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..