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 Nino-La Nina 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 Ninos - when the ocean gives back heat to the atmosphere - to La Ninas, 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-Nina 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 Nino [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.
While climatologists are wary about picking when the Pacific will switch back to an El-Nino dominated phase, the world may get an inkling of what is in store if an El Nino event is confirmed later this year.
The Bureau of Meteorology last week maintained its estimate of a 70 per cent chance of an El Nino this year. It noted, though, that warming sea-surface temperatures in the central and eastern Pacific had yet to trigger the consistent reinforcing atmospheric patterns such as a stalling or reversal in the easterly trade winds.
Even without the threshold being reached, however, El-Nino-like conditions had already contributed to the warmest May and June on record and equal-warmest April. Australia too has continued to see well-above average temperatures, with last year and the 12 months to June 30 setting records for warmth.
Data out this week from the US may confirm early readings that June's sea-surface temperatures were the biggest departure from long-term averages for any month.