Global warming and ocean currents may result in Southland's sea temperature in a 100 years time being as warm as water in the Marlborough Sounds is now.
Research by a group of New Zealand scientists suggests subtropical ocean currents will be swept further south and warm up sea temperatures off the eastern South Island by as much as 2degC in the next century.
As atmospheric carbon dioxide levels climb and winds intensify, the scientists say the East Australian Current, which moves down Australia's east coast and across to New Zealand, will become stronger and drive more heat southward.
The scientists from GNS Science, NIWA, Victoria University and Auckland University, base their findings on the study of microscopic marine plankton fossils called foraminifera, which were found in seabed sediment collected from the Tasman Sea and east of New Zealand.
Analysis of the foraminifera enabled the scientists to reconstruct the last major global warm period about 125,000 years ago, when ocean temperatures were about 2degC warmer than today.
"The ocean warming has already started, with temperatures off Tasmania having risen by 1.5degC in the past 70 years, which is more than twice the global average rate," said Giuseppe Cortese of GNS Science.
The warming off Tasmania had been accompanied by an invasion of sub-tropical marine life, which had replaced subantarctic species.
Dr Cortese said as temperatures increased off Australia's east coast, ocean circulation patterns changed - weakening heat transfer towards the North Island and strengthening it towards the South Island.
This would mean sea temperatures off Southland would become more like today's sea temperatures in the Marlborough Sounds in 100 years' time.
Dr Cortese said scientists were now turning their attention to how the changes would affect the marine food chain and ocean currents.
"From what we are seeing off the coast of Australia, both in recent observed trends and in our reconstruction from the past, such temperature changes are likely to have impacts on the whole marine ecosystem, and will ultimately impact on commercial fish stocks."