Sea level rise is a clear consequence of climate change. Warmer water expands, while melting land ice flows into the sea. But what fun would it be if the natural world did not make things a little more complicated? A number of processes can experience the trends varying on different coasts, from ocean circulation to rising or falling land uplift. Although we want to quantify each participant in sea level rise, it should come as no surprise that the figures do not always add up perfectly.
That researchers have become accustomed to a process called ‘closing the budget’ – working on additions of ice mass loss, thermal expansion of seawater, and changing water storage on land, and then comparing that with estimates of sea levels of timers and satellites. Research has closed the budget for the past decades, including the era of good satellite data that began in the early 1990s and extends back to about 1960. But in the first half of the 20th century, tidal meters are more sparse, as are observations of glaciers. and ocean temperatures. As a result, a mismatch remained between what we would expect based on estimated sea level rise and what we think we saw, based on estimated sea level rise. There was apparently more sea level rise than we could explain in that time period.
Please try again
Researchers have been working on these issues from every angle, clearing data problems and tackling complications such as changes in land elevation. That lets a team through Thomas Frederikse at Caltech’s Jet Propulsion Laboratory decided it was time to try again. Frederikse and his colleagues find that estimates are now consistent until 1900. And that includes some time periods of higher or lower rates of sea level rise along the way.
Combining each dataset they could get their hands on, these researchers calculated global and regional estimates from 1900 to 2018. They save all the error bars by generating 5,000 realizations with randomly selected possible values (known as a Monte Carlo analysis).
They separate the effect of temperature on expanding ocean water volume (“thermostatic” sea level rise) from the factors that add water to the oceans (“barrastic” sea level rise). These include the melting of mountain glaciers and the leaks of Greenland and Antarctica, as well as the depletion of groundwater resources. That compensates is the construction of dams, which provide storage for surface water on land that did not exist before.
Their rough average rate of sea level rise for 1900-2018 is 1.56 ± 0.33 millimeters per year. The sum of all the factors for changing sea level is 1.52 ± 0.33 millimeters per year – a comfortable match.
Estimates also agree within error bars for 1957-2018 and 1993-2018, but these periods show rapid sea level rise, with rates increasing to about 1.78 millimeters per year and then 3.35 millimeters per year.
What came out of it
For the first half of the 20th century, water added to the oceans accounted for more than 80 percent of the total sea level rise, although dam building in that number ate up in the second half of the century. Here’s a simpler comparison: since 1900, melting ice has caused twice as much sea level rise as expansion due to warming.
The two largest deviations in the middle of the 20th century in the 1940s, when sea levels went a little faster, and in the 1970s, when sea levels went a little slower. It is interesting that the global temperature trend differs somewhat similarly, but thermal expansion is not what causes these variations at sea level.
In the 1940s, the uptick seems to be due to greater ice loss from mountain glaciers and Greenland. The delay in the 1970s coincides with a spike in dam construction on land that “steeled” from the oceans by filling new reservoirs.
Zoom in to individual ocean basins needs even more factors to consider. While things like salinity effects on seawater volume are washing out in the global average, they are becoming significant in a region. There is not enough data on salt mining in the early 1900s to go back anywhere. But even though the number of tidal gauge and land elevation data differ, the researchers found that any sea level of sea guidance corresponds to observations dating back to 1960.
Not all pools are the same
There are some interesting differences between basins, with a slower rise in sea levels in the East Pacific than in the South Atlantic, for example. Strangely enough, the trend of North Atlantic sea level corresponds to the global average despite a strange situation in which competing factors roughly compensate. Ice sheets actually exert a gravitational pull on surrounding seawater, drawing more water closer and increasing local sea levels. Melting glacier ice relaxes this attraction, and lowers local sea levels even as meltwater in the ocean is dumped. The effect worked to reduce the rate of sea level rise in the North Atlantic Ocean. But at the same time, downward movement of the seawall (a relic from the ice age) had the opposite effect, causing water to pour in to fill the deepened basin.
In general, the researchers write, “Closing the budget of the twentieth-century sea level, as shown here, implies that no additional unknown processes, such as large-scale thermal expansion of the ocean or additional mass loss from the Antarctic ice sheet “, are required to explain the observed changes in the global sea level. Such additional processes were speculated to explain the non-closure found in previous studies on the global sea level budget.”
And if we have a good approach to events so far, that adds some confidence that we understand where and how hard the water will pile up as the oceans get warmer and more glacier ice disappears from the land.
Nature, 2020. DOI: 10.1038 / s41586-020-2591-3 (About DOIs).
