Climate-change sceptics often point to statistics that show global temperatures stopped rising 15 years ago. But it is only one element of many in a complex, interlinked system of meteorological processes that we are only beginning to understand.
Halt in global warming is a hot topic
First it was dismissed as imaginary, then it was called a statistical blip. Now it’s become one of the hottest topics among climate scientists: why has global warming stopped?
Since the late 1980s, we’ve been told that our planet is warming up with potentially disastrous consequences. Leading climate scientists have declared – with increasing confidence – that the fault lies with mankind, with our reckless use of greenhouse gas-generating fossil fuels.
Yet not everyone has gone along with this scientific consensus. Some have argued that the 0.8C rise over the last century is just natural variation. Others have insisted it’s due to faulty measurements.
But some have claimed that while global warming may have once been underway, it isn’t any more.
They point to the graphs of average global temperatures, which show a seemingly inexorable rise from the mid-1970s onwards, reaching a peak in 1998 followed by… stasis. For the past 15 years, the graphs show no discernible trend.
The claim that global warming is on hold was first made in 2006 by Prof Bob Carter, an earth scientist from James Cook University, Australia.
At the time, the claim was deeply controversial, and was challenged on the entirely reasonable grounds of insufficient evidence. Yet since then the evidence for a pause in the upwards trend has grown.
Climate scientists continued to argue it was a statistical blip, but now even leading advocates of man-made global warming accept that the pause is real, and demands explanation.
Last month, the leading science journal Nature – not known for its denial of climate change – looked at the current best guesses as to the likely cause of the pause.
Top of the list, ironically, is the phenomenon widely blamed for the record-breaking peak global temperature in 1998: El Niño.
Known to scientists as the El Niño-Southern Oscillation (Enso), this is a notorious family of weather patterns that breaks out every three to eight years in the Pacific Ocean.
Its origins lie in the complex interaction between the atmosphere and the ocean. Each year the sun’s warmth causes a huge build-up of heat in the Pacific, plus powerful convective air currents in the atmosphere.
By the end of the year, some of this heat is offloaded by currents from the western coast of South America, triggering winter rains in Australasia.
But every so often, there’s a much larger westward surge of heat, affecting ocean temperatures and air flow across the whole Pacific.
The result is a powerful El Niño effect, accompanied by a change in air pressure known as the Southern Oscillation – and worldwide climatic upheaval.
The 1997-8 El Niño event was particularly dramatic, and is widely blamed for everything from floods in Chile to droughts in Indonesia. It is also thought to be the cause of that sudden spike in global temperatures.
But almost immediately afterwards, colder waters from deep within the Pacific moved in to the same area. This is now seen as part of an even grander weather cycle, called the Pacific Decadal Oscillation (PDO).
And when something as vast as the Pacific gets cooler, it’s likely to have global consequences.
This has now been confirmed by computer models based on actual temperature data from the Pacific taken over several decades.
Researchers at the Scripps Institution of Oceanography in La Jolla, California, have shown that the resulting PDO cooling is enough to mop up the underlying global warming trend – making it appear to have stopped dead.
To climate-change sceptics, this will sound like fiddling with computer models until they give the “right” answer. So many weather cycles have now been identified that the explanations based on them look suspiciously like the gear-packed “epicyclic” models the Greeks used to prop up the idea of an Earth-centred solar system.
In fairness, climate scientists are discovering more complexity rather than merely inventing it. Even so, what’s needed is that acid test of any credible scientific theory: a verifiable prediction – which is what climatologists are now seeking.
The most obvious is that, if the PDO really is the cause of the pause in global warming, it will flip around, causing a sharp rise in global temperatures.
That turnaround may be underway right now.
According to reports in Nature, strong tropical winds are already driving warm water back into the El Niño zone of the Pacific. The western part of the ocean has now acquired a 20cm swell compared with the east – and you don’t need a computer to tell you that can’t last forever.
As climate scientist Dr Kevin Trenberth of the US National Centre for Atmospheric Research (NCAR) in Boulder, Colorado, memorably put it in Nature: “At some point the water will get so high that it just sloshes back”.
And when it does, the resulting release of heat will trigger a sudden resumption of global warming.
Or, rather, it should. For, as with so much in climate science, things aren’t that simple.
There’s another influence on our climate that’s puzzling scientists – one that could throw any prediction out of whack: the sun.
Like our climate, the sun goes through cycles, marked by the rise and fall in solar activity, as measured by sunspot numbers.
These cycles last roughly 11 years, and the current one reached its peak last year.
What bothers scientists is that it wasn’t much of a peak. In fact, it was the most feeble for more than a century.
This has led to speculation that the sun is starting to wind down to a level of activity not seen since the late 1600s – which just happened to coincide with a period when the Earth became much colder.
The causal link between solar activity, sunspots and global temperatures is far from fully understood.
What is clear, however, is that it could make a mockery of climate predictions – and with it those scientists hoping to revive the case for action on global warming.
Robert Matthews is visiting reader in science at Aston University, Birmingham