Surface warming ‘hiatus’ could stick around another five years, say scientists

Don’t be surprised if the slower pace of warming we’re seeing at the Earth’s surface lasts for another five years, scientists say.

A new paper out today puts the chances of the so-called “hiatus” staying until the end of the decade at about 15 per cent, or one in six.

But the heat hasn’t gone away. The scientists say most of it is lurking in the deep ocean and we can expect the pace of warming to pick up when this heat gets released again.

Slower surface warming

Since 2000, the temperature at the Earth’s surface hasn’t warmed as quickly as it has in previous decades, despite greenhouse gas emissions rising faster than they were before.

A growing body of evidence is homing in on the Pacific Ocean as the main culprit for why we’re seeing “unexpectedly modest” warming, as the Nature Climate Change paper puts it.

Scientists think a natural fluctuation is causing heat to find its way to the deep ocean in the Pacific, where it doesn’t warm the atmosphere as much it would if it stayed at the surface.

A number of recent studies have found that periods of faster and slower warming aren’t unusual in Earth’s temperature record. It’s what scientists expect as these natural cycles flip-flop between their different phases, superimposed on top of greenhouse gas warming.

But what are the chances of natural variability being strong enough to offset some, or even all of the warming expected from greenhouse gases?

The new paper by Dr Chris Roberts, an ocean and climate specialist at the Met Office Hadley Centre, and colleagues at the University of Exeter sheds some new light on this question.

Climate model projections (blue lines) of global mean surface temperature (GMST) since 1900 compared with observations (red lines). Source: Roberts et al., (2015)

Odds of a ‘hiatus’

The new paper uses a suite of climate models to examine the impact of natural variability on future temperatures. The authors find there’s a 28 per cent chance in the model simulations that natural variability could cause a five-year long ‘hiatus’.

The scientists define ‘hiatus’ as a period during which the rate of warming expected from greenhouse gases, 0.2 degrees Celsius per decade, is partially or entirely offset by natural cooling influences.

When the scientists used only climate models that closely matched real-world temperatures in the Pacific, the probability of a five-year long ‘hiatus’ went up to 30 per cent.

The chances of longer ‘hiatus’ periods occurring are considerably lower, the scientists note. According to the same subset of models, the probability of a 10-year event is 10 per cent, with a less than one per cent chance of an event lasting 20 years.

Put another way, that means the current period of slower warming, which has already lasted 15 years, is a particularly unusual event in the context of earth’s recent history.

The authors note that if greenhouse gases continue to rise in the 21st century, periods in which natural variability is strong enough to counteract most of the greenhouse gas warming will occur less frequently.

Current slowdown

The expected frequency of a ‘hiatus’ period doesn’t tell us much about how long we can expect the current slowdown in surface warming to continue, the authors note.

For the first time, the new paper looks at the chances of the current slowdown lasting another five years. They scientists find the odds of this happening are “non negligible”, putting the likelihood of the current event lasting till the end of the decade at 15 per cent.

There is disagreement between the models, however, with some giving a 25 per cent chance and others giving a zero per cent chance. The average across the board is 15 per cent.

Research suggests other factors such as small volcanic eruptions, a pronounced low in solar activity and an increase in aerosols may also be contributing to the current period of slower-than expected surface warming. While a redistribution of heat to the deep ocean is looking like the biggest contributor, these are likely to be playing a role to some extent, too.

Communicating the odds of the slowdown continuing for a few more years is important to avoid “allegations of overconfidence in global climate model projections”, say the researchers.

Expect a resurgence

There’s a good chance we’ll see an accelerated pace of surface warming as soon as the current surface warming slowdown comes to an end, the new paper says.

That’s because the heat stored in the deep ocean will be released back to the atmosphere when the natural cycle driving it flips in the other direction.

Climate models suggest we could see Arctic warming intensify, leading to “increased climate stress on a region that is already particularly vulnerable to climate change”, the paper says.

Mean surface temperature changes during 5-year “accelerated warming” periods following ‘hiatus’ decades. Source: Roberts et al., (2015)

As ever when discussing the current ‘hiatus’, it’s important to realise we’re talking about temperature changes at the Earth’s surface, namely, the air above land and the top of the ocean. While it’s fascinating to unravel the factors driving the slower pace of warming, and how long it’s likely to last, there’s plenty of evidence energy is continuing to accumulate in the climate system as a whole. You just have to look below the surface to see it.

Main: Clear blue ocean.