Warming blast on its way (Image: NASA)

Consider yourself warned. We can expect a burst of supercharged warming when the pause in rising global temperatures finally ends.

Global mean surface temperatures have not risen significantly since about 1998, which could be thanks to the oceans sucking up the extra heat.

If this turns out to be the case, Chris Roberts from the Met Office in the UK and colleagues have found that there is a 60 per cent chance the hiatus will be followed by a five-year period of rapid warming at twice the usual background rate of around 0.2°C per decade.


The models also suggest there is a 15 per cent chance the hiatus will continue for five more years.

Lots of factors are likely to be behind the slow-down, but the biggest appears to be the burying of heat in the oceans, particularly the Pacific.

If that is the main factor driving the hiatus, scientists argue it is likely to end with rapid warming as the heat is released from the ocean. This happens as the world shifts from a decade dominated by cold La Niñas to one in which warm El Niños rule.

“What tends to happen is you get these flips between a positive and negative phase,” says Matthew England from the University of New South Wales in Sydney, Australia. A decade dominated by El Niños that warm the world up rapidly is followed by a decade rich with La Niñas, which have the opposite effect.

Cooling trend

Until now no one had quantified the chance of that rapid warming happening. So Roberts and colleagues decided to see exactly what climate models have say about it.

A global warming hiatus occurs when various sources of climate messiness or “variability” combine to produce a cooling trend that temporarily slows, stops or even reverses warming.

Although hiatuses appear in global climate models, they don’t happen at the same time in each model so the combined and averaged results of models don’t usually include them.

So Roberts and colleagues worked out the probabilities of hiatus events by running each model separately, categorising the results and counting how often they happened over time and across models.

They found that following a hiatus, 60 per cent of the time the world experiences rapid warming at twice the background rate for five years – at around 0.4°C per decade rather than 0.2°C. That rapid warming won’t necessarily stop there. “We only looked at the five years following hiatus decades and did not extend our analysis to look at periods beyond,” says Roberts.

Looking at where that warming would occur, they found it was often amplified in the Arctic, adding stress on a region that is already particularly vulnerable to climate change.

Still on pause

It a small consolation then that rapid warming could still be a few years away. Although 20-year hiatuses were quite uncommon in the models – having a probability of less than 1 per cent – they were not unlikely when a hiatus has already lasted for 15 years, like the one we’re living through now. In that case, they continued to 20 years about 15 per cent of the time.

“Even if the best estimate is for temperatures to rise in the next few years, we shouldn’t be too surprised if the pause continues,” says Roberts.

But Axel Timmerman from the University of Hawaii in Honolulu isn’t so sure. He thinks there are strong indications that the pause ended last year, when ocean temperatures reached a record high, with signs that a major source of variability – the pacific decadal oscillation – is turning around. “I think the 2014 situation was so special, that it is fair to say that the hiatus… ended,” he says.

Roberts is not convinced. “I would argue that we need a run of several unusually warm years to be able to definitively identify the end,” he says.

Journal reference: Nature, DOI: 10.1038/NCLIMATE2531