Global sea-levels could rise more than two metres by 2100, scientists warn

By 2300, sea levels could have risen by more than four metres

Global sea-levels could rise on average by nearly two-and-a-half metres (eight feet) by 2100 if greenhouse gas emissions are not reduced.

That's according to a review of sea-level change and projections by scientists at Rutgers University and other scientists, published in the Annual Review of Environment and Resources.

The report also claimed that by the year 2300, sea levels could rise by more than 15 metres - or 50 feet - depending on emissions of greenhouse gases, such as carbon and methane.

That's because, since the start of the century, global average sea-levels have risen by about six centimetres.

There's much that's known about past and future sea-level change, and much that is uncertain

Under moderate emissions, estimates of global average sea-level rise from different analyses range from 40 centimetres to 80 centimetres by 2100, and from 80 centimetres to 1.6 metres by 2150. By 2300, sea levels could rise by between 1.8 metres and 4.3 metres.

With 11 per cent of the world's 7.6 billion (and rising) people living in areas less than 10 metres above sea level, rising seas pose a major risk to coastal populations, economies, infrastructure, and ecosystems around the world.

Significantly rising sea levels would also have a major impact on the area of land that can be dedicated to crops.

Sea-level rise varies naturally over location and time. However, scientists have developed a range of methods to reconstruct past changes and therefore estimate the potential for future rises in sea levels.

Despite the different approaches, they claim that a clear story is emerging over what is likely to happen over the coming decades, depending on the level of emissions of carbon and other gases implicated in global warming and climate change.

From 2000 to 2050, global average sea-levels will most likely rise by between 15 centimetres and 25 centimetres, but with a rise of 45 centimetres or more regarded as extremely unlikely.

Beyond 2050, projections are more sensitive to changes in greenhouse gas emissions.

"There's much that's known about past and future sea-level change, and much that is uncertain. But uncertainty isn't a reason to ignore the challenge," said study co-author Robert E. Kopp, a professor in the Department of Earth and Planetary Sciences at Rutgers University.

"Carefully characterising what's known and what's uncertain is crucial to managing the risks sea-level rise poses to coasts around the world."

Scientists used case studies from Atlantic City, New Jersey and from Singapore to discuss how current methods for reconstructing past sea-level change can constrain future global and local projections.

They also discussed approaches for using scientific sea-level projections and how accurate projections can lead to new sea-level research questions.

A large portion of sea-level rise in the 20th century, including most of the global rise since 1975, is down to human-caused global warming, the study states.

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