Doubts cast on warmest decade

Doubts are being raised about the significance of new figures showing the past decade has been New Zealand's warmest on record.

Average national temperatures over the past decade were just a few hundredths of a degree Celsius higher than in the 1980s, the previous warmest.

A National Institute of Water and Atmospheric Research (Niwa) statement yesterday reaffirmed its statements that the decade was likely to be the hottest recorded.

However, the new statement failed to include figures.

Blue Skies Weather forecaster Tony Trewinnard said it was doubtful a few hundredths of a degree was such a statistically significant amount on which to base such statements.

Niwa principal climate scientist James Renwick told The Press temperatures at seven key stations over the past 10 years were one-tenth of a degree above the 1971-2000 norm.

"The next warmest were the 1980s, about 0.07C above that normal, so it's only a few hundredths of a degree difference," he said.

Asked if the difference was significant, Renwick said: "Well, it is a very small amount. [But], officially, it was warmer – the long-term trend is for warming."

A warmer atmosphere provides more energy for weather systems, which leads to more extreme weather.

In winter, that increases the chances of severe snowstorms, like those being experienced in Europe, North America and parts of Asia.

However, Trewinnard questioned how Niwa could claim a long-term warming trend when the 2000s were only marginally warmer than the 1971-2000 norm and the 1980s, and when the 1990s were cooler than both those decades.

"There isn't a long-term warming trend by his [Renwick's] own numbers, nationally," he said.

"That's not supported by the data.

"If you're comfortable with saying three-hundredths of a degree is different, then that's fine, but are those claims really supported by the data?

"It's like a political party claiming it leads in the polls when its lead is less than the margin of error in the poll."

Three-hundredths of a degree difference in a decade was the same as 0.3C difference in a century, which was still 10 times less than some of the international predictions of rising temperatures, he said.

"Suppose, for the sake of argument, nobody had ever thought about climate change and global warming, but we just wanted to look at climate trends," Trewinnard said.

"If you had this data, I'm absolutely certain your conclusion would be that temperatures were much the same for the 2000s as the other decades."

- © Fairfax NZ News