Antarctic science: Sea ice and climate change

Blocks of ice removed as part of melting a hole through the sea ice form an arty sculptured against one of the last Antarctic sunsets of the year. Photo / NIWA

This summer, the Herald will be travelling to Antarctica to meet Kiwi scientists working at the front line of Antarctic and climate science. As part of a lead-up series of interviews, Jamie Morton talks to NIWA’s Dr Craig Stevens, who has been part of a team camping at McMurdo Sound to investigate why Antarctic sea ice is not shrinking at the same rate as Arctic sea ice.

Firstly, can you describe your research programme on the ice, how this project has been developed, and how many scientists are involved?

We have a team of seven down here for this sampling but there are several colleagues based back in New Zealand who are also part of the research group.

I like to think of it as a continuum whereby we allow ideas to evolve as we work between field observation and computer modelling to see how our understanding needs to improve.

Our work looks at some very detailed aspects of a very large-scale problem.

We are measuring the properties - like temperature, salinity, turbulent energy, currents - in small volumes of seawater to try and improve models that work at the global scale.

This is because a lot of the action happens at the interface between ice and ocean.

The seasonal growth and decay in sea ice coverage is arguably the largest natural annual geophysical change on the planet, with the shrinking Arctic ice being one of the clearest manifestations of climate change.

This tripod holds an oceanographic mooring with 16 instruments attached to it at various depths in the ocean below with the lowest being about 375m deep. Photo / NIWA

But why is this shrinking Arctic ice not paralleled in the Antarctic?

We are researching the possibility that the answer lies in the production of supercooled seawater beneath the giant Antarctic ice shelves.

Because it is so cold, the ice shelf water influences sea ice growth far beyond the shelves that make up 40 per cent of Antarctica's coastline.

The main thrust of your work down there concerns "supercool" seawater, which helps drive sea ice growth. In layman's terms, can you elaborate: what exactly is "supercool" seawater and what large-scale part does it play in the Antarctic environment?

Supercool seawater is formed beneath ice shelves at depth when "warm" sea water melts the ice underside.

This melted water is very cold and a little fresher than normal seawater.

As a consequence it "drains upwards" until it reaches the edge of the shelf.

As it rises the reduction in pressure means that the freezing temperature rises and so it finds itself colder than the temperature at which it should form ice. A consequence of this is it forms ice crystals - platelet ice - that coats the underside of the sea ice and shallower parts of the ice shelves.

What are you hoping to discover or confirm, and where, and how physically, are you conducting your fieldwork on the ice?

Specifically we are trying to identify how rough the super-cool influenced underside of the sea ice is.

This is an important number to be able to put into models to get a better handle on heat transfer.

We are also developing techniques to improve temperature and salinity measurements of seawater that is colder than its freezing point.

NIWA's Dr Craig Stevens at the starting phase of melting a hole through the sea ice, to undertake oceanographic measurements. Photo / NIWA

I am primarily a field observationalist.

There is a lot of emphasis placed on predictive models, as you would expect.

However, these models are simply collections of our present understanding and are always challenged by the real world so there is much to do behind the scenes to enhance various aspects.

So yes, we base ourselves physically on the sea ice for several weeks and conduct a sequence of precision measurements.

Can you tell us a little about the environment and conditions you've been operating in? How challenging has it been and what have you been up against down there?

Conditions in Antarctica are pretty much always challenging.

However, collectively our team has been working here for many years - decades in some cases.

This, plus the support of Antarctica New Zealand, means we are pretty much as effective as we can be within the constraints of weather.

Once our field camp is in place we are impervious to weather as we are container based rather than tent based.

For scientists, the growth of sea ice extent around Antarctica has remained somewhat of a challenge to understand. Why is this? And how is it that extent has been growing as the rest of the planet's climate and sea surface warms?

Um, it's hard to measure!

It's hard to get here, it's hard to measure thickness of sea ice.

Possibilities for the modest growth - in area - around Antarctica relate to changed winds and the paradox that melting ice shelves generate very cold surface plumes that then enhance sea ice growth.

Changes in sea ice growth can also contribute to climate models. How might patterns observed here be relevant to the rest of us back here and around the world?

Sea ice growth is a vital part of any climate model.

It's is clear that our present models don't do so well at predicting present sea ice extent and so forward projections are questionable.

We are looking at ways of improving these models. Sea ice area is only one part of the puzzle.

Thickness is difficult to measure from satellite. In part because of exactly the thing we are looking at.

Ice crystals on the underside make for a complex material so that estimating thickness and volume from freeboard - the amount floating above water - is tough.

The data are highly relevant to coastal populations around the world.

Computer predictions suggest we could see many metres of sea level rise over the next few centuries - entire cities will need to shift, entire economies will be diverted to these moves.

Planning is essential if as a set of societies we wish to minimise impact.

These predictions are based on still incomplete understanding about what the important factors are.

We are researching aspects associated with how melting ice shelves affect sea ice patterns.

Following your fieldwork, what's the next step?

Lots of analysis of data, thinking about what it means, giving science and public talks, writing papers and getting them reviewed and published.

The work is part of a continuum as we have complementary experiments planned for next year.

Having developed techniques and understanding we need to apply this in different regimes.

So we are part of a large ice shelf experiment that will be drilling through the ice shelf next season and we have ocean sensors monitoring the temperatures and currents several hundred kilometres to the north.