area has been modelled to be very likely by 2050 (Lin et al., 2012). The suggestions of Woodruff et al., 2013 appears valid as it is very likely in the future a combination of SLR and increased storm activity could significantly affect coastal areas. This is supported by Mousavi et al., (2011) who modelled that even by 2030 as a result of SLR, surges could be up to 0.3 m higher in the USA, and by 2080 0.8 m higher. Considering they found this to likely be on the lower end of estimates due to unmodelled uncertainties, the effects of storm surging and the economic and human damage that it can bring will be significant.

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It is apparent that SLR is having a significant effect on the destructiveness of storms. Raising the base sea level means storms like Sandy and Matthew cause far more damage than they would have 100 years ago and with projected increased storminess, governments need to get a handle on the potentially catastrophic effects. So Florida, get your act together!

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.

Kiribati- A climate change reality

A short follow up post today with a very important video building on last week's post. The video below shows quite how dangerous SLR could be to low-lying islands. It aims to raise awareness of this creeping menace that is very difficult to stop.

 

For an assessment of the potential effects of implicating mitigation strategies Tol, 2007 provides an interesting suggestion of the possible costs by 2100. Low-lying islands such as Kiribati feature highly in spending as a percentage of GDP and percentage of land at risk of SLR. The main feature of the paper is the assertion that money is better spent on adaptation rather than purely trying to block SLR, and in the long term this can greatly reduce cost that damage caused by SLR would cause.

On the frontline of Sea Level Rise

So the blog is going to have a bit of a change of direction… Having covered all of the major physical processes that affect SLC I now feel able to spend some time investigating case studies across the world of the places and people directly at risk of SLR.

Low Lying tropical islands are most at risk of SLR, indeed the inhabitants of these islands may become some of the first Climate Change refugees. Despite the recent COP21 Paris agreement on emissions reductions, the Prime Minister of Kiribati suggests it will not be enough to prevent these islands slipping into the sea. 

Welcome to the Maldives, a paradise archipelago of atolls and islands in the Indian Ocean. The Maldives hit the headlines a few years ago when their government held a cabinet meeting under the sea in order to highlight the potential effects of SLR. The Maldives will be at the forefront of SLC, its highest point lies just 2 m above sea level. 

The Maldives

Although this is by no mean a laughing matter, I came across a cartoon that rather accurately depicts the potential plight of these small island nations.

Cartoon about potential SLR. Source

In the Solomon Islands, for some it is already too late. 5 islands have been reclaimed by the sea (see below) as it experiences above GMSL average rise. This created headlines worldwide, with most major news outlets picking up on this issue. The global SLR rise of approximately 3 mm/yr, is dwarfed in this area with estimates of an acceleration up to 7mm/yr (Albert et al., 2016). Although SLR is clearly the dominant cause of threat to these low-lying islands, studies have indicated that human activities have also accelerated the degradation of the land. Inappropriate development choices, including the building of sea walls has accelerated erosion and led to the abandonment of islands as it became entirely unsustainable to live there. It is however not all doom and gloom; Webb & Kench, 2010 suggest that many islands are dynamic in their response to SLC and used quantitative analysis to reveal that 86% of low-lying islands in their study were either stable or growing in area despite SLR. This is because islands respond to a variety of factors, of which SLC is just one of them. Although this may indicate a positive trend for now, with SL rising continually and the rate of rise likely to increase a threshold will be passed by which many of these islands will become inundated. 

One of the Solomon Islands that has been lost to SLR. Source

Even if SLR doesn’t leave islands uninhabitable there have been suggestions that SLR will cause a slightly different effect that will still leave inhabitants in serious trouble. As sea level rises, the level of groundwater rises as well (Gulley et al., 2016). Therefore as this paper suggests the important freshwater can leak into lakes or create new ones which results in increased evaporation. As my last post indicated, groundwater depletion can have a significant effect on SLR but the effect here would be twofold: water is a precious resource in these areas, and the land use change could also be significant.

Contributions to Sea Level Rise: Direct Human Impact

The past few posts have focused on the variety of contributions to SLC that are as a feedback response to Global Warming and the current climate change we are experiencing. This post looks at how humans directly affect SLC through water impoundment and reservoir building.

This is the least studied section of SLC due to significant uncertainties about the future levels of groundwater extraction and depletion (IPCC, 2013). Groundwater is extracted from aquifers for agriculture and supply water for urban areas. Its unsustainable extraction means aquifers are not replenished and therefore reduces water supply, and this has started to gain more attention in the press due to its effect on SLC.

Current situation

Groundwater depletion is assessed to currently contribute 0.57mm/yr to SLC, a significant increase since 1900 when it was only projected to have contributed 0.04 mm/yr (Wada et al., 2012). This increase is attributed to increased water demand as a result of increased population and more intensive farming methods. This also fits with Konikow (2011) who suggested since 1900 there has been an average of 0.11mm/yr. Church et al., (2011) found similar values to those of Wada et al., (2012) but found significant uncertainty (±27%) in these estimates as it relies on a combination of groundwater models and observations that are difficult to constrain. In addition there is significant global spatial variability (see graph below). 

Groundwater depletion since 1960 with individual regional depletion. Source

Possibly more importantly the majority of studies have noted a recent acceleration due to increased groundwater uptake. As the video below shows, groundwater depletion is increasing rapidly in India, an area of significant groundwater depletion (above) which will affect both SLC and water security. 

This is offset by increased water impoundment behind dams for hydroelectric power or as a water source. Dams are thought to have reduced sea level by up to 30mm since 1900, as a result of preventing water from the sea. This will have reduced the impact of mountain glacier melting (see previous post) but not to the extent to have a massive effect on SLC. The projected proliferation of dam building across the world, expecting a 25% in global dam building means water movement will be even more controlled (Zarfl et al., 2015).

The future

It is likely that for the next couple of decades groundwater depletion will continue to increase. Over the next 50 years some projections suggest it will be of equal importance of melting glaciers and ice sheets to SLR. However there will become a point where groundwater is so depleted (see graph below) to a point where groundwater extraction is limited. 

Past Groundwater depletion contribution to SLC (black) and projected future groundwater depletion contribution to SLC from a range of models. Source

This is riddled with uncertainty due to a lack of knowledge of current groundwater reserves. The more concerning aspect of groundwater depletion is the potential impact on water resources, many countries with limited resources and rapidly growing urban areas rely almost entirely on groundwater extraction for water and agriculture. The majority of studies suggest that all of the extracted water will eventually end in the ocean; however more recent research has put that figure at more like 80% (Wada et al., 2016). This would greatly decrease potential SLC contribution, and has suggested the IPCC projections of SLC as a result of groundwater depletion are overestimated by a factor of 3.

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: Greenland Ice Sheet

Having focused in the last post on the contributions of glaciers and ice caps to SLC, this week focuses on the one of the two largest stores of freshwater the Greenland Ice Sheet. Along with the Antarctic Ice Sheet it holds 98% of the freshwater stored in ice and so its stability or otherwise could be crucial to SLC in the future. The behaviour of ice sheets creates the most significant uncertainty for future SLC, as their response to climate change is poorly understood and they are notoriously difficult to model.

The volume of ice in Greenland is equivalent to 7 m of SLR

From this statement it is clear that melting of ice from Greenland (below) has potential to catastrophically affect the world. However what is the likelihood of significant ice loss from this source?

The Greenland Ice Sheet with some major outlet glaciers indicated. Source

Current situation

The Greenland Ice Sheet, despite being significantly smaller than the Antarctic Ice sheet has in recent decades contributed more to global SLR due to increased surface melting (Rignot et al., 2011).  Straneo & Heimbach (2013) suggest mass loss has quadrupled in the last 20 years and contributes up to 25% of global SLR. This is attributed to increased surface runoff due to summer surface melting (see below) as a result of the warming temperatures in the region. Temperatures have increased rapidly by up to 5 degrees in recent years which increases the propensity to melt (Applegate et al., 2014). Until recent years this has been offset by increases in precipitation (Hanna et al., 2007).

Increased area of surface melting in the last 10 years. There is some interannual variability but a trend for increased melting. Source

However the increased run off has been linked to a subsequent increase in sliding at the base of the glacier which increases velocity and therefore calving in the ocean terminating glaciers (Zwally et al., 2002). The graph below summarises the recent situation of the Greenland Ice Sheet, with the decrease in Surface Mass Balance (red) particularly worrying and suggesting SLR.

Graph showing mass flux of Greenland Ice Sheet. Blue line (Precipitation), Green line (Melting), Red line (Surface Mass Balance), Orange Line (Run off). Dashed lines are trends since 1990. Source

Greenland has only a few outlet ice streams that reach the sea, and act as the major deliverer of melt to the sea. Indeed some of Greenland’s outlet glaciers such as Jakobshavn have been dubbed ‘the fastest glaciers in the world’. As the image below shows, Jakobshavn has retreated rapidly in the past 150 years. This is worrying as Jakobshavn drains 6.5% of Greenland’s Ice sheet area. This picture is repeated across a number of outlet glaciers with rapid retreat and increasing contribution to SLR.

The retreat of the Jakobshavn Glacier since 1950. Source

Increasing ocean temperatures around Greenland result in rapid calving at the front of the glaciers as this video shows, attributed to rapid submarine melting due to the increased ocean temperatures (Rignot et al., 2010). This speedup continues with it estimated to move up to 46 m per day! As the clip from the film 'Chasing Ice' shows, calving can be rapid and dynamic causing SLR.

The Future

The recent acceleration in ice loss from the Greenland Ice Sheet means it is contributing more and more to GMSL rise. This contribution is projected to increase as more surface melt due to occurs over more sections of the Greenland Ice Sheet, which increases ice loss. The IPCC suggests it will contribute 0.01- 0.17 m to GMSL rise (or 0.2 - 2 mm/yr) by 2100 but these estimates may be an underestimation due to limits in the understanding of glacier flow, leading to uncertainty as shown in the graph below. 

Projections of Greenland's contribution to SLR by 2100. C is Solid Ice Discharge (ie. calving), D is Surface Mass Balance (melting). Source

Irreversible Loss?!

Some GCMs have projected a non-linear response to warming for the Greenland Ice Sheet; as the surface melts it lowers and this warms the near surface which would further melt the ice sheet. This could lead to vastly increased SLR as the Ice Sheet dynamically thins. This is as yet uncertain and projected as unlikely in the 21st Century but further forward could lead to massive reduction in size of the Greenland Ice Sheet and significant contribution to SLR.

Contributions to Sea Level Rise: Mountain Glaciers & Ice Caps

In this momentous week where Donald Trump has been elected as President of the USA; Climate Scientists, and all those who believe in the argument that frankly among climate scientists is no longer an argument, it is worrying to point out this video where Trump dismisses Climate Change as a hoax or just ‘weather’. A good point of view piece is from Green MP Caroline Lucas on why Trump's election is a worry. On the off chance Donald is reading this blog (…) I will point out the Climate Central website which projects the impact of SLC on coastal cities including his beloved New York… 

But I shall not dwell on this and move onto one of the major contributors to SLR, that from glacier melting. To clarify this I am following the IPCC in distinguishing between the Antarctic and Greenland Ice Sheets (see map below) and all other glaciers and ice caps, this post focuses on these smaller glaciers and their contributions to SLR both during the instrumental period and in the future.

Location of the major mountain glaciers and ice caps (1-18) that contribute to SLC, Greenland & Antarctica (19 & 20) are not discussed in this post. Source

Mountain glaciers and regional ice caps have been observed to be one of the most important recent contributors to SLC. The video below shows how Glacier National Park, USA could become no Glacier National Park by 2050. 

This picture is repeated across mountain glaciers and ice caps around the world, dynamical and extreme retreat has occurred in most mountainous regions since the 1850s, producing meltwater that contributes to SLC. As the graph below shows recent SLC contribution is similar to that of the Greenland and Antarctic Ice Sheets combined.

This paper from Jacob et al., (2012) analyses the contribution of Glaciers and Ice Caps to SLC and suggests a value of 0.41±0.08 mm/yr as its contribution to the GMSL rise. This paper heralded a distinct advancement in the understanding of these smaller ice sources and their contribution to SLC by using GRACE altimetry to constrain mass balance (whether a glacier is advancing or retreating). It shows an acceleration across a number of regions including Alaska but suggests a decreased influenced from the Himalayas. This is due to enhanced spatial resolution as they took sub regions of the Himalayas compared to seeing the Himalayas as one region as done by Matsuo & Heki (2010), which resulted in a perceived overestimate of Himalayan contribution. This is a forward step for understanding contribution to SLC from these smaller, spatially disparate sources that have had significant impact in recent times to SLC. Further support for the large contribution to recent SLC is given by Gardner et al., (2012) who suggest current contribution of 29±13% to global SLR is from these Glaciers and Ice caps. Ice loss is particularly strong in the Andes, and Arctic Canada and Alaska (see below) and as my post from a couple of weeks ago suggests, this could cause far greater local and regional SLC.

Contributions of different glaciated regions to SLC. Source

Future Contribution

As significant ice mass loss continues this will contribute to global SLR, however as glaciers shrink in size they are likely to come to a point when they reach a balance. Some glaciers such as those remaining in Africa are likely to disappear completely although their contribution to SLR is likely to be negligible. Radic & Hock (2011) present a study suggesting many will have experienced serious loss but some may only lose 20% of ice mass. GMSL contribution by 2100 is expected to be 0.124±0.037m yet variable contributions and uncertainty mean that this is by no means certain. Therefore by 2100 the contribution may be almost negligible as melt may have stabilised due to lack of available ice to melt. In addition the effects on water availability, for instance billions of people rely on the Himalayas as the primary source of water could have severe consequences.

Projected volume (left axis) and Sea Level equivalent (right axis) of glaciers for a series of models until 2100. Source

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...