How the Driest Regions on the Planet Add to Sea Level Rise
Source: Wang, Jida; Song, Chunqiao; Reager, John; Yao, Fangfang, Yao; Famiglietti, James ; Sheng, Yongwei; MacDonald, Glen; Brun, Fanny; Schmied, Hannes; Marston, Richard; Wada, Yoshihide, 2018. “Recent Global Decline in Endorheic Basin Water Storages.” Nature Geoscience Vol. 11, pp. 926-932. Find the original article here.
Land water lost, sea water gained
As humans, we continuously engineer our environment. We build structures, drain water sources, and create urban and suburban areas that transform the way our world looks and how it functions. But, we can’t forget that the changes we make have consequences. One such repercussion is the alteration of global water storage and the movement of water on the earth.
Due to climate change, the last few decades bore witness to a measurable decline in water storage in all terrestrial or land-based environments across the globe. Examples of water loss from land include melting glaciers or frozen soils, depleted aquifers, drained reservoirs or lakes, and many more. When this water is lost from the land system, it is still part of the hydrosphere and will move to a different part of the global water cycle. So where is it going?
In this case, lost terrestrial water ends up in the world’s ocean. An estimated 20-35% of sea level rise in the last decade was due to water lost from the aforementioned terrestrial sources.
Rising seas are a major threat to 40% of the world’s population because approximately 2.8 billion people live within 100 kilometers of a coastline. The global ocean level currently increases by 3.4 mm every year, a height equal to the thickness of 2 quarters. That may seem minimal, but according to the Intergovernmental Panel on Climate Change, the rate of sea level rise is only set to get larger in the future and we can expect anywhere from 10 to 30 inches of sea level rise by the year 2100.
However, a portion of the water lost from land does not directly enter the ocean from its source because it is lost from some of the most isolated and driest areas on the planet. New evidence from Jida Wang and a team of interdisciplinary researchers describe how changes in the water balance in dry regions can actually have an impact on ocean water volume.
Overworked and Water Stressed
The subject of the study is endorheic basins. These areas are essentially landlocked regions with topographic features like mountains and ridges that block flow of water at the surface from going directly to the ocean. An example of these basins in North America is the Great Basin, which includes Death Valley and the Great Salt Lake. The largest in the world is located in central Asia, including the Caspian Sea and the Aral Sea, both of which are important fishing areas for the people of central Asia.
Endorheic basins are defined by hydrologic isolation, with a steady state of water inputs and outputs, so water stored on the surface is a scarce resource. The basins are typically warm and water loss through evaporation is high, but if there is enough rainfall the losses can be balanced out. Altering that delicate equilibrium of water coming in and out can have major consequences because humans and wildlife living in these basins rely heavily on a very limited water supply.
Although they only make up 20% of total land area on Earth, endorheic basins include 50% of the world’s water-stressed regions. This trend is not slated to improve. Warming and drying due to climate change has disrupted the careful water balance in endorheic basins, tipping the scales towards water loss in many dry, landlocked regions.
When terrestrial water loss in basins was investigated separately, researchers found that terrestrial water loss rates were twice as fast as compared to other areas. According to this study the mass of water lost from endorheic regions is 2-4 times higher than previously estimated.
And because of long-term climate patterns, extreme water loss is affecting most endorheic basins whether they are highly engineered by humans or not. High rates of evaporation, due to human activities like river damming and diverting surface water, contribute to the problem, but decade long droughts and temperature increases are likely the major reason water loss is high in these regions.
All water makes its way to the ocean
If endorheic basins are isolated from the ocean, where does all this water go? Wang and his colleagues estimate that approximately 80% of the water that is evaporated from isolated basins is eventually brought to the ocean in the form of rain or snow. With those numbers, approximately 9% of measured sea level rise, or 0.3 mm/yr, can be attributed to water that was previously stored in extremely dry and isolated regions.
The most important takeaway from this study is that although we have a lot of information about our global water status, there is still a lot that we do not understand about where our water comes from, where it goes, and what impact it has once it gets there.
Continued drying of our land basins around the world will put more stress on our limited water sources and could lead to conflict. Studies such as this help us find more pieces of the puzzle that make up a closer to complete picture of understanding and in turn wisely using our water systems.