Research shows surprise global warming 'hiatus' could have been forecast

http://www.theguardian.com/environment/2014/sep/09/research-global-warming-hiatus

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Australian and US researchers have shown that the slowdown in the rate of global warming in the early 2000s, known as a so-called “global warming hiatus”, could have been predicted if today’s tools for decade-by-decade climate forecasting had been available in the 1990s.

Although global temperatures remain close to record highs, they have shown little warming trend over the past 15 years, a slowdown that earlier climate models had been largely unable to predict.

This has been used by climate change sceptics as evidence that climate change prediction models are flawed.

Gerald Meehl, a senior scientist at the National Centre for Atmospheric Research in the US, along with the Centre for Australian Weather and Climate Research in Melbourne, decided to challenge the assumption that no climate model could have foreseen the hiatus.

“We wanted to know: if we could be transported back to the 1990s with new decadal prediction capability, a set of current models and a modern-day supercomputer, could we simulate the hiatus?” Meehl said.

Studies have shown global warming had not stalled but was occurring in the deeper layers of the world’s oceans instead of the surface, he said, which were absorbing the heat and obscuring levels of warming. In other words the “pause” was actually a slight slowing of the rate of increase in surface temperatures, with 93% of the extra heat trapped in the oceans.

Decadal climate prediction uses the state of the world’s oceans and their influence on the atmosphere to predict how global climate will evolve over the next few years.

It is a relatively new area of climate science driven by supercomputing, increased sophistication of global models and the availability of higher-quality observations of the climate system, particularly the ocean.

When Meehl and his team applied these decadal climate prediction models to past-observed conditions in the climate system starting in the late 1990s, the three-to-seven-year forecasts consistently simulated the leveling of global temperature that was observed after the year 2000.

While Meehl said all the factors that might be driving the hiatus were still being studied, his research, published in the journal Nature Climate Change, suggested natural decade-to-decade climate variability was largely responsible.

“Because this is a brand new way of doing predictions we need to be careful as to how reliable these predictions are,” he said. “But there are indications from some of the most recent model simulations that the hiatus could end in the next few years.”

Professor Matthew England, the deputy director of the climate change research centre at the University of New South Wales, said the study was important because it revealed initialised climate models could have predicted the recent slowdown in surface atmospheric warming.

“This gives us confidence in the skill of these models for longer-term climate projections, as they appear to be able to capture the interplay between decadal variability and long-term warming,” he said

“Like a weather prediction system, the models just need appropriate initial conditions, and then their skill on the decadal time-scale comes through.”