The global consequences of groundwater extraction have received less attention than many other environmental issues.  Here are the main points, in Question and Answer form.

Does groundwater extraction contribute to sea-level rise?
Yes, at a global level.  A basic principle of hydrology is that water may change its state (liquid, ice or vapour) or location, but is never lost.  Water extracted from groundwater aquifers must go somewhere.  Its initial destination will depend on its use. When used in agriculture, some will enter the atmosphere as water vapour via transpiration by crops and evaporation from soil, and some will enter rivers via run-off from soil.  When used for domestic or industrial purposes, much of the resulting wastewater will again enter rivers.  But water vapour in the atmosphere condenses to form clouds and fall as rain, much of which falls on the sea, and rivers flow to the sea.  It is true that water also passes in the opposite direction, by evaporation from the surface of the sea into the atmosphere, where it may be carried by wind and contribute to rainfall on land and percolate through the soil into aquifers.  But that is a natural process that occurs anyway.  Groundwater extraction can in some circumstances result in a net transfer of water from aquifers to the sea, contributing to sea-level rise.

What circumstances determine whether extraction from a particular aquifer will contribute to sea-level rise?
Aquifers differ both in the extent to which they are recharged by rainfall and local water flows, and in the extent to which they discharge water naturally via springs, seeps or underground flows.  Where an aquifer is recharged at a fairly stable rate, it is a renewable resource and if the rate of extraction matches the rate of recharge there is unlikely to be any effect on sea level.  Some of the water extracted will still reach the sea, but because the water table in the aquifer will be below its natural level, there will probably be an offsetting reduction in natural discharge, some of which would otherwise also have reached the sea via the atmosphere or rivers.  Where however an aquifer in an arid region receives little or no recharge, extraction will have a cumulative effect in lowering the water table, and the transfer of water to the sea will not be offset by any other effect.  Between these extremes are intermediate cases in which extraction is partly offset by natural recharge.  The key indicator of an aquifer’s contribution to sea-level rise is therefore its rate of depletion, not its rate of extraction.

How can rates of groundwater depletion be measured?
One approach (the volumetric approach) involves estimating the change in groundwater volume of water over time.  Volume changes can be inferred from changes in water tables, or from changes in mass as measured by its gravitational pull.  A limitation of this approach is that it requires a separate measurement for each aquifer.  Unless measurements are available for all large aquifers around the world, estimates of global depletion must be extrapolated, using questionable assumptions, from those measurements that are available.  An alternative (the flux-based approach) involves estimating the various water flows into and out of aquifers.  Estimates of groundwater extraction at country level are available from the International Groundwater Resources Assessment Centre (1).  A limitation of this approach is that inflows, and natural outflows, are difficult to measure accurately.  Where rates of abstraction and recharge are both high, moreover, net depletion will be calculated as a difference between numbers that are both subject to uncertainty.  Neither of these approaches can be expected to give more than very rough estimates of total global depletion.

What is the current rate of global groundwater depletion?
Using a primarily flux-based approach, Wada et al estimated global depletion for the year 2000 at 204 cu km (cubic kilometres) (2).  Using a volumetric approach, Konikow estimated average annual global depletion at 102 cu km during 1991-2000 and 145 cu km during 2001-2008 (3).  Taking a figure between these estimates, a best estimate of depletion for the year 2000 might be 150 cu km.  Consideration then needs to be given to growth in depletion to date.  Konikow’s estimates suggest annual growth of about 4% which, if continued, would imply depletion in 2013 of very roughly 250 cu km.

What is the current annual sea-level rise attributable to groundwater extraction?
Very roughly 0.7 mm (millimetres).  This figure is obtained by dividing estimated annual groundwater depletion – 250 cu km – by the surface area of the world’s oceans and seas –  360,000,000 sq km.

How does this compare with total annual sea level rise and the contributions of other factors?
It represents just under a quarter of total annual sea-level rise which is estimated by the Intergovernmental Panel on Climate Change at 3.2 mm (this is an average for the period 1993-2010).  Most of the remainder is due to climate change, comprising 1.5 mm for melting of glaciers and polar ice, and 1.1 mm for thermal expansion of seawater (4).

In economic terms, does the contribution of groundwater extraction to sea-level rise represent an externality?
Yes.  Cumulative sea-level rise imposes costs of protection or migration on people whose homes or livelihoods are at or only slightly above sea level.  Those who extract groundwater do not have regard to those costs.

Is there a case for policy intervention to mitigate sea-level rise by discouraging groundwater extraction?
Probably not, unless the rate of extraction becomes much higher.  The situation parallels, albeit on a less dramatic scale, that of carbon emissions and climate change.  Just as a carbon tax is widely advocated as an efficient method of reducing carbon emissions, so a per unit tax on extraction of non-renewable groundwater would in principle discourage extraction for uses with a private value per unit less than the tax.  However, the practical difficulties would be considerable.  Many countries already have policies to control extraction with the aim of sustainable management of their groundwater resources, but implementation and enforcement have often been difficult (5).  Much groundwater extraction is by small farms with individual tubewells, so the costs of monitoring extraction and collecting a tax would be high.  Estimating the costs of sea level rise attributable to groundwater extraction to inform setting of a tax rate would be contentious.  Equity also needs to be considered.  Those extracting groundwater (eg in India and Pakistan) include many poor farmers (6), while those potentially affected by sea level rise include not only poor people on low-lying island nations in the Pacific and elsewhere, but also inhabitants of coastal cities in developed countries (eg Miami, New Orleans) (7).  This is not to say that governments should do nothing.  To the extent that sea-level rise is due to anthropogenic climate change, policies such as carbon trading schemes or carbon taxation contribute to its mitigation, and this is part of their justification (although mitigation of climatic effects on human living conditions and agriculture may be more important justifications).  There may also be justification for policies to help people adapt to sea-level rise, such as assistance with the costs of moving or, for those on low-lying islands, emigration.  But the case for addressing sea-level rise by discouraging groundwater extraction is not strong.

Notes & references

1. International Groundwater Resources Assessment Centre Information System  http://www.un-igrac.org/publications/119

2.  Wada Y, van Beek L, Weiland F, Chao B, Wu Y & Bierkens M (2012) Past and future contributions of global groundwater depletion to sea-level rise  Geophysical Research Letters 39 L09402 p 2 of 6    http://chinawaterrisk.org/wp-content/uploads/2012/07/Past-and-Future-Contribution-of-Global-Groundwater-Depletion-to-Sea-level-Rise.pdf

3.  Konikow L F (2011) Contribution of global groundwater depletion since 1900 to sea-level rise Geophysical Research Letters 38 L17401 p 4 of 5   http://water.usgs.gov/nrp/proj.bib/Publications/2011/konikow_2011b.pdf

1. Church J, Clark P et al (2013) Working Group 1 Contribution to the IPCC Fifth Assessment Report, Climate Change 2013: The Physical Science Basis Chapter 13 Sea Level Change p 16  http://www.climatechange2013.org/images/uploads/WGIAR5_WGI-12Doc2b_FinalDraft_Chapter13.pdf

5.  Comprehensive Assessment of Water Management in Agriculture 2007  Water for Food, Water for Life: A Comprehensive Assessment of Water Management in Agriculture  London: Earthscan, and Colombo: International Water Management Institute  pp 416-7

6.  Comprehensive Assessment, as above pp 407-9

7.  Global Green USA  Sea Level Rise: The Risk, The Facts
http://www.globalgreen.org/articles/global/95