Have We Been Underestimating Sea Levels?

For many years scientists have been carefully tracking how the oceans are rising as the planet warms. Rising sea levels are one of the clearest long-term consequences of climate change, driven by two main processes: melting glaciers and ice sheets, and the thermal expansion of seawater as it warms.

But new research suggests something rather unsettling.

It appears that the true level of the oceans around the world may have been underestimated.

What the New Research Found

A recent study published in the scientific journal Nature analysed 385 peer-reviewed studies published between 2009 and 2025. Researchers compared the sea levels assumed in many models with the actual measured coastal sea levels.

Their conclusion:
The baseline sea level used in many models may be too low.

On average, the research suggests that global ocean levels are about 30 cm higher than previously assumed.

In some regions — particularly parts of South-East Asia and the Indo-Pacific — the difference may be as much as 100–150 cm higher than earlier estimates.

That is a very large discrepancy.

Why This Matters

Sea-level rise projections are used to estimate how vulnerable coastal cities and communities are to flooding.

The global climate authority, the Intergovernmental Panel on Climate Change, estimates that global sea levels could rise by between 28 cm and 100 cm by the year 2100 depending on emissions.

But if the starting point for sea level is higher than previously believed, the risk calculations change.

This means:

• Coastal flooding could occur earlier than expected

• Storm surges could travel further inland

• Some low-lying communities may face greater long-term risks

Why Sea Levels Are Hard to Measure

Measuring sea level globally sounds simple, but it is actually quite complicated.

Scientists use several different methods:

• Satellite altimeters measuring the height of the ocean surface
• Tide gauges at coastal monitoring stations
• Geodetic measurements that track whether land itself is sinking or rising

And this last point is crucial.

In many places the land is slowly subsiding (sinking), particularly in river deltas and coastal cities where groundwater extraction is common. When the land sinks while the sea rises, the effect can make the ocean appear to rise even faster locally.

The Most Vulnerable Regions

Some of the regions most exposed include:

• South-East Asia

• Pacific island nations

• Low-lying delta regions such as Bangladesh

• Parts of the Indo-Pacific

In these areas, millions of people live only a few metres above sea level.

What This Means for the Future

The research does not necessarily mean the oceans suddenly rose overnight. Rather, it suggests our baseline measurements may have been off, which affects how we estimate future impacts.

What it does reinforce is that coastal planning must account for uncertainty.

Governments may need to reconsider:

• Flood defences

• Coastal development planning

• Managed retreat from vulnerable areas

• Wetland and mangrove restoration as natural protection

A Reminder of the Scale of Climate Change

Sea level rise is one of the most slow but unstoppable consequences of global warming. Even if emissions stopped tomorrow, melting ice and warming oceans will continue to influence sea levels for decades.

Understanding the true baseline is therefore essential.

Because when you are planning for the future of coastal cities, 30 cm can make a very big difference.