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.