Jamie Morton

Jamie Morton is science reporter at the NZ Herald.

Smart science: Ice free past points to daunting future

This week, science reporter Jamie Morton explores a range of homegrown research projects. Today's feature looks at the history of the Antarctic ice sheet and its implications for global warming.

A key question has been how much warming is required to melt the West Antarctic Ice Sheet. Photo / Supplied
A key question has been how much warming is required to melt the West Antarctic Ice Sheet. Photo / Supplied

Herald series: Smart science Kiwis behind the microscope

Researching the history of Antarctica has given scientists like Victoria University's Dr Rob McKay an alarming window into the future.

It also gives a warning about some potential future surprises a warmer climate may have in store for us.Dr Rob McKay We know Antarctica today as a frozen wilderness hardly habitable for humans.

Yet 49 million years ago, we could have lived along the continent's eastern coast quite comfortably, basking in a typical North Island climate with a mean annual temperature sitting at around 14C.

Scientists regard this as the warmest point or "hothouse" of the Eocene epoch that lasted from 56 to 34 million years ago.

There was little to no ice on Earth within the hothouse, when New Zealand lay approximately 1000km closer to the South Pole than it is now.

Australia was even closer, the seaway that separated Antarctica from Tasmania stretching just 400km in width.

This effectively meant there was no Southern Ocean and the ocean currents in this part of the world were vastly different from today.

When combined with high concentrations of greenhouse gases in the atmosphere, these ocean currents created such a warm environment that New Zealand and the polar regions would have experienced temperatures similar to what we see in the subtropics today.

It was when continental drift eventually pushed Australia away from Antarctica that a gateway opened up, creating the Southern Ocean and allowing a single cool water current to circulate around the continent. This ultimately plunged the planet into an ice age - and scientists suspect there is a natural mechanism that saw an initial rapid cooling, driven by plate tectonic change, plucking vast amounts of CO2 from the atmosphere.

Researching this era has given scientists like Victoria University's Dr Rob McKay an alarming window into the future, provided climate change continues to warm the world.

Today, atmospheric CO2 has just reached 400 parts per million due to human emissions - the highest level seen on Earth in the past 3 million years - and they are continuing to rise at a comparably rapid 2 ppm per year.

During the Eocene hothouse, CO2 levels reached more than 1000 parts per million - an endgame scenario if human emissions go on unabated.

"While this will not give us an absolute like-for-like comparison for what may happen in the future, as the natural system may adjust slower compared to the very rapid input of CO2 we are putting into the atmosphere today, it does give an indication of what aspects of the climate or cryosphere are most sensitive to a warmer climate," Dr McKay said.

"It also gives a warning about some potential future surprises a warmer climate may have in store for us - one example could be the release of methane as Arctic permafrost thaws, which would act to rapidly amplify warming.

"One of our tasks in the geological community is to try to identify such events in the geological past and see how the Earth as a whole reacted."

Dr McKay, based at Victoria's Antarctic Research Centre, said a key question had been how much warming was required to melt the West Antarctic Ice Sheet.

Marine sediments collected from underneath a floating part of the sheet eventually revealed that in Pliocene climates, three million to five million years ago, the sheet did in fact melt.

He said that if parts of the sheet directly connected to the ocean were to melt, the sea level would rise by 3 metres.

The Greenland Ice Sheet would add a further 7 metres if it was also to completely melt.

Determining the potential rate of future melting by ice sheet modelling was proving difficult, but scientists knew from geological records of melting ice sheets that at the end of the last ice age, 20,000 years ago, the rate of sea level rise due to melting ice sheets averaged 1 metre per century. And it wasn't just Antarctica that offered insights into the past melting of ice sheets.

One of the world's best research sites outside the continent happened to be the Whanganui region, where cycles of beach sands and ocean muds are preserved in rock outcrops.

"These sediment cycles relate to the sea going up and down as ice sheets grew and melted due to natural ice age cycles," Dr McKay explained.

They demonstrated that sea levels during the Pliocene period were about 20m higher than today, suggesting the Greenland, West Antarctic and parts of the East Antarctic ice sheets melted when we were in similar climate conditions as today.

But it could take a few thousand years to reach this state, he said, as most of the East Antarctic Ice Sheet appeared to be relatively stable at the moment.


The series

Monday: The flu and us: The Shivers Project
Tuesday: What lies beneath: Mapping our underground
Yesterday: Secrets of the ice part one - life in Antarctica
Today: Secrets of the ice part two: unlocking Antarctica's past
Tomorrow: Our drone future: Miniature air vehicles

- NZ Herald

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