18 April 2002

The oceans around New Zealand are warming at a rate not seen since the 1930s.

Figures from NIWA show that the ocean temperature around New Zealand has increased rapidly over the last 10 years.

This is in contrast to the preceding 20 years when the temperature actually decreased.

According to a report just released by NIWA, New Zealand’s oceans began to cool slightly in the 1970s and 1980s, while the average global temperature steadily increased. Over the last century the average ocean temperature around New Zealand increased by about 1.0 °C, a somewhat greater increase than the global average (0.6 °C).

NIWA scientist Dr Craig Stevens said the lower ocean temperatures around New Zealand during the early 1990s were caused by stronger and more frequent El Nino events. El Nino and La Nina events can cause the temperature of the tropical ocean to fluctuate by 3 to 4°C, and the temperatures around New Zealand fluctuate by a smaller amount in the opposite way.

“We are seeing sharp rises in temperature punctuated by periods of cooling when El Nino prevails.”

Dr Stevens said the results show that the temperatures of New Zealand’s oceans do not always follow the global average, and it was important that we continue collecting local information.

“It’s crucial that we kept track of long-term changes in ocean temperature.”

“Although the overall changes in New Zealand’s ocean temperatures in the past 100 years have been similar to the global average, they have occurred at different times and rates. It’s important that we focus on local trends when looking at climate change rather than borrowing results from overseas.”

The gradual warming is also causing the ocean to expand and sea level to rise. NIWA scientists have detected a rise in sea level around New Zealand of 1 to 2 mm per year, and they expect this rate to increase over the next few decades as the ocean continues to warm.

Dr Stevens said they are trying to predict the behaviour of a complicated system over periods of 100 years or more, and computer models are their best chance of getting things right. “But our models are only as good as the information we feed into them.”

He said they expected the oceans around New Zealand to gradually warm over the next 100 years or so, although the temperature would continue to fluctuate because of natural variability.

“If we know how our ocean will behave in the future, then we stand a good chance of planning and adapting to a changing marine environment.”

Ends