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Even 1.5C warming will still leave world’s coasts exposed to extremes

One-in-100-year extreme sea-level events along the world’s coastal regions will become annual occurrences by the end of the century even if the most ambitious climate change action is taken.

The findings, which include work by Australian researchers, are published on Tuesday in Nature Climate Change journal. The paper builds on the Intergovernmental Panel on Climate Change’s latest report and sought to estimate what was the lowest amount of global warming that would trigger at least a 100-fold change in the frequency of the present-day once a century extreme event by 2100.

The research combined observations at 179 spots with climate modelling to assess more than 7200 locations around the world. About half the world’s coastal areas will endure such extreme sea-level events “even under 1.5 degrees of warming, and often well before the end of the century”, the paper said.

“We combined different data sets across international research groups,” said Ebru Kirezci, an engineering post-doctoral student at the University of Melbourne and one of the paper’s authors. “Governments and policymakers should do more work on protection and mitigation scenarios.”

For Australia, the areas likely to see the fastest rise of extreme sea-level events will be in the east and the south, but also areas of south-west Western Australia, Dr Kirezci said.

Such events lead to beach erosion, inundation of inland areas and other damage.

According to the recent IPCC report, global sea levels have risen an average of about 20 centimetres between 1901 and 2018 as the warming oceans swell and more land-based ice melts into the sea.

The average annual rate has accelerated from 1.3 millimetres in the first seven decades of the 20th century, to 1.9 millimetres in the following 25 years to 3.7 millimetres between 2006 and 2018, the IPCC stated.

The paper found areas with relatively narrow sea-level changes now, such as in parts of the tropics that are not subject to big cyclones and the Mediterranean, will register the shift to more extremes sooner. By contrast, some northern hemisphere coastlines will see little change even at 5 degrees of warming.

John Church, a former senior CSIRO sea-level researcher now working at the University of NSW, said the paper contained “new and useful information”.

Even at 1.5 degrees, the lower end of the Paris Agreement, “we’re going to have to adapt”, Dr Church said. “It’s an important message to get out.”

Dr Church’s past work showed that in the 20th century, the risk of extreme events along the west and east coasts of Australia had roughly tripled.

“I’d guess since 2000 they have increased by another factor of three,” he said.

Dr Kirezci said governments needed to prepare for the inevitable sea-level rises by considering sea walls and other barriers. Governments should also assess which coastal communities are most at risk and consider relocating them inland, and also investigate early warning systems to improve safety.

Her next line of work has been to examine the socio-economic damage likely for populations in every country.

Sea-level increases vary around the world because of two main processes, Dr Church said. One is a change in mass distribution, such as in Greenland where the land actually rises as its heavy ice sheets melt.

Changes in currents, such as the strengthening of the East Australian Current, also contribute to difference rates of sea-level rises, he said.