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That sinking feeling

R&D Now

Scientists fear that polar ice sheets are on the brink of collapse from the effects of human-induced global warming. Which one of the planet’s three major ice sheets will be the first to go?

Image credit: Tanya Patrick
Image credit: Tanya Patrick

Now isn’t the first time the Earth has experienced elevated atmospheric CO2 and rising global temperatures. Around three million years ago, during the mid-Pliocene, global warming was the result of volcanic activity. Then, during the last interglacial period between 120 and 130 thousand years ago, global temperatures again rose to about one or two degrees Celsius higher than today’s, which led to a reduction in the volume of the planet’s ice sheets and higher sea levels.

So, what do these ancient paleo events mean for life on Earth, particularly the world’s coastal populations?

Marine geoscientist Dr Mick O’Leary at Curtin’s Department of Environment and Agriculture has some critical answers. For the past five years he has worked on the major project ‘Pliomax’, an international research collaboration of US, Australian and German research organisations, led and funded by the US National Science Foundation. The research aim was singular: to estimate accurately the mid-Pliocene’s peak sea level.

O’Leary’s study of Western Australia’s ancient fossil shorelines revealed that WA’s coastline has been minimally affected by tectonic movements, which can also be responsible for sea level rises, and enabled the Pliomax project to use WA’s geomorphology to investigate the sea-level rise that occurred during the mid-Pliocene.

“There’s been a global temperature rise of one degree in the past 100 years, and atmospheric CO2 has now reached the same level as during the mid-Pliocene warm period,” O’Leary explains.

“The mid-Pliocene therefore offers a natural analogue for a warmer, higher CO2 world.

“In particular, knowing the sea levels and volume of polar ice during the mid-Pliocene and last interglacial periods gives us a better idea of how vulnerable the Greenland and two Antarctic ice sheets are to current global warming.”

Ocean, ice and gravity

As global temperatures rise, so too does the fear that within the next century, either the Greenland or one of the two Antarctic ice sheets will reach breaking point. Which ice sheet melts first, and when, will determine the pattern and scale of sea level rise across the planet. The reason for this, O’Leary explains, is gravity.

“The three ice sheets exert a powerful gravitational attraction, pulling ocean water towards them. The larger the ice sheet, the larger the gravitational pull, and vice versa.

“So, if an ice sheet diminishes due to melting, water will migrate away from the ice sheet, due to its own reduced gravitational effect and the increased effect of the other two ice sheets.”

He describes this as the ‘climate change paradox’. A melting ice sheet could actually make the sea level fall along coastlines located near the ice sheet, while the regions furthest from the ice sheet will experience sea level rise greater than the global average.

These impacts have been further characterised by recent fluid dynamics simulations of the collapse of the three polar ice sheets, which showed that each would produce a distinct pattern of sea level change.

“If the East Antarctic ice sheet melted, the sea level across much of Australia, for example, would be lower than the global average, whereas Pacific Island communities would experience a rise of up to 1.5 metres,” O’Leary says.

“But if the West Antarctic ice sheet was to melt, Australia would experience a sea level rise of more than one metre; however the rise in Broome would be 20 centimetres greater than that in Sydney, which is closer to the ice sheet.

“If the Greenland ice sheet collapsed, Australians would see a rise similar to the global average, while people living in Northern Scandinavia and Scotland would actually experience a fall in their sea levels.”

Together, the two Antarctic ice sheets are about twice the size of Australia, and hold about 90 per cent of the Earth’s frozen water. If these melted, the Earth’s average sea level would rise by 60 metres, drowning civilisations around the world. In Sydney, only the tips of the Opera House sails would be visible above the waves.

It’s therefore a major climate change misconception that all coastlines will experience a sea level rise of between 60 centimetres and a metre this century. These predictions, O’Leary cautions, are based on modelling that calculates the volume of ice melt divided over the area of the oceans – it’s a globally averaged measurement. The real behaviour of the ocean surface in response to an ice sheet melt will be far more complex.

Furthermore, while satellite data over the past 20 years suggests an average sea level rise of three millimetres per year, there are significant local and regional variations – including hotspots in the western Pacific and over northern Australia that are experiencing rises three times the global average. This may be due to current patterns, thermal expansion or atmospheric effects. The extreme La Niña event in 2011–12, for example, delivered a massive pool of warm water, driven by enhanced trade-winds flow, and higher than average sea levels to the north and west of Australia.

In addition, the gradual melting of individual ice sheets and glaciers over time causes changes in the Earth’s gravitational field and rotational state. Shoreline positions may also be altered by tectonic movements, and by the expansion of land previously under the weight of glacial ice.

“These phenomena can all contribute to geographic variation in sea level change. So there’s massive uncertainty in current climate and ice sheet models,” says O’Leary.

To isolate the effect of ice-sheet melts on sea-level rise, he’s now following up his work on Pliomax and studying Australian coastal sediments and morphology, for a project led by Macquarie University and funded by the Australian Research Council.

“We’re taking a novel approach by using the coastal imprint combined with simulations of ice sheet collapses to identify which ice sheets contributed to past sea level rises.

“Basically, we’ll be able to ‘fingerprint’ the meltwater sources for sea-level rise during the last interglacial period using inverse modelling of the distinct geometry of Australia’s sea-level record.”

The research outcomes will help answer three major scientific questions: which polar ice sheets are the most vulnerable to warming, how fast would sea levels rise, and what are the likely impacts on the world’s coastlines during the 21st century?

Polar ice and tropical life

More than 30 years ago the United Nations was already focused on modern-day climate change.

In 1988, the UN Environment Programme and the World Meteorological Organization established the Intergovernmental Panel on Climate Change (IPCC). In its fifth assessment report in 2013 the IPCC assumed a ‘steady as she goes’ approach to emissions.

The scientific community, however, is not confident that emissions will moderate, let alone reduce. It’s taken only one century for the global sea level to rise as much as 20 centimetres – about the same amount as the preceding 2,000 years, which was due mainly to glacier melts and thermal expansion of the oceans. There’s a greater than 90 per cent probability this century will see a global sea level rise of between 0.8 and one metre.

While many island nations are dreading the higher seas that will erode their coastlines, swamp communities and wash away crops and livelihoods, there’s a wider perception that there’s time to reverse the trend and that surely ‘science will save us’. However, already there’s been a global temperature rise of one degree, and, as O’Leary says, even a modest rise of two degrees could result in a significant rise in global sea levels.

It’s no surprise, then, that the United Nations Refugee Agency predicts that events linked to climate change will become the biggest drivers of community displacement, and is anticipating a humanitarian crisis of ‘climate-change refugees’.

Unfortunately, current generalisations about rising sea-level are disadvantaging many communities that rely on scientific reports to guide coastal planning and mitigation strategies. But any way you look at it, a potential ice-sheet collapse is a critical concern for Pacific island nations. As O’Leary emphasises, “The further you are to a melting ice sheet, the higher the sea will be rising around you”.

“Many of these distal, tropical communities have high-density populations that could face environmental and economic devastation. Whole cultures would be swallowed by the sea.”

We can only hope that governments worldwide have their ears to the Earth ready for the answer to one of the century’s biggest environmental questions: just how vulnerable are the planet’s ice-sheets to global warming?

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