The UK's Met Office has downgraded its forecast for warming at the Earth's surface over the next five years. Headlines this week announced that global warming is "at a standstill". Climate sceptics crowed. But the Met Office said the outlook for later in the century remains unchanged. New Scientist looks at the facts
Has global warming stopped, or hasn't it?
Atmospheric warming has certainly slowed greatly in the past decade. The Met Office says this appears to be due to natural cycles that are counteracting the warming effect of greenhouse gases. After incorporating new analysis of natural cycles into its latest model of atmospheric and ocean circulation, it has concluded that we are in for a few more years of little change.
Having calculated annual global temperatures for the next five years, its best guess is that they will be, on average, 0.43 °C higher than the average for 1970 to 2000. That's down from its previous prediction of a 0.54 °C rise. If the new prediction proves right, then 2017 will barely be warmer than most years in the past decade.
The forecast comes with a big error bar, however. The average warming for the next five years could be as much as 0.59 °C, or as little as 0.28 °C.
What has changed in their thinking?
There is a growing awareness among climate scientists of the importance of natural variability in predicting climate change, especially in the short term, where it can completely obscure the global warming signal. This realisation has been bubbling up for a while. Four years ago, New Scientist reported evidence – including research by the Met Office's Doug Smith – that natural cycles were pushing the atmosphere into a cold phase. Back then, we said the research "suggests that surface air temperatures will remain steady for the next six years or so, as cooler sea surface temperatures keep the lower atmosphere cool despite ever higher greenhouse gas levels".
So what are these natural cycles?
Mostly they involve the movement of heat between the atmosphere and the oceans. The oceans are the sleeping giant of climate change. They act as a huge heat sink: 90 per cent of the heat generated by accumulating greenhouse gases is absorbed by the oceans. How fast this happens is variable, depending on ocean currents and other fluctuations.
Scientists have known for a long time that in El Niño years, when warm water spreads out across the equatorial Pacific, heat leaves the ocean for the atmosphere. But there are also longer-term cycles. The biggest cycles are known as the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation. Recently, both have been causing the oceans to absorb more heat, shutting off atmospheric warming.
There are other possible confounding influences. The 11-year solar cycle has a small effect. So do volcanic eruptions and smog that shades the earth. Longer term, changes in Earth's orbit are thought to trigger ice ages. But all the evidence is that in recent times and over the coming decades, ocean-atmosphere interactions are the only influence comparable in scale to greenhouse gases.
Are these cycles just something scientists have invented to explain away the lack of recent warming?
No. The Met Office admits that we still know far too little about how these natural cycles work, and how big they are. And climate scientists are open to the charge that they ignored the potential impact of natural variability when it was accelerating global warming. According to Brian Hoskins of Imperial College London, it now looks like natural cycles played a big role in the unexpectedly fast warming of the 1990s.
Even so, the fundamental physics about how greenhouse gases trap heat in the atmosphere hasn't changed. And we know that, even as atmospheric warming stalls, the oceans are continuing to warm. That may could explain why Arctic sea ice melted so dramatically last summer, even though air temperatures were not exceptional.
So press reports that global warming is at a standstill are not true, even in the short term. Right now the oceans are taking up almost all the extra heat. That is most unlikely to persist.
Now we're getting headlines about the warming stopping as well as headlines about record-breaking temperatures. What do we make of it all?
There is lots of natural variability at local scales. Last year the US had its hottest year on record, but for the planet as a whole, it was only the ninth warmest. Australia is feeling the brunt of a record-breaking heatwave at the minute, but in northern India they are dying from unusually cold temperatures.
What's the outlook?
Scary. If oceanic cycles do what the Met Office and others expect, then global average air temperatures will stay fairly stable – though still hotter than they have been in the past – until later this decade. The cycles will then flip into a new phase and the oceans will probably start releasing heat instead of soaking it up. Combined with continued accumulation of greenhouse gases in the atmosphere, that could mean that sometime round 2020, warming will start to race away again as the atmosphere makes up for lost time.
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