https://www.newscientist.com/article/mg ... ouse-wars/FEATURE 19 July 1997
Greenhouse warsBy Fred Pearce
PAT MICHAELS, a belligerent sceptic about global warming, is in ebullient mood. “The truth is that what we sceptics say is always pilloried by the climate modellers, and then adopted as their own five years later. That would make a good theme for your article.”
Michaels, a climatologist at the University of Virginia, is convinced that the tide is about to turn in his favour, and that the efforts of the mainstream climate modellers, stalwarts of the UN’s Intergovernmental Panel on Climate Change (IPCC), are about to collapse under the weight of their own inconsistencies. “They can’t go on forever tinkering with their models, trying to make them fit reality. Ever heard of Ockham’s razor? It says the simple explanation is usually the best. Apply that in this case and you conclude that the climate just is not as sensitive to the greenhouse effect as they predicted.”
So what exactly is the problem for the climate modellers? Well, something strange has happened to global warming. For almost twenty years, while temperatures at ground level round the world have continued to rise inexorably, the warming has failed to penetrate the atmosphere. In wide areas some three kilometres above Earth, the atmosphere has actually been cooling. This is not what is predicted by the computerised climate models on which all estimates of global warming depend. They all say the warming should spread right through the troposphere, the bottom ten kilometres or so of the atmosphere.
Global warming sceptics have spent almost a decade challenging some of the basic tenets of the climate models. Ever since global warming became headline news in the late 1980s, they have been complaining that the prevailing view is skewed and overstates the problem. Their prime motivation seems to be indignation, coupled with a maverick instinct to buck the latest fashion. But they have also managed to secure some lucrative lecturing fees and consultancy deals with commercial concerns – such as the coal industry – who are anxious to undermine international efforts to control emissions of greenhouse gases such as CO2.
It is a year this month since the US’s under-secretary of state Timothy Wirth castigated the sceptics at a UN climate conference. They were “bent on belittling, attacking and obfuscating climate change science”. But the stubborn failure of most of the troposphere to warm continues to hearten the greenhouse outlaws. Are they right? Has the climatic apocalypse been postponed? Are the modellers on the run from reality?
On a tour of some of the main players, you dodge a constant crossfire of personal and professional abuse. In his home near Boston, the sceptics’ guru Dick Lindzen, a meteorologist from the Massachusetts Institute of Technology, accused the IPCC of being dominated by “guys from the bottom of the heap, such as geographers”.
Back in Virginia, Michaels slams the IPCC scientists for manipulating data, then settles into an explanation of how the key to Vice-President Al Gore’s challenge for the presidency next time round will be corrupt environmental journalists who he will use to peddle a fraudulent version of climate change.
In the greenhouse wars, the battle is bloody and many a good scientist has been conscripted by both sides. Take John Christy. For ten years he was a Baptist minister in Kenya before he took up science. Now, as professor of atmospheric science at the Global Hydrology and Climate Center, part of the University of Alabama in Huntsville, he catalogues data from satellite instruments operated by the US government’s National Oceanic and Atmospheric Administration (NOAA).
Since 1979, long before global warming was an issue, NOAA’s instruments have been measuring microwave radiation released by the atmosphere. The radiation comes largely from oxygen molecules, which release more as they warm, giving an overall picture of temperatures in the troposphere. So far, and in stark contrast with the ground-based meteorological stations, the satellites have apparently picked up little evidence of warming.
Naturally, the sceptics have adopted Christy’s data to suggest that global warming is a myth. In response, collectors of ground-based data have hit back. Jim Hansen, the NASA climate modeller who first put global warming onto the front pages back in 1988, claims that if the satellites can’t see evidence of warming “there’s something wrong with their data”.
Both are wrong, says an exasperated Christy. First, his short time-series – just 18 years – is skewed by warming at the start, caused by a short-term natural warming in the Pacific, and by cooling towards the end, from the eruption of Mount Pinatubo in the Philippines. Filter out those effects and he reports “a very slight warming trend”. But most importantly, says Christy, “the satellites don’t measure surface temperatures, but average temperatures through the troposphere”. Thus the surface and satellite data complement rather than contradict. Put them together, he says, and they show that the surface of the planet is warming, but the bulk of the troposphere, the so-called free troposphere, is not.
Ups and downs
Add archive data from weather balloons recently assembled at the British Meteorological Office by David Parker, another assiduous and independent-minded data-cruncher, and something even more surprising emerges. From the early 1960s to the late 1970s, there was strong warming in the free troposphere, at a time when the surface was cooling slightly. Then everything switched: the surface warmed strongly while at 1.5 kilometres and upward, temperatures were at least stable, if not falling.
Nobody can explain this reversal. Certainly, the temperature record of the past three decades does not match the predictions of the models, agrees Parker. In the models, the surface and the free troposphere are very closely coupled, so their temperatures should move together. But if this is right, why are the temperatures in these two regions moving in different directions? “The surface and mid-troposphere appear to be much less coupled than the models assume,” says Parker. “If the models don’t get tropospheric heating right, we are in trouble.”
After years of trying, the greenhouse sceptics finally feel that here they have found the Achilles heel in the climate models. If the models are wrong about how surface warming influences temperatures in the troposphere, they are also likely to be wrong about another fundamental feature: the movement of water vapour between the surface and the free troposphere. And that, argue the sceptics, means the models may have misrepresented, or even have invented, one of the vital mechanisms behind global warming itself: the positive water-vapour feedback.
Feedbacks are what turn the greenhouse effect from a benign curiosity into a potential apocalypse. Even sceptics agree that putting more greenhouse gases, such as CO2 from burning fossil fuels, into the atmosphere will tend to warm the planet. But even the doubling of CO2predicted for the late 21st century would only add about 1 °C to global temperatures. According to the models, however, this initial warming will be magnified by a series of positive feedbacks. In the most important of these, they predict that surface warming will increase evaporation from the oceans and push more water vapour into the atmosphere. Because water vapour is itself a strong greenhouse gas, this would amplify the effect of the CO2 – a positive feedback that might roughly double global warming.
But will it? Christy’s collaborator on the satellite data, Roy Spencer of NASA’s Marshall Space Flight Center, is doubtful. “I don’t think warming will be as big as people think,” he says. “The positive feedback theory assumes that, in practice, a warmer troposphere will actually hold more water vapour.” But he points out that if, as the satellite data suggest, the free troposphere is largely cut off from the surface, water that evaporates from the oceans will not necessarily mean more water vapour in the free troposphere.
This matters, according to Spencer, because water molecules in the troposphere could have a much bigger warming effect than ones that stay close to the surface. Higher up, the air is extremely dry, and Spencer argues that because of this, adding or subtracting relatively small amounts of water there could greatly alter the amount of heat trapped.
Not everybody agrees about the importance of the upper troposphere in driving water vapour feedbacks. One of Britain’s leading meteorologists Keith Shine, from the University of Reading, says: “The surface is at least as important here. Sure, a single molecule is more important aloft, but there are so many more molecules at the surface.”
But if Spencer is right, this would seem to pose a serious problem for the conventional view that water vapour would amplify the effect of CO2. The extra water vapour generated by warming at the surface would never make it to the regions where it could significantly increase the greenhouse effect.
Clouding the issues
And there is more. Some sceptics also argue that the complex physics of clouds could actually reduce the amount of water vapour that reaches the free troposphere – and so damp down global warming. The modellers are on weak ground in this discussion because individual clouds are too small to be modelled in any detail inside global climate models, which assume that their dynamics will be unchanged by warming.
The sceptical position here is that warming might make clouds more efficient at producing rain, leaving less water vapour behind to moisten the free troposphere. It works like this: most of the water vapour that evaporates from the oceans does not stay in the air for long. It forms clouds and quickly returns to the surface as rain. But some re-evaporates from clouds and heads off into the clear air of the free troposphere. Change the amount of water that leaves the clouds as rain, says Spencer, and you change the amount of water vapour left to re-evaporate from the clouds, and hence the amount of water in the free troposphere. ( See Diagram.
Two opposing views on the effect of water vapour on global warming.
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This is where Lindzen comes in. Though he is highly regarded for his innate ability, Lindzen’s ideas are notoriously difficult to pin down. Colleagues complain that too few of them turn into peer-reviewed papers and too many emerge as invective in newspaper articles. Shine says: “Lindzen is painfully clever. But he keeps changing his arguments.” Even so, John Houghton, co-chair of the IPCC’s science panel, once described him as the “most serious” of his sceptic foes.
Lindzen argues vehemently that water vapour operates a negative rather than a positive feedback. He says “all the data show” that if clouds are warmer, they will turn a greater proportion of their moisture into rain. Result: less water re-evaporating from clouds, a drier free troposphere and a negative water-vapour feedback. In other words, changes in the tropospheric water vapour would compete with, not reinforce global warming caused by CO2.
Here, even some fellow sceptics back off. “It’s intriguing, but it’s a theory, that’s all,” says Spencer. “There is no actual evidence for a negative feedback.” Shine agrees. “The idea that precipitation efficiency is better in warm clouds is at best contentious. I think what is alarming us more about Lindzen’s comments is his assertion that the answer is known.”
But Lindzen’s opponents know that he is attacking them at a weak point. Evidence of a positive feedback is not overwhelming, either. The IPCC’s most recent review, its Second Assessment published in 1996, admits to serious gaps in knowledge about water vapour and concedes that feedback “remains a substantial uncertainty in climate models”.
Simon Tett, one of the Met Office’s top modellers and a leading IPCC author, agrees that recent studies suggest the positive feedback may have been overestimated. “I believe the upper troposphere is probably drier than the models suggest,” he says. Though the evidence for a negative feedback is, if anything, even weaker, the sceptics still believe things are moving their way.
Lindzen for one argues that if the models get the detail wrong, they will get the big picture wrong, too. But modellers say the proof of the pudding is in the eating. “We think there is good evidence that our models reproduce past climate change reasonably well,” says Tett. “That is good evidence that they are basically correct.”
But here too the modellers are being challenged. “The simply fact,” says Michaels, “is that even at the surface the world is not warming up as much as the modellers say it should.” Hansen agrees: “Models driven by greenhouse gases alone give warming about twice as large as observed over the past 150 years – about 1 °C rather than the observed 0.5 °C to 0.6 °C.” For recent years, says Christy, “the models suggest a warming three times what we see”.
Michaels was one of the first scientists to propose an explanation for this. In an article he wrote for New Scientistin the early 1990s, he suggested that sulphate smogs emanating from industrial areas were casting a thin pall over sufficiently large areas to mask much of the warming (“Global pollution’s silver lining”, 23 November 1991, p 40). By late 1994, modellers at the Met Office had adopted the idea and claimed that combining predictions of warming from CO2 and cooling from sulphate produced a good fit of actual climate change – a fit that persuaded John Gummer, Britain’s environment secretary of the day, to pronounce himself convinced that global warming was for real.
Yet today Michaels is a vociferous opponent of this theory. The problem is geography. Sulphate only survives a few days in the air. Since most sulphate is emitted in the northern hemisphere, its cooling influence should be largely limited to that hemisphere. So if sulphate cooling is important, Michaels says, the southern hemisphere should be warming faster than the north. Until the late 1980s, that was what was happening. But since 1987, warming has virtually ceased in the southern hemisphere – notably in the mid-latitude region between 1 and 1.5 kilometres above the surface, where warming was previously most intense – while it has surged ahead in the north.
For Michaels, that condemns the sulphate theory to the dustbin. And he scorns modellers who don’t follow his lead. One of Michaels’ most violent verbal attacks on fellow scientists is against Ben Santer of the US government’s Lawrence Livermore National Laboratory in California. In a paper in Naturelast summer, Michaels implied that Santer bolstered a claim that sulphates were cooling the planet by arbitrarily ending his analysis of temperature trends in 1987, the year the southern warming ceased. Santer’s data finished in 1987, but Michaels argues that there are other datasets that Santer could have used to extend his findings. In a subsequent letter to Nature, Michaels argued that the models and reality were diverging. “Such a result… cannot be considered a `fingerprint’ of greenhouse-induced climate change,” he claimed.
And that charge drew blood. For Santer was the main author of a key chapter in the IPCC’s Second Assessment which concluded, largely on the basis of this work, that there was “an emerging pattern of climate response to… greenhouse gases and sulphate aerosols in the climate record”. In other words, the human fingerprint could now be seen in climate change.
But is it that simple? Santer, who admits to being “troubled” by Michaels’ assault on his integrity, responds vigorously. He denies selective use of the data and says that the datasets available when he wrote his paper for the years after 1987 were not compatible with his own data. He says that the relative cooling of the southern hemisphere since 1987 does not contradict the models. In fact, the models explicitly predict it. His case is that what we are seeing is the interaction between two different effects happening on two different timescales.
Blowing hot and cold
Basically, the first effect, global warming, is bound to happen more slowly in the southern hemisphere than the north. This is because most of the southern hemisphere consists of oceans, which heat up more slowly than the landmasses which dominate the north. But the picture has been confused by the second effect, sulphate cooling, which peaked in the north in the mid-20th century. It slowed warming in the northern hemisphere so much that the southern hemisphere, oceans and all, raced ahead. But since 1987, the growing force of the greenhouse effect has reasserted itself, and the north has again taken the lead. “The contention by Michaels that model predictions and observed data differ fundamentally is simply incorrect,” says Santer.
Michaels dismisses this. He says the data actually show that the southern hemisphere is not simply warming more slowly, as Santer maintains. It has actually been cooling since 1987, which no models predict. This, he says, makes a nonsense of both the theory of sulphate cooling and the models of global warming – a double-whammy that he relishes.
And, if anything, the more recent raw data quoted by both sides tends to bear Michaels out: the past five years was more than 0.2 °C cooler on average than the previous five. But, says Tett, “that’s probably just natural variability. You can’t dismiss a 30-year warming trend on the basis of a blip at the end.” Again, the jury is out.
Meanwhile, the argument is moving on. In a paper published in Sciencein November last year, Tett added ozone depletion in the stratosphere to the equation. It has been clear for some years that ozone depletion in the stratosphere is causing cooling there. More recently, modellers have suggested that this cooling is being transmitted to the upper levels of the troposphere as well. “Between 1960 and 1995, adding ozone depletion dramatically improves the fit between reality and the model,” Tett said. This is compelling, in particular because it helps explain why the upper troposphere has failed to warm as the models first predicted.
Michaels thinks there might be something in this. But he scoffs that constant tinkering with the climate models to make them fit reality is deeply unscientific. “It’s like a doctor who prescribes an aspirin for a headache and then claims the body’s problem was a lack of aspirin. They don’t have a proper diagnosis.” There is a clash of scientific cultures here.
Stripped of the polemic, Michaels analyses the two possible explanations for the slowness of global warming to date. Either, as the modellers say, the warming is being masked by something else, such as sulphate or ozone depletion. In which case, the mask must eventually slip as the greenhouse effect intensifies. Or, as the sceptics say, the climate is simply less sensitive to the warming effect of greenhouse gases than the models predict. And the positive feedbacks are largely illusory.
The modellers’ belief that they can create the future in climate simulations are undoubtedly shaken by the constant revisions to the models. The problem for the sceptics, however, is that they still lack a coherent story about how the atmosphere is working. And whenever they can find any uncertainty in the way that the atmosphere works, they tend to use this to claim that there will be no problem with greenhouse warming.
As Shine says of Lindzen: “He always falls back on uncertainty. Sure there is uncertainty, but he then claims that all the uncertainty will work in his direction. Why should it?”
Parker admits that “there are a lot of things we don’t know”. But, he adds, that doesn’t disprove global warming, or the models. “Sceptics tend to elevate one element in a complex system above all the others. You cannot do that, however clever you are. You have to integrate every influence to find out what they might mean when all acting together. And the models are the only way of doing that.”
Is there any common ground? Of all people, Michaels insists there could be. “When it comes to it, the modellers and the sceptics are not so far apart,” he says. Indeed, if pressed, Michaels, Lindzen, Spencer and other sceptics suggest a doubling of CO2 in the atmosphere would raise average temperatures by between 1 and 1.5 °C. And 1.5 °C is the bottom end of the modellers’ range of predictions.
But Michaels has, as ever, a twist. “You can’t make a case for a global apocalypse out of a 1.5 °C warming. It destroys the issue. If politics weren’t driving this we could all meet on common ground.”
But, of course, he thinks the politics is all on the other side.
Further reading: see for example “The missing climate forcing” by J. Hansen and others, Phil.Trans. Roy. Soc. London B, vol 352, p 231 (1997)
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Updated on: 19 Oct 2023, 22:24
56%
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