A recent study led by scientists from Rutgers University has revealed that global sea levels are rising at an unprecedented rate compared to the past 4,000 years. This rapid increase is a major threat to coastal areas, with many of China's largest and most important cities being especially at risk.

The research team examined thousands of geological records, including coral reefs and ancient mangrove swamps, to understand sea level changes over the last 12,000 years. Their study, published in Nature, found that since around 1900, global sea levels have increased by about 1.5 millimetres per year. While this might seem like a small amount, it is much faster than the average rate seen in any period over the past four millennia.

What Is Causing This?

This is driven by two main factors: warming oceans and melting ice. As the world warms due to climate change, the oceans absorb much of the additional rising heat. This causes the water to expand, raising sea levels. At the same time, glaciers and large ice sheets, particularly in Greenland and Antarctica, are melting which adds more water to the oceans.

While rising seas are a global issue, China has a particularly complex situation. Many of its most important and large cities, such as Shanghai, Shenzhen, and Hong Kong, are built on river deltas. These areas are flat, low-lying, and made up of soft sediments, which naturally causes them to sink over time.

Moreover, human activity is making the problem even worse. One major cause is the extraction of groundwater. In many parts of the country, water is drawn from underground aquifers to meet the needs of cities and farms. This removal of water weakens the ground beneath, causing the land to sink. This process is known as subsidence.

Coastal Cities at Risk

In Shanghai, for example, the ground has sunk by more than a metre during the 20th century, mostly because of excessive groundwater use. This rate of sinking is much higher than the rate of sea level rise alone. When both happen together, the risk of flooding increases significantly.

The researchers used geological data, satellite records, and maps showing human activity to understand how rising seas and subsidence affect China's coastal regions. They focused particularly on the Yangtze River Delta and Pearl River Delta. These two areas support tens of millions of people and are crucial to the global economy.

However, these same features also make them vulnerable to the effects of rising seas. Even a small rise in sea level can result in more frequent and severe flooding, stronger storm surges, and damage to infrastructure.

The study also highlights the broader implications. Many coastal cities in China are not only important nationally but also play a key role in global manufacturing and trade. Disruptions in these areas, due to extreme weather or long-term flooding, could affect supply chains worldwide.

Immediate Action Needed

Some cities are already taking steps to manage the problem. For instance, Shanghai has implemented stricter rules on groundwater use and has started putting water back into underground aquifers. These actions have helped slow the sinking of the ground in recent years.

The researchers also created vulnerability maps that identify the areas that are most at risk. These tools can support governments and city planners in making better decisions about development, infrastructure, and flood prevention.

While this study focuses on China, its insights are relevant globally. Many major cities, such as Jakarta, New York, and Manila, are built on low-lying coasts and face similar challenges. As the climate continues to change, understanding both natural and human-made risks will be critical for protecting coastal communities.

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