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The construction of large-scale dams has played a surprising role in limiting rising seas, scientists say.
In the past century, melting glaciers and the thermal expansion of seawater levels have risen.
But this new study finds that dams almost stopped the rising sea in the 1970s because of the amount of water they prevented from entering the oceans.
Without them, the annual rate of increase would have been about 12% higher.
Measuring how much the seas have risen in the past 100 years is a daunting task for scientists.
Researchers found that there was a gap between how much water they knew they had gone into the oceans compared to how much those oceans had actually risen in the past century.
In this new work, the authors review information on sources and measurements to come up with a new, more accurate estimate.
Like the melting of glaciers and the thermal expansion of the sea by heating energy entering the water, the researchers found that water storage facilities such as dams and reservoirs during the period had a significant impact on sea levels.
There are currently about 58,000 large dams in the world, with many of them 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 built
In the 1950s and 1970s, a construction boom was completed with several large-scale constructions, including the Kariba Dam in Zimbabwe, the Bratsk Dam in Siberia and the Aswan High Dam in Egypt.
When the full influence of these giants flowed in the 1970s, their ability to block water in the sea flowed the continuing rise of global sea levels.
“A large portion of this dip comes from sea level [rise] was almost stopped because of the amount of water stored in dams, “said lead author Dr. Thomas Frederikse, of Nasa’s Jet Propulsion Laboratory in Pasadena, California.
“So by building dams, we stopped rising sea levels for almost a decade or so.”
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Many dams, such as the Itaipu between Paraguay and Brazil, hold large amounts of water back to generate electricity
The study finds that the general sea level increased between 1900 and 2018 by about 1.56mm per year.
The largest contributors to rising seas in most of the 20th century have melted glaciers that respond more quickly to a warmer world.
Over this entire period, the authors believe that sea levels would have been about 12% higher without the impact of dams and reservoirs.
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When things go wrong with dams, like these in Michigan, they can be a disaster for the environment and the local people.
However, the influence of dams in holding the waters began to disappear in the 1990s.
Concerns grew about the environmental impact of large dams, and the number of projects began to decrease.
At the same time, the growing impact of climate change led to an increase in sea levels due to increased ice loss from Greenland and a greater thermal expansion of the waters, as more heat energy went into the oceans.
All these factors have seen the rise in sea level accelerate in the past 30 years and it is now running at 3.35mm per year.
Could a new dam construction program help save the world from rising waters?
Earlier this year, a team of researchers produced a paper on how Europe could prevent the worst effects of rising seas by building a large dam across the North Sea.
But Dr Thomas Frederikse does not think that additional barriers would work now.
“At the height of the dam building, we were able to slow down the sea level rise by about 0.8 mm per year.
“And now we’re seeing sea levels rise by about four millimeters a year over the last 10 years.”
“That means you have to build five times the number of dams we built in that period to stop the current rate of sea level rise.”
“I think that’s impossible.”
The better and much cheaper option according to Dr Frederikse is to reduce CO2 emissions, faster and deeper than at present.
The study was published in the journal Nature.
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