Q. Firstly, why is Antarctica so important to the issue of climate change and, broadly, what impact might it have on the planet under future scenarios?
Antarctica holds 70 per cent of the worlds freshwater as ice, and if it entirely melted global sea-level would rise sea-levels by 60m. We know that about 20m of sea-level rise is held in ice that currently sits below sea-level and is highly vulnerable to rapid melting as the Southern Ocean continues to warm.
Ninety three per cent of the heat from anthropogenic global warming has gone into the ocean and the majority is being stored in the Southern Ocean. Melting Antarctic ice is will be the major contributor to global sea-level rise by the end of the century, but a lack of understanding of exactly how fast it will melt is currently hampering accurate predictions of sea-level rise.
The latest research that uses vastly improved computer models for the Antarctic ice sheet suggests that the Antarctic contribution will be significantly larger than current estimates for 2100 in the most recent Intergovernmental Panel on Climate Change (IPCC) assessment report.
Q. When did science first start turning to this continent for answers around climate change?
A. The Antarctic continent has been preserved as a place for peace and scientific research under the Antarctic Treaty for more than 50 years, however because of its remoteness, and that the majority of our population lives in the Northern Hemisphere, its role in the climate system has been relatively poorly known.
In the mid 1970s, ship-based scientific drilling of the ocean floor around Antarctica provided the first evidence that the ice sheet had existed for 35 million years but that it had been highly variable in response to climate change.
However before this time Antarctica was much warmer and didn't have an ice sheet. Geological exploration in the 1980s in the Antarctic Peninsula found fossil evidence of plants and animals that showed that Antarctica supported temperate rain forests 50 million years ago and that temperatures in summer got up to 20?C.
Also in the 1970s, US glacialogist John Mercer published a paper in Nature that recognised the potential for the West Antarctic Ice Sheet, that largely sits below sea-level and is vulnerable to ocean warming, to collapse if carbon dioxide levels in the atmosphere kept increasing and the climate kept warming. This was controversial at the time because measurements recording increasing carbon dioxide in the atmosphere had only just begun and anthropogenic global warming was not taken very seriously.
Another major breakthrough came in the early 1990s with the first deep ice cores that provided a continuous record of atmospheric greenhouse gases frozen in air bubbles spanning the past half a million years. The records showed past natural climate cycles lasting 100,000 years each, during which the world warmed and cooled by 5-6?C as the concentration of the greenhouse gas carbon dioxide increased and decreased between 200 and 300 parts per million (ppm).
Today carbon dioxide levels are at 400 ppm, a level not seen for 3 million years during a time known as the Pliocene when the world was on average 3?C warmer.
Over the last 15 years satellites have been measuring changes in the elevation, mass and flow of the ice sheet, these observations are showing some concerning trends.
The ice sheet is thinning and retreating at its margins especially where the ocean has warmed the most.
Q. How has Antarctica responded to ancient climate change scenarios and what has brought these about?
A. Another major breakthrough was made in the early 2000s when the ANDRILL Project, led by New Zealanders, recovered a geological drill core that from the Ross Ice Shelf that showed, that when the world last had 400 ppm the Ross Sea was 5?C warmer and the West Antarctic Ice Sheet melted raising global sea-levels.
Using latest generation ice sheet models it was estimated that Antarctic melting at this time contributed about 15m to global sea-level rise. It appears this is the stability threshold for the marine based Antarctic ice sheets and some scientists have argued that we are already committed to unstoppable loss of the West Antarctic Ice sheet.
At atmospheric carbon dioxide concentrations of greater than 1000ppm, Antarctica cannot sustain an ice sheet. Geological information suggests carbon dioxide levels were last at 1000ppm 50 million years ago when Antarctic supported temperate rain forest.
Q. What large-scale effects are we observing in Antarctica today and what physical drivers, such as warmer water, is causing this?
A. Ice shelves are the floating extensions of the ice sheets, and form when the ice flows off the continent under gravity onto the ocean. They play an important role in stabilising the ice sheet. Effectively, they block up large bays - like the Ross Ice Shelf - and hold back the ice sheet from sliding into the ocean.
The ice shelves are melting all around Antarctica as the ocean and the atmosphere warms. This is especially true around the Antarctic Peninsula and in the Amundsen Sea region where many ice shelves have collapsed and disappeared in the last 10 years. Once the ice shelves melt the ice sheet flows faster and melts faster. It's like taking the cork out of bottle. And of course this land based ice raises global sea level when it slides into the ocean.
Q. What are some of the key ways we can monitor change in the Antarctic environment?
A. Over the last 15 years satellites have been measuring changes in the elevation, mass and flow of the ice sheet, and these observations are showing some concerning trends.
The ice sheet is thinning and retreating at is margins especially where the ocean has warmed the most. We know from these observations that the ice sheet is losing mass at about 120 gigatonnes a year and the rate of mass loss is accelerating with a doubling time of less than 10 years. As a consequence the Southern Ocean is freshening and this has the potential to affect global ocean circulation. The highest rates of melting and retreat of the ice sheet margin are occurring in regions where the ocean is warming the most.
Q. What are the big uncertainties in terms of modelling and projecting future melt and sea level rise from Antarctica?
A. Accurately modelling the dynamic processes that cause ice shelves to collapse and marine ice sheets to retreat is the key to producing better projections of future melt. This involves getting a better understanding of the processes of ocean warming and circulation as well as the shape of the bedrock under the ice sheet. Much progress has been made in both areas. We know that once the ice shelves melt then the ice sheet itself retreats, and once it starts retreating into a deep bedrock basin, which much of west Antarctica sits on, then it can retreat quickly and in an unstoppable manner. Computerised ice sheet models are now incorporating these processes collectively known as "marine ice sheet instability" and as result are reducing the uncertainties around future sea-level projections.
The latest ice sheet models suggest the 2°C of global warming, the UN Paris target is the threshold for loss of Antarctica's ice shelves. In other words if we don't stabilise global warming below 2°C then we are likely to see many metres of sea-level rise over the coming centuries. The latest results show that Antarctica may contribute an addition 50cm above 1m predicted for the warmest IPCC scenario by 2100.
Q. What are the general similarities and differences between Antarctica and the Arctic, in terms of the impact climate change is having?
A. Climate change is having a much stronger affect on surface temperature in the Arctic than the Antarctic. Some parts of the Arctic have warmed more than 4 times the global average of 0.8°C since 1850. This is because the Arctic is a shallow ocean surrounded by continental land and the Antarctic is a cold ice covered continent surrounded by a deep ocean. The feedbacks are much more powerful in the Arctic especially the sea-ice feedback, whereby the heat uptake of the Arctic Ocean is melting the sea-ice which is then replaced by more dark ocean which absorbs more heat. This amplifying process is called the ice albedo feedback. Arctic sea-ice has retreated significantly in the last 10 years and it is estimated that the Arctic Ocean will be seasonally ice free by 2050 or 2100 depending on how we deal with future carbon dioxide emissions.
The Antarctic on the other hand has a significantly lower surface temperature and, while some areas have warmed dramatically, such as the Antarctic Peninsula, the continent as a whole has not experienced amplified warming so far. This is because the Southern Ocean is a net heat sink. It is deep and well mixed and has taken a large amount of the heat from global warming. Consequently the sea-ice apron that forms seasonally on the ocean around Antarctica has not got smaller, in fact it has increased in area. This is in part to do with the ozone hole which has an affect on the position of the low pressure systems causing sea ice to grow in the Ross Sea but retreat in the Weddell Sea. So there is also a complicated pattern of cooling and warming. Models predict that as the ozone hole decreases over the coming decades and carbon dioxide increases, the Antarctic will experience amplified warming like the Arctic.
Finally, even although the surface warming is subdued, changes in atmospheric circulation around Antarctica are causing surface waters to warm, as to 2°C warmer deep waters held in the bottom of the Southern Ocean upwell onto the continental shelf around Antarctica where they are melting the ice shelves and the ice sheets.
Q. Finally, why should New Zealanders care about what's happening in Antarctica?
A. Because we are not very far away and our climate is intimately connected to changes that occur in Antarctica and the Southern Ocean, such as sea-level rise and changes in rainfall patterns and temperature.
These changes are already impacting our environment, communities and economy.
• Professor Tim Naish will be joining a panel on the Paris climate talks organised by the New Zealand Institute of International Affairs (NZIIA) and to be held at Victoria University on Friday morning. Other panelists include Maty Nikkhou O'Brien and Sir Douglas Kidd of the NZIIA, Victoria University climate scientist Dr Jim Renwick, Motu Economic and Public Policy Research fellow Catherine Leining, Generation Zero researcher and campaigner Paul Young and Herald economics columnist Brian Fallow.
People wishing to attend the panel discussion, running from 8am to 9am at Lecture Theatre 1, in the Old Government Buildings at Victoria's Pipitea Campus in Lambton Quay, should register at nziiaclimate.eventbrite.co.nz.