Sea levels are rising faster than they have in 4,000 years

Sea levels have always fluctuated throughout Earth’s history. But something different is happening now – sea level rise is speeding up, and it’s not looking good for many of the world’s largest cities.

By digging into records preserved in ancient coral reefs and coastal landscapes, scientists discovered something alarming. Today’s sea levels are rising faster than at any time in the past 4,000 years.

That’s not just a fun fact for the history books – it’s a serious warning, especially for places like China, where several of the world’s biggest cities sit right along the coast.

Why sea level rise matters

Since 1900, the average global sea level has gone up about 1.5 millimeters (0.06 inches) a year – a little over one-sixteenth of an inch. That might not sound like much, but over time, it adds up.

And here’s the real issue: that speed beats any century-long sea level rise in at least the past four millennia.

Two things are driving the change. First, as the planet heats up due to climate change, the oceans warm and expand. Second, glaciers and ice sheets are melting faster than before. When ice melts on land, it adds water to the oceans.

“Getting warmer makes your ocean take up more volume,” said Yucheng Lin, who conducted the research as a postdoctoral associate at Rutgers University.

“And the glaciers respond faster because they are smaller than the ice sheets, which are often the size of continents. We are seeing more and more acceleration in Greenland now.”

Rising seas and sinking land

The study looked closely at China’s coastline. Cities like Shanghai, Shenzhen, and Hong Kong are built on soft, low-lying land in river deltas. These deltas are great for farming and business, but they naturally sink over time. And human activity is speeding that up.

“We’ve been able to quantify the natural rate of sea level rise for this area,” said Lin. “But human intervention, mostly groundwater extraction, makes it happen much faster.”

When people pump groundwater from underground, the land can slowly sink – a process called subsidence. In Shanghai, for example, parts of the city dropped more than three feet in the 1900s because of this.

That’s way faster than sea level rise alone. And when you combine sinking cities with rising oceans, the risk of flooding jumps dramatically.

“Centimeters of sea level rise will greatly increase the risk of flooding in deltas,” Lin said. “These areas are not only important domestically, they’re also international manufacturing hubs. If coastal risks happen there, the global supply chain will be vulnerable.”

Managing water to fight back

There’s some good news. Shanghai realized it had a problem and started to manage how much groundwater people were using. The city even began reinjecting fresh water underground to help stop the sinking.

“Shanghai now is not sinking that fast anymore,” Lin said. “They recognized the problem and started regulating their groundwater usage.”

The study created maps to help governments see where the risks are highest. That way, cities can plan ahead and protect themselves better.

Rising tides threaten cities worldwide

The study’s message is bigger than one country. Many coastal cities around the world – from New York to Jakarta – face similar threats. Low-lying areas near oceans are popular places to live and build. But those same features make them vulnerable.

“Lin’s work illustrates how geological data can help us better understand the hazards that coastal cities face today,” said Robert Kopp, a professor at Rutgers University who co-authored the study.

“Deltas are great places, good for farming, fishing, urban development and naturally draw civilizations to them,” Lin said. “But they are really flat yet prone to human-caused subsidence, so sustained sea level rise could submerge them really fast.”

If cities start taking action now – by managing how much groundwater they use, improving infrastructure like drainage systems and sea walls, and planning ahead for heavier flooding – they can still get a handle on the situation. It’s not too late to make a difference, but it won’t fix itself.

The longer they wait, the harder it gets. Sea levels won’t slow down just because we’re behind.

Once the water starts creeping into streets, homes, and factories, the damage – both physical and economic – will be much tougher to reverse. This is a problem that builds slowly, then hits all at once.

The full study was published in the journal Nature.

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