Ready or not

Global sea levels are rising, but how high remains unknown.

Written by      

On 23 January, 2011, the Antipodean moon was only three days past full, and had just swung by at the nearest point of its monthly trajectory. For most, it simply meant a picturesque few nights, until the Waitemata Harbour inundated the Auckland waterfront. King tides happen several times a year, but this one, bullied by a low-pressure system over Northland, took on a special energy. Every hectopascal below average air pressure can raise the sea level by a centimetre, and as the low bore down on Auckland, it shunted ahead of it a tide half a metre higher than predicted.

Around noon, a 4.13-metre storm tide washed over parts of downtown, some eastern suburbs, and Herald Island, near Hobsonville. Then it swamped eastbound lanes on the North-western Motorway. Waves surged over seawalls and beach boulevards, flooding shops, homes and roads. Stormwater systems, choked with sea water, backed up. You have to go back to March 1936 to find the previous highest storm tide—3.99 metres. Some people said climate change had arrived; others simply blamed a freakish meteo-astronomical conspiracy, but the inundation was abetted by one cold, hard fact: Auckland sea levels have risen by 17 centimetres since 1900.

Late last year, Parliamentary Commissioner for the Environment Jan Wright released a report on sea-level rise around New Zealand. In it, she warned that a mean increase of 30 centimetres by 2050 was “now inevitable”. The report was based largely on data and projections worked by the Intergovernmental Panel on Climate Change (IPCC), which were themselves based on prevailing trends as we understood them as at 2013. By 2100, found the IPCC’s Fifth Assessment Report, global sea levels will have risen by around a metre. If that doesn’t sound like much to you, consider that more than 200 million people are currently living in that doomed metre. But in March came news that put even that frightening figure up for revision: the Totten Glacier, which drains more than 570,000 square kilometres of the East Antarctic Ice Sheet, is melting.

Glaciologists have suspected this for some time, but had hitherto been unable to show the mechanism. It was already well known that the neighbouring West Antarctic Ice Sheet had been wasting for decades—since 1992, it has lost an estimated 65 million tonnes of ice each year—and twin studies published late last year found that melting had accelerated in the previous 10 years. Between 1992 and 2013, meltwater from the West Antarctic has added about 4.5 millimetres to sea levels across the planet. Seventy per cent of that gain came in the second half of that decade.

Seas have been rising very gradually for two millennia, according to data extracted from sediment cores, and more rapidly from the late 19th century, measured against tide gauges at ports around the world. As the Earth’s seas warm under the influence of climate change, it will power further rises. The United States climate agency National Oceanic and Atmospheric Administration posits four scenarios for the future (dotted line), depending on the response of the Earth’s great ice sheets.

This colossal thaw is the work of a self-perpetuating geophysical loop: glaciers in both the East and West Antarctic feed ice shelves that slope gently into the sea. As the ocean warms, they can rapidly thaw. With that bulwark gone, the remaining ice sheet behind then flows more quickly into the sea, and so on. As the ice sheets retreat, they allow the intrusion of ever-warming sea water beneath them (in this particular region, warm offshore sea water actually sits beneath cold layers of fresh water, because it’s more dense). Meltwater from the glacier itself penetrates further through the structure until, some researchers believe, the glacier could abruptly and dramatically collapse.

It’s important to remember here that the Antarctic Ice Sheets have been around for more than 30 million years, and melt data has been drawn from just 20 of them, so the uncertainties are many. The processes are very real, but whether they point to a permanent, accelerating spiral or simply a temporary anomaly, nobody can say for sure. But confirmation that the Totten Glacier is suffering the same fate is sobering: most of the ice sheet it drains is grounded below sea level, which means the whole region is vulnerable to sudden collapse. The East Antarctic Ice Sheet holds four-fifths of all the ice on Earth, and has survived earlier warming periods. Ironically, it has thickened slightly of late, because climate change drives more precipitation, which in that part of the globe falls as bolstering snow. But scientists don’t yet know how it will respond to collapse at the coast.

Between them, the Antarctic Ice Sheets hold enough water to raise global sea levels by more than 58 metres—West Antarctica alone would lift the high-tide mark worldwide by more than three metres, and at least one study estimates that fully one-third of that ice sheet could be gone within two centuries (though other researchers have disagreed strongly with that timeline).

Totten is believed to hold the same sea-raising potential. And here’s one of those perverse contradictions that only apocalyptic physics can produce: should Antarctica melt, it will disproportionately flood the northern hemisphere. The United States could experience sea-level rise some 25 per cent higher than the southern hemisphere average, because the sheer mass of Antarctica as it stands exerts so much gravitational pull on the oceans that it’s literally holding sea water around it. As it loses mass, that pull weakens, and more water is released back into equatorial circulation, from where it will be redistributed north.

For this and other reasons (a huge ice sheet in Greenland’s northeast, for instance, previously believed stable, is now known to be wasting), many feel that the IPCC’s projections are conservative. It’s a matter of record that sea levels rose by at least five metres above present day during the last interglacial 125,000 years ago, but we may not have to wait that long for those high levels to return—they occurred in temperatures similar to those projected for the late 21st century. The last time atmospheric carbon dioxide concentrations hit 400 parts per million, between three and five million years ago, associated warming collapsed the Greenland and West Antarctic ice sheets. Global sea levels were 10 to 20 metres higher at that time.

Rising sea levels threaten many cities around the world, and millions of dollars worth of infrastructure. New Zealand’s three main centres are all near the coast. What would Auckland’s CBD look like if, say, the West Antarctic Ice Sheet collapsed? New Zealand Geographic modelled the result of a spring high tide in the present-day city, and the projected hike of at least two metres that would result from the ice sheet collapse. Most of the waterfront reclaimed over the past two centuries of urban development would be inundated.

Sea levels rose last century by an average of 1.7 millimetres a year, but since 2000, that rate has increased to 3.3 millimetres a year, very possibly swelled by Antarctic meltwater. New research suggests the sea level may in future rise as rapidly as one metre every 25 years. That has a monumental consequence: even a modest increase—half that projected by the IPCC—is predicted to boost the frequency of coastal inundation in New Zealand by a multiplier of 1000 times. Twelve of New Zealand’s 15 largest towns and cities already sit alarmingly close to the high-tide mark (and some areas of Christchurch are 50 centimetres lower after the earthquakes), and already they’re having to deal with more coastal erosion, storm surges, king tides and salt water invasion of artesian supplies.

For the time being, most Kiwi councils have set planning guidance around the IPCC projection of one metre of rise by 2100, which alone will see much of Wellington’s waterfront inundated. But elsewhere, authorities are urging more foresight: Britain’s Department for Environment, Food and Rural Affairs has adopted an extreme scenario of 1.9 metres, while the Climate Change Research Centre at the University of New South Wales advises planners to consider two metres.

By 2050, then, Auckland can expect to suffer 2011 flooding perhaps yearly, and most New Zealand coastal towns and cities will fare the same. Just what that means for national infrastructure, beach amenities, property values, demand for higher land and household and commercial insurance, nobody has dared guess. Certainly, it will be one of the greatest physical transformations Kiwis have ever seen, though just one of the many tough adaptation challenges climate change will set us.

Of course, sea-level rise will not stop come 2100. What happens in the 22nd century depends heavily on what we do now. If we burn our remaining reserves of coal, oil and gas, we’ll release another 4.5 trillion tonnes of greenhouse gases. That would warm the planet to an estimated average 26ºC—too hot for humans in many places, and certainly too hot for water to freeze. For the first time in more than 30 million years, Earth would be ice-free.

More by