Increased knowledge of past sea-level measurements is considered key in creating ever-more accurate sea-level forecasts. The factors responsible for rising sea levels are already well known.
These include melting glaciers adding water to the seas and warmer temperatures causing water to expand.
We impounded so much freshwater, humanity nearly brought sea level rise to a halt
NASA’s Thomas Frederikse
Conversely, features such as dams obstructing water’s flow into the sea are recognised as slowing the rise.
And in combination, all these factors should roughly match the sea level we see today.
However, the sea level budget has stubbornly fallen short of observed sea level rises, causing scientists to question why the budget does not balance.
A new NASA study published this week aims to balance this budget.
By learning more about historic measurements, scientists can even more accurately forecast how each of these factors will affect sea-level rise and how this rise will continue to impact areas.
After re-examining each of the known contributors to sea-level rise from 1900 to 2018, the NASA research incorporates refined estimates and satellite images to better understand historic measurements.
The scientists discovered estimates of global sea-level variations based on tide-gauge observations had slightly overestimated global sea levels before the 1970s.
Tide gauges are used to measure sea level height at coastal stations scattered around the world.
The NASA researchers also learnt glacier meltwater was contributing more water to the world’s oceans than previously thought.
Somewhat surprisingly, they also found the relative contribution of glaciers to sea-level rise is slowly decreasing.
In addition, glacier and Greenland ice sheet mass loss are now thought to be a key factor in the increased rate of sea-level rise before 1940.
And finally, NASA’s latest study found sea-level rise slowed to a crawl during the 1970s when dam construction was at its peak.
Dams create reservoirs that can impound freshwater that would normally flow straight into the sea.
Thomas Frederikse, a NASA postdoctoral fellow and the study’s lead researcher, said: “That was one of the biggest surprises for me.”
He added: ”We impounded so much freshwater, humanity nearly brought sea-level rise to a halt.”
Since the 1990s, however, Greenland and Antarctic ice sheet mass loss and thermal expansion have accelerated sea-level rise, while freshwater impoundment has slowed.
And as the planet’s climate continues to warm, the majority of this thermal energy is absorbed by the oceans, causing the volume of the water to expand.
Melting ice sheet and thermal expansion are now thought to account for approximately two-thirds of global mean sea-level rise.
Mountain glacier meltwater currently contributes another 20 percent, while declining freshwater water storage on land adds the remaining 10 percent.
Sea levels have risen on average 1.6mm (0.063 inches) per year between 1900 and 2018.
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