Experts dispute El Nino influence

The El Nino weather pattern is to blame for most changes in the climate, a report has claimed.

The study, published in the Journal of Geophysical Research, found about 70 per cent of changes in global temperature were caused by weather patterns known as the El Nino/Southern Oscillation.

Now nine New Zealand and overseas scientists, mostly contributors to the United Nations' Intergovernmental Panel on Climate Change, have submitted an article to the same journal panning the study.

They say the El Nino factor was "grossly overstated", because the authors filtered out long- and short-term changes to focus on the time interval most likely to be affected by El Nino - about 2-7 years.

El Nino and La Nina are atmospheric winds and ocean temperature patterns affecting weather and climate.

Climate scientists agree El Nino years tend to be hotter than average globally, but cooler in New Zealand.

The 1990s were a hot decade partly because of a high number of El Ninos, while the 2000s were cooler because of the opposite, La Nina, effect.

IPCC graphs of temperature trends over time periods of 30-40 years - the trends filtered out by the latest study - show global average temperature creeping up over the last 50 years, with spikes and falls in individual years caused by El Nino and other influences.

The study's lead author, information technology specialist John McLean, said he never set out to look at long-term temperature trends.

He had to filter out short-term influences so the results were not skewed by random hot and cold days.

He said the study's most important finding was not the exact percentage of influence it assigned to El Nino, but that global average temperatures tended to closely follow El Nino after a time lag of about seven months.

However: "If they just said they'd shown El Nino was important year-to-year, that would be fine ... But (they're) saying it explains ... temperature trends," said Niwa climate scientist Dr Renwick.

He said it was common for studies to filter results to focus on a particular time interval. But having filtered out long-term changes, the authors could not claim the results explained 30- or 40-year trends.

The study has been widely cited by sceptics of man-made global warming as evidence there is little room for the influence of greenhouse gases.

Mr McLean and one co-author, geologist Bob Carter of James Cook University in Queensland, are on the advisory panel of the Australian Climate Science Coalition, a body dedicated to questioning man-made climate change.

The third author, Auckland University's Chris de Freitas, is associated with the New Zealand Climate Science Coalition - a New Zealand coalition of climate sceptics - which Dr Carter also works with.

Jim Salinger of the World Meteorological Organisation, who helped write the rebuttal with Dr Renwick, said studies on the topic since the 1970s concluded El Nino drove between 15 and 30 per cent of global temperature variation.

The latest study found it accounted for 72 per cent of changes in global average temperature over a 20 years and 68 per cent over 50 years.

Mr McLean and his co-authors will have a chance to respond to the criticism when the rebuttal is considered for journal publication. Dr Renwick said it might not be printed until next year. "No one is disputing El Nino is a big player but [its importance] hasn't increased."