Sea ice extended further north in the Southern Ocean during the last Ice Age than previously thought, a New Zealand research team has found in a study that could improve predictions of climate change.

The team from the University of Otago, led by PhD student Ceridwen Fraser, delved deep into the genetic code of modern-day bull kelp from samples taken from many sub-Antarctic islands, as well as New Zealand and Chile.

The findings showed that southern bull kelp, Durvillaea antarctica, had only recolonised the sub-Antarctic islands in the past 20,000 years after the retreat of sea ice.

The kelp live in the shallow inter-tidal zone and were destroyed by the scouring motion of sea ice across the sea bed.

"We found this pattern that there is a lot of genetic diversity further north and next to no diversity further south, which suggests that it's just recently been colonised by the species," Mr Fraser told Reuters from Dunedin in New Zealand yesterday. The findings challenge current data of the estimated extent of sea ice based on sediment core samples from the Southern Ocean seabed. In some areas, there is abundant data, in others very little because of the remoteness of the vast ocean.

None of the sediment data suggests sea ice extended as far north as the sub-Antarctic islands between about 50 and 55 degrees latitude such as Macquarie island, south of New Zealand, or South Georgia, in the far south Atlantic.

"If the sea ice was more extensive than previous studies suggested then that's going to call for a re-assessment of other things we understand about climate change and how all these systems interact," Mr Fraser said, referring to ocean and wind circulation patterns and the amount of solar radiation sea ice reflects back out to space.

"A very small change in the extent of sea ice, a couple of degrees latitude, is going to make a huge difference in the amount of radiation that is reflected and ocean circulation patterns," she added.

"This helps us understand better what sort of patterns we might expect in the future and also the rate of climate change," she said of the findings, which suggested temperatures were a little cooler than some previous studies had reported. Research supervisor Jonathan Waters said it was the first time scientists had looked at modern-day genetic data to study past climate in the southern hemisphere. The aim was to expand the concept to study other species in the sub-Antarctic zone.

"It's important to understand historic climate and how inaccurately estimating it can give us false impressions of what can happen in the future," he said.