Scientist warns of more extreme flood events

There would be more flood events if ice shelves melted and sea levels rose as a consequence. Photo / NZPA

Climate change could be putting the planet on a path to an era not seen for 3 million years, a New Zealand scientist has warned.

Professor Tim Naish, director of Victoria University's Antarctic Research Centre, said sea levels in 2100 could be alarmingly higher than today if carbon emissions continue at their present rate.

Today, atmospheric CO2 has just reached 400 parts per million due to human emission, and the last time the planet experienced such levels was 3 million to 5 million years ago, during the Pliocene era, when the climate was 3°C warmer.

At this point in time, both the West Antarctic Ice Sheet and parts of the East Antarctic and Greenland ice sheets had melted and sea levels were at least 10m higher.

"Some are arguing that we are already committing our atmosphere and our climate to a time that we haven't seen for three million years, which saw the loss of those ice sheets," said Professor Naish, speaking in Hobart at the first joint conference between Antarctica New Zealand and the Australian Antarctic Division.

The last time Earth was 1°C-2°C warmer was as recently as 125,000 years ago, and the global mean sea-level was 6m to 10m higher.

"If you maintain carbon dioxide levels of 400ppm and commit the planet to an atmosphere of that composition for the next 100 years, then you stand a chance of losing both the Greenland Ice Sheet and the West Antarctic Ice Sheet, having sea level potentially 10m (higher) than it is today," he said.

To avoid that scenario, the world would need to meet the United Nations target of stabilising temperatures at about 2°C above pre-industrial levels.

"At the moment, that is going to be very difficult to achieve," he said.

"To achieve it we could burn fossil fuels for another decade or two, and then we would have to reduce carbon emissions to zero by 2100 - that's a very steep emissions pathway, and a very, very difficult challenge."

The consequences of projected melting of the ice sheets, raising sea levels, would be seen in more frequent flood events, he said.

The long-term average rate of sea level rise, calculated over thousands of years, during natural transitions to warm periods was 1m a century.

But geological records suggested rates of up to 4m a century was possible if ice sheet stabilities came into play, and scientists believed that happened around the 400ppm mark.

The most recent assessment report by the UN's Intergovernmental Panel on Climate Change (IPCC), in 2007, estimated sea levels could rise up to 60cm by 2100 - but observations had found previous IPCC projections too conservative.

More recent projections have put the figure closer to 1m, but Professor Naish said even an increase of half a metre could see weather events presently described as annual occurring daily, while "100-year events" could take place several times a year.

The rule of thumb, he said, was a 10cm rise tripled the frequency of flooding events.

"Often people say that with climate change, sea level rise is one of those things that doesn't sound like very much and it plays out over a such a long time, that should we really be worried about it?

"The absolute magnitude might not seem much, but the fact is when you have a storm, and when you have a king tide, this happens upon a much higher sea level - and it has a big effect on coastal flooding."

Efforts to better understand climate change have seen a surge in Antarctic research, including a range of projects just given the green light by the newly-established New Zealand Antarctic Research Institute.

History lessons

• The last time atmospheric CO2 levels were at 400 parts per million, as they are now, was three to five million years ago when the world was 3°C warmer, the West Antarctic Ice Sheet had melted, as had parts of the Greenland and East Antarctic ice sheet, and sea level was at least 10m higher.

• The West Antarctic Ice Sheet is sensitive to warming in the Southern Ocean, and if CO2 stays above 400ppm, we could be committed to the loss of that ice sheet and the Greenland Ice Sheet.

• The last time the planet was one to two degrees warmer, around 125,000 years ago, global mean sea level was between 6m and 10m higher.

• The rate of present sea level rise is unusually high in the context of the last few thousand years, but not in the context of the natural glacial to interglacial world. The Earth's response to the present and projected global warming has started our climate on a path not seen for around three million years.