Proof soon on forecast of warmest decade

Tension is rising over whether the decade just ended was the warmest on record in New Zealand.

December's national average temperatures will be critical in determining the facts, which are of great interest to both sides of the climate change debate.

Since Christmas the National Institute of Water and Atmospheric Research (Niwa) has predicted the 2000s will be the warmest since New Zealand readings began.

It says the nine-year period 2000-2008 was 0.05 degrees Celsius hotter than the 1980s, the warmest decade to date.

But some media, including the government-funded Science Media Centre, have jumped the gun, mistakenly turning Niwa's prediction into "fact".

Blue Skies Weather forecaster Tony Trewinnard and Niwa say December temperatures will be the determining factor.

The official national December summary should be out tomorrow or Wednesday.

Niwa principal climate scientist James Renwick said December would "have to have been unusually cold" to jeopardise the expectation it was New Zealand's warmest decade on record.

Last month, Niwa said average temperatures for the month would have to be more than 2C below normal to shift the 2000s into second place behind the 1980s.

Renwick said he had seen some preliminary figures that showed last month was not that cold nationally.

"It was a pretty average December – maybe a little bit cooler than normal in some places but not unusually cold.

"I imagine that prediction will come true."

Statistics from the Niwa database reveal that Christchurch Airport has bucked the warming trend of recent decades.

Those figures show the 2000s were the coolest of the last five decades in terms of mean temperature, with an average of 11.45C.

The airport's 1960s' mean temperature was 11.53C, the 1970s' was 11.59C, the 1980s' was 11.81C and the 1990s' was 11.5C.

Trewinnard said Christchurch seemed to have broken more records for cold months over the last 10 years than in previous decades.

"Everyone says our winters are colder than they were, that our summer weather is more changeable. Given that background, intuitively it didn't feel like the 2000s were any warmer than the 1990s, and indeed might be cooler than the 1980s, which were very warm."

- © Fairfax NZ News