May 4 2016
According to a new study, extraction of land water, including groundwater, does not contribute to sea level rise as significantly as previously thought. In fact, such extractions contribute three times less to sea level rise.
Image Credit: Iakov Kalinin
The study, which was featured in the Nature Climate Change journal, does not alter the overall picture of sea level rise in the future. However, it does provide a better understanding of the interactions in the oceans, the atmosphere, and between water on land, all of which could aid in improving the potential models of sea level rise.
Projecting accurate sea level rise is important, because rising sea level is a threat to people who live near the ocean and in small islands. Some low-lying areas will have more frequent flooding, and very low-lying land could be submerged completely. This could also damage substantially coastal infrastructure.
Yoshihide Wada, Researcher IIASA
Over the 20th century and the early 21st century, sea level has increased 1.7 mm per year and this trend is projected to continue as ongoing climate change warms the Earth further.
The rising sea level is attributed to several factors, including thermal expansion, melting glaciers, and ice caps, and the extraction of groundwater and other land water for human use. Compared to the contribution of thermal expansion and ice melt, the contributions of land water are small, yet this trend has been rising. As a result, there have been concerns that this trend could aggravate the existing issue of sea level rise induced by climate change. However, researchers are unsure about the level of contribution made by different sources toward the rising sea level. Sea level, in fact, has increased more than researchers could justify from the known sources. This has created a gap between modeled and observed global sea-level budget.
Historic works, including estimates applied in the IPCC Fifth Assessment Report, presumed that almost all of the groundwater reached the ocean. The latest research improves on preceding studies by considering feedbacks between the ocean, land, and atmosphere. It observed that the estimated number is closer to 80%, meaning that the gap between observed and modeled sea level rise is even greater. This indicates that other processes are also contributing more amount of water than previously thought.
During the 20th century and early 21st century, cumulative groundwater contribution to global sea level was overestimated by at least 10 mm.
Yoshihide Wada, Researcher IIASA
The study also showed that between 1971 and 2010, land water contribution to global sea level rise was slightly negative, which means that groundwater contains more amount of water. This negative trend is also attributed to reservoir impoundment behind dams.
Between the period of 1993 and 2010, the study estimated that terrestrial water contributes 0.12mm per year to the rising sea level. However, the study does not change the overall picture that future contribution of groundwater to rising sea level will increase as extraction of groundwater increases. Also, increased depletion of groundwater negatively impacts beyond sea level rise.
The water stored in the ground can be compared to money in the bank. If you withdraw money at a faster rate than you deposit it, you will eventually start having account-supply problems. If we use groundwater unsustainably, in the future there might not be enough groundwater to use for food production. Groundwater depletion can also cause severe environmental problems like reduction of water in streams and lakes, deterioration of water quality, increased pumping costs, and land subsidence.
Yoshihide Wada, Researcher IIASA