Berlin: If global temperatures continue to go up unabated, sea levels around the world could rise by up to five metres in coming centuries, which will severely affect low-lying countries like Bangladesh and several small islands nations, a new study has claimed.
The study, published in journal Nature Climate Change, is the first to give a comprehensive projection for such long perspective, based on observed sea-level rise over the past millennium, as well as on scenarios for future greenhouse-gas emissions.
"Sea-level rise is a hard to quantify, yet critical risk of climate change," said lead study author Michiel Schaeffer of Wageningen University in the Netherlands.
"Due to the long time it takes for the world`s ice and water masses to react to global warming, our emissions today determine sea levels for centuries to come," said Schaeffer, also the director of Climate Analytics in Germany.
Study co-author Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany described the potential impacts of rising temperatures as very significant.
"As an example, for New York City it has been shown that one metre of sea level rise could raise the frequency of severe flooding from once per century to once every three years," Rahmstorf said.
Also, low lying deltaic countries like Bangladesh and many small island states are likely to be severely affected, Rahmstorf added.
However, the scientists said, limiting global warming could considerably reduce sea-level rise. While the study suggests that even at relatively low levels of global warming, the world will have to face significant sea-level rise, it also demonstrates the benefits of reducing greenhouse-gas emissions, they said.
Limiting global warming to below 1.5 degrees Celsius and subsequent temperature reductions could halve sea-level rise by 2300, compared to a two-degree scenario, they said.
But, if temperatures are allowed to rise by 3 degrees, the expected sea-level rise could range between two and five metres, with the best estimate being at 3.5 metres, they warned.
The scientists further stated that the warmer the climate gets, the faster the sea level climbs and coastal communities have less time to adapt if sea-levels rise faster.
"In our projections, a constant level of 2-degree warming will sustain rates of sea-level rise twice as high as observed today, until well after 2300," Schaeffer said.
"But much deeper emission reductions seem able to achieve a strong slow-down, or even a stabilisation of sea level over that time frame."
The researchers also pointed out that past multi-century projections of sea-level rise by the Intergovernmental Panel on Climate Change (IPCC) were limited to the rise caused by thermal expansion of the ocean water as it heats up, which the IPCC found could reach up to a metre by 2300. However, this estimate didn`t include the potentially larger effect of ice melting.
The new study is using a complementary approach, called semi-empirical, that is based on using the connection between observed temperature and sea level during past centuries in order to estimate sea-level rise for scenarios of future global warming, they stated.
"Of course it remains open how far the close link between temperature and global sea level found for the past will carry on into the future," Rahmstorf said.
"Despite the uncertainty we still have about future sea level, from a risk perspective our approach provides at least plausible, and relevant, estimates."