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: 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..