Time series of anomalies in sea-surface temperature and temperature over land in the Australian region. Anomalies are the departures from the 1961–1990 average climatological period. Sea-surface temperature values are provided for a region around Australia (from 4°S to 46°S and from 94°E to 174°E). Graphic: Australia Bureau of Meteorology

By Peter Hannam, Environment Editor
21 July 2014

(Sydney Morning Herald) – A common refrain by climate sceptics that surface temperatures have not warmed over the past 17 years, implying climate models predicting otherwise are unreliable, has been refuted by new research led by James Risbey, a senior CSIRO researcher.

Setting aside the fact the equal hottest years on record - 2005 and 2010 - fall well within the past 17 years, Dr Risbey and fellow researchers examined claims - including by some members of the Intergovernmental Panel on Climate Change - that models overestimated global warming.

In a study published in Nature Climate Change on Monday, the team found that models actually generate good estimates of recent and past trends provided they also took into account natural variability, particularly the key El Niño-La Niña phases in the Pacific.

“You’re always going to get periods when the warming slows down or speeds up relative to the mean rate because we have these strong natural cycles,” Dr Risbey said.

In roughly 30-year cycles, the Pacific alternates between periods of more frequent El Niños - when the ocean gives back heat to the atmosphere - to La Niñas, when it acts as a massive heat sink, setting in train relatively cool periods for surface temperatures.

By selecting climate models in phase with natural variability, the research found that model trends have been consistent with observed trends, even during the recent “slowdown” period for warming, Dr Risbey said.

“The climate is simply variable on short time scales but that variability is superimposed on an unmistakable long-term warming trend,” he said.

While sceptics have lately relied on a naturally cool phase of the global cycle to fan doubts about climate change, the fact temperature records continue to fall even during a La-Niña dominated period is notable, Dr Risbey said.

The temperature forcing from the build-up of greenhouse gases in the atmosphere “is beginning to overwhelm the natural variability on even shorter decadal time scales”, he said.

“We will always set more heat records during an El Niño [phase] … than we will during the opposite but we’re still setting records even during the cold phase because we’re still warming,” Dr Risbey said. [more]

Climate models on the mark, Australian-led research finds



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