Twelve million years of weather

By Catherine Masters

Scientists are gathering in Tallahassee, Florida, next week to chew over some dramatic climate findings.

What they have discovered below the ice in West Antarctica, they say, will provide important new clues about where the world's weather may be heading.

While they are not revealing everything before their workshop begins, they have already seen evidence that at times in the not-so-distant past there was no ice at all where they recently drilled through the Ross Ice Shelf.

This shelf is 85m thick and the largest floating ice shelf on the planet. The scientists drilled through it and into the seabed.

What they have pulled up is a "remarkable" and "unprecedented" record of earthly climate change stretching back 12 million years. At 1284m the core is deeper than ever drilled before on the Antarctic margin.

What surprises and intrigues them, says Dr Tim Naish, a New Zealander and one of the project's chief scientists, is the appearance of sudden change from glacial to ocean conditions.

This tells the scientists there have been times of rapid ice retreat - or even total collapse to leave open ocean. The discovery is a warning of what may be to come as the world warms up.

"When up to a kilometre-thick ice sheet is removed from southern Ross Sea, then substantial thinning of ice must also have occurred on Antarctica itself," says Naish, from the Institute of Geological and Nuclear Sciences and Victoria University's Antarctic Research Centre.

"These observations from warm times in the past will help us understand the magnitude of change that may occur as a consequence of global warming."

It is early days in the analysis of the core and Naish, speaking from America, was not about to make predictions. But as the world warms due to man-made CO2 emissions and the ice melts, the evidence of the core indicates sea-level rise could be much higher than so far predicted.

The Intergovernmental Panel on Climate Change [IPCC], a body of the world's scientists, recently warned of up to half a metre of sea level rise by the end of the century.

But the panel did not take into account the accelerated rate of thinning anticipated for the Antarctic and Greenland ice sheets over the next century. If this had been included in calculations, Naish believes predictions may have been of up to two metres of sea level rise in the next two centuries.

The core was extracted during the Antarctic summer by the Antarctic Geological Drilling (Andrill) Program, of which Naish is a co-founder. The technical achievement in drilling so deep is remarkable, he says.

"What's quite exciting is that there are periods of time in that core where Earth's climate was warmer than it is today and the sort of climate that we're heading towards in the next 100, 200 years."

Twelve million years ago, at the bottom of the core, it was cold, says Naish. The bottom looks pretty similar to the top, which represents a million or so years.

The middle of the core, however, is much more dynamic and variable. Scientists can see periods where the ice sheet and ice shelf disappeared, around three to four million years ago.

"That's the bit we're excited about because that is, if you like, the window into our climatic future."

The scientists make their findings based on what they see in core sediments. They can tell when the ice wasn't there for extended periods because they see biological sediments made up of algae called diatoms.

Their presence indicates a very productive, warm ocean. When ice sheets were present, scientists see stony sediment similar to what you would see at the outbed of a glacier.

From their findings scientists will be able to reconstruct past temperature and vegetation, and by dating the rocks they will be able to tell how fast changes took place in the past.

Once the environment is reconstructed scientists can computer model what would happen if temperatures rise two to three degrees.

"We run the computer model into the future and it might collapse the ice shelf, it might retreat the ice sheet, it might rise sea level and we can say this is realistic. Then we can go back and say, 'yes, that's what we saw, for example, three million years ago in our geological record, we saw those sorts of conditions so we think our computer model is working properly."'

IF the findings are extraordinary to non-scientists, Naish says they are also amazing to the scientists involved.

"Because we've never drilled rocks of that age before this is the only record of that warmer climate around the Antarctic that we have.

"We were just blown away to see these dramatic changes in the sediments essentially telling us the ice shelf had retreated and probably a substantial part of the West Antarctic ice sheet had also retreated."

It cannot be determined from the core just how high sea level rose in the warmer times, but other scientific records indicate anything from five to 20m higher than the present day. Naish does not believe sea level would have been 20m higher "but you know, if West Antarctica melted it's got five metres in it - and if Greenland melted it's got another five metres in it. That's 10m."

He is not predicting 10m of sea level rise. But he repeats that the IPCC is inherently conservative - something the climate change sceptics might find hard to believe.

'But it's true, they're a very conservative group and the likely scenarios we're looking at are likely to be worse than the IPCC are predicting.

"We're seeing evidence of dramatic changes during times that might be similar to where we're heading."

The warmer weather in the area is indicated during the earth's Pliocene period, a recognised time of global warmth. There have have been big questions about how the ice sheets behaved during that time.

"Now I think we have the record to really address those questions."

Without the ice, shorelines could have been several metres higher than they are now and there would have been significant changes in ocean circulation, a key mechanism for transporting heat around the latitudes.

Naish points out a significant proportion of the world's population already live close to the shoreline so any rise is significant for low-lying countries.

"Couple that with more severe storms and hurricanes such as hit Louisiana. You're just rising the datum on which many natural processes are already happening, so a few metres is very serious for tens of millions of people on the planet."

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