An international team of researchers, including scientists from the Australian National University, has developed the most accurate reconstruction to date of past Antarctic ice sheet melt, helping to forecast future rising sea levels.

The Antarctic ice sheet is the largest block of ice on earth, containing more than 30 million cubic kilometres of water. Its melting could have a devasting impact on future sea levels.

To find out just how big that impact might be, the research team turned to the past, using historical records from around Australia.

“If we want to know what is going to happen in the next 100 years, we need to have an accurate model for how ice sheets respond to climate change,” Dr Mark Hoggard, an earth scientist at ANU, said.

The study focuses on the mid-Pliocene, three million years ago. By examining fossilised corals and sea-level markers in Australia, the researchers determined that during the mid-Pliocene, the Antarctic ice sheet likely contributed 9.8 metres to sea level.

“Previous forecasts of the Antarctic contribution to global mean sea level rise were anywhere between 20 and 52 cm by 2100,” Dr Hoggard said. “But by getting a better idea of sea levels during the Mid-Pliocene era, our study reduces this estimate to between 5 and 9 cm.

“The Mid-Pliocene period, 3 million years ago, is considered the best equivalent to conditions expected this coming century in terms of carbon dioxide levels and temperature.”

The mid-Pliocene is the most recent time in Earth’s history when mean global temperatures were 2 to 3 degrees Celsius warmer, and atmospheric carbon dioxide concentrations higher, than pre-industrial levels; and sea level was an estimated 15 to 25 metres above modern levels, the Intergovernmental Panel on Climate Change states.

Dr Hoggard said accurately determining sea level during this period can help reveal how the Antarctic ice sheet behaved in the past, and therefore how it might behave in the future.

To determine the historic sea level, researchers first looked at the geological record of Australia to find fossilised corals and other sea-level markers that indicate how high the shoreline used to be.

“This is not a perfect method, as fossil markers are not only affected by the movement of the sea, but also the movement of the land,” Dr Hoggard said.

Over millions of years, Earth’s tectonic plates move up and down in a process called dynamic topography.

“If you stand on the shoreline of Australia today and see that our sea level is rising, it could be one of two things,” Dr Hoggard said. “It could be sea level genuinely rising, or it could be the land you’re standing on subsiding.

“For the first time, we have corrected for these up and down movements across a whole continent, so we can see where the sea level markers really sit.”

Previous estimates had sea level during the Mid-Pliocene somewhere between six and 60 metres above current sea level in Australia. Now, it can be more accurately pinned at 16 metres; the Antarctic ice sheet likely contributes 9.8 metres in height.

Dr Hoggard credited the accuracy of these predictions to significant advances in science over the past 10 years.

“Thanks to better models, improvements in computational power, and a greater understanding of the geological processes, our ability to map the movement of tectonic plates over the mantle has been revolutionised,” he said.

“Right now, this is probably the best reconstruction we’ve got.”

Reducing this uncertainty will allow for more accurate modelling of future sea level rise.

While a lower estimated contribution by the Antarctic ice sheet is good news, the researchers point out there is still plenty of work to be done. Models must be refined to address the potential impacts.

“If you live in a Pacific Island nation like Tuvalu where the highest point of elevation is only 4.6 metres, small changes in the baseline sea level can have devastating impacts when disaster events like cyclones or storm surges hit,” Dr Hoggard said.

“Ensuring we have more accurate models can help improve policy, especially when looking at coastal and low-lying communities which can be impacted by just centimetres of sea level change.”