dpa | The global sea level has risen much faster from 1900 to 2020 than at any other time in the past 4,000 years. This finding comes from a study that examined changes in the global sea level over the last nearly 12,000 years.
The current rise in the global sea level is mainly due to two effects, according to the group led by Yucheng Lin of Rutgers University in Piscataway, New Jersey, in the journal Nature: for one, the water in the oceans is warming—and expanding as a result; for another, melting of mountain glaciers and the ice sheets in Greenland and Antarctica is adding more water to the oceans.
“Glaciers respond more quickly because they are smaller than the ice sheets, which often are the size of continents,” Lin is quoted in a university statement. “In Greenland, we are currently seeing an ever-stronger acceleration.”
After the end of the last Ice Age, the sea level rose particularly steeply during the period from about 11,700 years ago to 8,200 years ago — on average by 10.7 millimeters per year. About 6,000 years ago the annual rise was then around 2.8 millimeters. About 3,000 years ago it was only 0.4 millimeters per year and then declined further.
Acceleration since 1990
Over the past 4,000 years, the sea level fluctuated only slightly. It was not until the 19th century that a change occurred: in the first half, the sea level rose only relatively little, by an average of 0.1 millimeters per year; in the second half, it was 0.76 millimeters per year.
The scientists quantify the average rise from 1990 to 2020 at 1.51 millimeters. Other studies indicate that the rise during this period has increasingly accelerated.
For the study, the team examined thousands of data from various sources, including ancient coral reefs and mangroves, which serve as natural archives of past sea-level heights. These data were fed into a modeling software that Lin himself developed. In this way, it is possible to distinguish among the various causes of sea level rise or fall, it says.
Contribution of Dense Urban Development
Thus, the subduction of one tectonic plate beneath another can lift the upper plate. Conversely, many coastal cities lie near river deltas, where the sedimentary substrate is compressed by the mass of buildings and roads, leading to subsidence and thereby a further rise in sea level.
The researchers illustrate this with examples from the southeast coast of China. Parts of Shanghai sank more than a meter in the 20th century — not only due to natural subsidence of the ground, but also because of heavy groundwater extraction.
Other coastal cities fared even worse: the former Indonesian capital Jakarta has subsided so far that parts of the city now lie below sea level — water must be pumped out there continuously.
