Figures showing temperatures flatlining have given the climate debate fresh ferocity. Jonathan Leake, Science Editor of The Sunday Times, unpicks the row in ‘So, do we freeze or fry‘
John Prescott was apocalyptic. “Our polar ice caps are melting,” the then deputy prime minister thundered. “Only this weekend Mexico was hit by freak snowstorms . . . a world of drought and crop failures, rising seas, mass migration and disease . . . rising greenhouse grasses [sic] . . .”
The year was 1997 and Prescott had just come back from Kyoto in Japan to give the House of Commons his account of the latest climate talks.
Prescott’s terrifying warnings were backed by Britain’s leading climate scientists. Just before Kyoto a Met Office report warned that climate-related floods would put 50m people at risk of death from starvation in the coming decades. Whole island nations would disappear, it added, while the American Midwest, which helps to feed 100 nations, was likely to face drought and the North Pole might melt.
That was 15 years ago — what has happened to world temperatures since then? Last month came the suggestion that the answer was, embarrassingly, nothing. Research based on Met Office figures pointed to temperatures having been flat since 1997.
It was the kind of admission that those who doubt climate science pounce on. “Forget global warming,” trumpeted The Mail on Sunday, because “the planet has not warmed in 15 years”. It then cited other research, into the declining energy output of the sun, to suggest the real danger was from a big freeze, raising the prospect of a reprise of the frost fairs held on the frozen Thames in the 17th century.
Two days earlier The Wall Street Journal had published a letter from 16 scientists advancing similar arguments. It said: “The lack of warming for more than a decade . . . suggests that computer models have greatly exaggerated how much warming additional CO2 can cause.”
Since then the same cry has been taken up by innumerable bloggers, exemplified by David Whitehouse, formerly the BBC’s science editor, now an adviser to the Global Warming Policy Foundation, which frequently challenges the views of climate-change scientists. He, it turns out, was a source of the research that sparked the whole row.
“We set out to see how long it had been since the temperature had risen, and 15 years was what emerged from the data set,” he said. “It raises serious questions about how the Met Office models future climate.”
It seemed a strong argument but the climate scientists came out fighting, starting with a furious blog posted by the Met Office itself, which attacked the Mail on Sunday article as “entirely misleading”.
That was followed by another letter in The Wall Street Journal, this time signed by 35 leading climate scientists, who pointed out that few of the signatories to its sceptical predecessor were actually involved in climate research.
“Do you consult your dentist about your heart condition?” it asked, adding: “Climate experts know that the long-term warming trend has not abated in the past decade. In fact, it was the warmest on record.”
What were the rest of us meant to make of this? Some scientists appear to be warning we will fry, while other sources fear we will freeze. For the public the outcome is, increasingly, confusion. Where might the truth lie?
Perhaps the simplest first step is to put aside the arguments and get back to the data. Is it really true that global temperatures have not risen since 1997?
The simple answer is: they have risen, but not by very much. “Our records for the past 15 years suggest the world has warmed by about 0.051C over that period,” said the Met Office. In layman’s terms that is 51 thousandths of a degree.
These figures come from the Met Office HadCruT3 database, which takes readings from 3,000 land stations around the world, along with oceanic readings from a similar number of ships and buoys.
However, HadCruT3 is just one of several global temperature databases, each overseen by different scientists and calculated in slightly different ways. This allows each group to cross-check results, confirming findings or spotting errors.
One, held at the National Climate Data Centre (NCDC), run by America’s National Oceanic and Atmospheric Administration, suggests that global temperatures rose by an average of 0.074C since 1997. That’s small, too — but it is another rise.
A third and very different data set is overseen by John Christy, professor of atmospheric science at the University of Alabama in Huntsville. He gathers figures from three satellites that orbit the Earth 14 times a day. They measure the average temperature of the air from ground level to a height of 35,000ft, a method completely different from those of the Met Office and NCDC. Oddly, given his reputation as a climate sceptic, he found the biggest rise of all.
“From 1997-2011 our data show a global temperature rise of 0.15C,” he said. “What’s more, our satellites have been taking this data since 1979, and over that period [the] global temperature has risen 0.46C, so the world has been getting warmer.”
Overall, then, the world has got slightly warmer since 1997. Perhaps the real question is: why has it warmed so much less than was predicted by the climate models?
For most climate scientists the answer is simple. “Fifteen years is just too short a period over which to measure climate change,” said Peter Stott, head of climate monitoring at the Met Office. “The world undergoes natural temperature changes on all kinds of time scales from daily variations to seasonal ones. It also varies naturally from year to year and decade to decade.”
Whitehouse accepts this point. “The records do show that global temperatures have risen by about 0.4C over the past three decades, most of it in the 1990s,” he said.
“I accept that CO2 is a greenhouse gas that might warm the world but the key issue is how strong the effect is and how the data compare with the models used to predict the future.”
This is an interesting admission, turning what had appeared to be an attack on the keystones of climate science — that greenhouse gases cause global warming — into a “shades of grey” debate over whether global warming will happen slowly and steadily or in jerks, accelerating in some decades but then slowing or even reversing a little in others.
For the critics of climate science this is a crucial point — but why? The answer goes back to the 2001 and 2007 science reports from the Intergovernmental Panel on Climate Change that had predicted the world was likely to warm by an average of about 0.2C a decade. The implication was that temperatures would rise steadily, not with 15-year gaps. The existence of such gaps, the critics argue, implies the climate models themselves are too flawed to be relied on.
Other leading climate scientists have raised similar issues. One is Judith Curry, professor of atmospheric sciences at the Georgia Institute of Technology. She argues that global climate is affected by so many factors, ranging from solar output to volcanic eruptions, that predicting how the world will warm is impossible.
Crucially, however, Curry accepts that greenhouse gas emissions are likely to lead to long-term warming. She wrote on her blog: “We don’t know what the climate will be for the next several decades. In terms of when global warming will come ‘roaring back’, it is possible this may not happen for the first half of the 21st century.”
For Curry and many others one of the key unresolved issues lies in the behaviour of the sun, whose output appears to be undergoing a steady but small decline. Most scientists accept that this will reduce global warming. The debate is over just how strong this effect will be, with people such as Curry suggesting it could be powerful while others see it as small.
Among the latter is Mike Lockwood, professor of space physics at Reading University’s meteorology department, who believes the sun has been in a “grand solar maximum” since the 1960s, thought to be the longest-lived peak in its output for more than 9,000 years.
“A decline in activity is long overdue,” he said. “How deep will it go? We think there is about an 8% chance that it will drop below the famous Maunder minimum.”
This was a 60-year period, starting in about 1645, when the sun had very few sunspots; it was marked by an unusually high proportion of cold winters in Europe.
That sounds ominous but Lockwood calculates that even a decline in activity on that scale would now have little effect because the impact would be far smaller than the opposing effects of surging greenhouse gas emissions.
What about the most evocative image of all — the prediction that the Thames might freeze over? This did happen in 1963, but far upstream in the stretches around Windsor. The idea that the lower tidal reaches might be in similar danger generates little but scorn from all sides.
Lockwood said: “The disappearance of frost fairs is nothing to do with climate. It is because the old London Bridge — really more of a weir — was pulled down and the embankments were put in. So the river now flows much too fast to freeze and is also a lot saltier. Even a return to Maunder minimum solar conditions would not cause the Thames to freeze again so far downstream.”
This article first appeared in The Sunday Times on Sunday 5 February 2012.