Global sea level to rise by up to 1.2 metres despite Paris agreement, say scientists

Global sea levels are set to rise dramatically, threatening the homes of some 100 million people, even if the strictest greenhouse gas emissions targets are met, according to a new study.

The research, compiled by climate scientists from a number of international institutions, analysed the long-term impacts of different emission levels and concluded oceans will rise by over one metre even if the world sticks to the Paris agreement.

Overall, the researchers estimated a global rise of between 0.7 and 1.2 metres – adding that if emissions are not curbed as soon as possible it will be even greater.

More than 100 million people are currently thought to live within one metre of the high tide level.

The Paris agreement was signed by nearly 200 countries including the US under former president Barack Obama's administration in December 2015. Donald Trump has since begun the process of withdrawing the US from the deal.

It calls for emissions to peak as soon as possible before going into decline, in an effort to limit the increase in global average temperatures to “well below 2C above pre-industrial levels” and to “pursue efforts to limit the temperature increase to 1.5C".

The new research lays out the consequences of delaying the action it specifies.

According to the analysis, for every five years this peak is delayed after 2020, an extra 20cm can be added to estimates for global sea-level rise.

Although the projections look far ahead to 2300, the scientists emphasised the dramatic impact short-term greenhouse gas emissions in coming decades would have on sea levels for centuries to come.

"Our results show that there are quantifiable consequences of delaying action,” said Dr Carl-Friedrich Schleussner, a researcher at the Potsdam Institute for Climate Impact Research (PIK) and one of the study’s co-authors.

“For millions of people around the world living in coastal areas, every centimetre can make a huge difference – to limit sea-level rise risks, immediate CO2 reduction is key."

The atmosphere can respond relatively quickly to changes in greenhouse gas emissions, meaning global surface temperatures can probably be stabilised or even reduced if drastic cuts are introduced.

However, the ocean is far slower in its response.

"Man-made climate change has already pre-programmed a certain amount of sea-level rise for the coming centuries, so for some it might seem that our present actions might not make such a big difference – but our study illustrates how wrong this perception is," said Dr Matthias Mengel, another researcher at PIK who led the research.

"Every delay in peaking emissions by five years between 2020 and 2035 could mean additional 20 centimetres of sea-level rise in the end – which is the same amount the world's coasts have experienced since the beginning of the pre-industrial era."

The results of this analysis were published in the journal Nature Communications.

The rise in global sea levels is the result of melting glaciers and ice caps around the world, as well as the warming and expansion of ocean water.

As it stands, current efforts by nations to reduce emissions are not enough to avoid the more significant rises in sea levels predicted by the new analysis.

On top of this, the instability of ice sheets in Antarctica could mean greater-than-anticipated rises in sea levels even if the most ambitious emissions targets are met.

"Indeed, the uncertainty of future sea-level rise is at present dominated by the response of Antarctica. With present knowledge on ice sheet instability, large ice loss from Antarctica seems possible even under modest warming in line with the Paris agreement," said Dr Mengel.

"Even a sea-level rise of up to three metres until 2300 cannot be ruled out completely, as we are not yet fully certain how the Antarctic ice sheet will respond to global warming."

Commenting on the report, climate scientists welcomed the findings as significant, though not entirely unsurprising.

"This is a great example of how delays to mitigation can make the costs of climate change add up,” said Professor Dave Frame, director of the New Zealand Climate Change Research Institute, who was not involved in the research.

However, Professor Frame noted that arguments about sea level rise might not be the “game-changer” some scientists would hope for when it comes to convincing people about the harmful effects of climate change.

"If people aren't prepared to mitigate on behalf of their children, whom they love, it's hard to see how information about people 300 years away will do more to alter their behaviour,” he said.

Professor Tim Naish, director of the Victoria University of Wellington’s Antarctic Research Centre, argued the research is significant for those who are already feeling the impact of rising sea levels.

“Pacific Island nations were strong advocates that the Paris agreement ensure parties make a genuine attempt to restrict global warming to nearer 1.5C above pre-industrial levels, as a number of studies were already showing that sea-level rise in a 2C world could flood some of most vulnerable lowest lying island nations such as Kiribati and Tuvalu,” he said.

"This is exactly the kind of work that people need to hear about,” added Associate Professor Pete Strutton, a biological oceanographer at the University of Tasmania.

“We need to realise that climate change is happening. Even if we stop emitting today, the effects of our past emissions will be felt for centuries to come and every year that we delay action has consequences for the future."