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The construction of large-scale dams has played a surprising role in limiting the rising seas, scientists say.
Over the past century, melting glaciers and thermal expansion of seawater have caused ocean levels to rise.
But this new study reveals that dams almost stopped the rising seas in the 1970s because of the amount of water they kept from entering the oceans.
Without them, the annual growth rate would have been about 12% higher.
Measuring the rise of seas over the past 100 years is a difficult task for scientists.
The researchers found that there was a discrepancy between the amount of water they knew to have gone into the oceans and how much of those oceans increased over the past century.
In this new work, the authors revisited the information on sources and measures to arrive at a new, more precise estimate.
In addition to melting glaciers and thermal expansion of seas due to the penetration of thermal energy into the waters, the researchers found that water storage facilities such as dams and reservoirs had a significant impact on sea level throughout the period.
There are currently around 58,000 large dams around the world, many of which have been built in the past 60 years.
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The Kariba Dam, on the border between Zambia and Zimbabwe, was the largest in the world when it was built
The 1950s to 1970s saw a construction boom with several large-scale constructions completed, including the Kariba Dam in Zimbabwe, the Bratsk Dam in Siberia, and the Aswan High Dam in Egypt.
When the full impact of these giants took effect in the 1970s, their ability to keep water from entering the sea slowed the continued rise in global sea level.
“Much of this decline is due to the fact that sea level [rise] was almost halted due to the amount of water being stored in the dams, ”said lead author Dr. Thomas Frederikse, from NASA’s Jet Propulsion Laboratory in Pasadena, California.
“So by building dams, we almost stopped the rise in sea level for about ten years.”
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Many dams, such as the Itaipu between Paraguay and Brazil retain large amounts of water to generate electricity
The study reveals that the global sea level rose by about 1.56 mm per year between 1900 and 2018.
The main contributors to the rising seas during most of the 20th century were the melting of glaciers which reacted more quickly to a warmer world.
Over this entire period, the authors estimate that the sea level would have been about 12% higher without the influence of dams and reservoirs.
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When things go wrong with dams, like the one in Michigan, they can be a disaster for the environment and the locals.
However, the influence of dams on water retention began to wane in the 1990s.
Concerns were growing about the environmental impact of large dams and the number of projects was starting to decline.
At the same time, the growing influence of climate change has caused sea level to rise through increased ice loss from Greenland and greater thermal expansion of the waters as more thermal energy has gone into the oceans.
All of these factors have seen sea level rise accelerate over the past 30 years and it now turns at 3.35mm per year.
So could a new dam-building program help save the world from rising waters?
Earlier this year, a team of researchers produced an article on how Europe could avoid the worst impacts of rising seas by building a massive dam across the North Sea.
But Dr Thomas Frederikse doesn’t think adding more barriers would work now.
“At the dam, we were able to slow the rise in sea level by around 0.8 mm per year.
“And now we are seeing sea level rise over the past 10 years by about four millimeters per year.”
“So that means you have to build five times the number of dams we have built during that time to stop the current rate of sea level rise.”
“I think it’s impossible.”
The best and much cheaper option, says Dr Frederikse, is to cut CO2 emissions faster and deeper than at present.
The study was published in the journal Nature.
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