Climate scientists are like an exotic tribe â€“ fascinating, sometimes hard to understand and rarely visited. The editor of a science journal warned me that I would find little to interest mainstream readers â€“ the boffins would agree on pretty much everything. The debate, she implied, was over.

She was wrong.Â While climate scientists agree the world is warming due to manâ€™s activities, there are still large areas of conflict, notably over how certain we can be about the predictions.

How did we choose?

Assembling any top 10 is fraught with difficulties. Assemble one for climate scientists and the problem is exacerbated by the complexity of their research and the range of specialist fields that make up a broad science.

Here, we focused on those people who specialise in climate prediction rather than climate changeâ€™s impact on the environment. The criteria for inclusion was original research, influence on peers and sound judgment.

So, while we were willing to consider talking to climate change naysayers â€“ â€œdenier” seems a rather Stalinist term â€“ we found that even their initially compelling arguments were rarely backed up by peer-reviewed research. None made it into the top 10, although since he is a contrarian whose earlier work remains relatively respected, we chose to profile Richard Lindzen, a professor at the Massachusetts Institute of Technology, as well.

Whoâ€™s missing?

One regret is the absence of Chinese experts. Three were approached but either refused to be interviewed outright or failed to make themselves available after seeing a list of questions.

What can we learn?

The fact that experts in the hard sciences disagree about climate change spells trouble for the politicians and policymakers at Copenhagen.

A majority of scientists claim to be limiting their carbon footprint â€“ but measured against the scale of the problem is such individual action meaningful?

â€œIf we believe in free will we can at least make some difference,” says Tim Lenton. Referring to the hundreds of thousands of air miles travelled by climate scientists going to and from professional meetings, however, he adds that â€œwe all have to â€™fess up to the fact that weâ€™re deeply hypocritical and contradictory beings.”

Others argue that the focus on personal behaviour is damaging: â€œHeart-warming stories of volunteers insulating the primary school roof have become a distraction,” says Myles Allen, who believes the future lies in carbon capture and storage.

â€œWe currently enjoy a free lunch, because energy, food and cement prices do not include the costs of decarbonisation. The sooner politicians are upfront with people about this, the better.”


Name: Stefan Rahmstorf
Age: 49
Nationality: German
Position: Head of Earth System Analysis, Potsdam Institute for Climate Impact Research
Expertise: Oceans

Mr Sea Level Rise fixes me with a steely gaze. We are sitting in a coffee shop close to the Oxford college where he is addressing a climate conference with his boss, John Schellnhuber. The two men are popular with the media, but some colleagues complain that they are too close to the policymakers and activists.

Rahmstorf, who is visiting from Potsdam, a university town strong in the sciences, projects a sense of both order and impatience. He argues that the Intergovernmental Panel on Climate Change has massively underestimated the rise in sea levels. Having tracked data for the past 120 years, he found that, above a certain equilibrium, a rise in temperature caused a proportionate increase in sea level. And when he compared the observations against the models for the years 1961 to 2003, the sea level had risen 50 per cent faster (1.8mm per year) than the models had predicted it would (1.2mm).

American oceanographer Carl Wunsch (see below) dismisses Rahmstorfâ€™s findings, saying adequate measurement of the oceans only goes back 20 years: â€œPeople who claim to know what the ocean was doing 100 years ago are telling you fairy tales.” But Rahmstorf insists his curve fits recent figures. Since satellite measurements began in 1993, sea level has risen by 3.4mm a year. The IPCC notes the modelsâ€™ inaccuracy in its full report, he says, yet makes no attempt to alter its projections. Neither will it include estimates for the melting icecaps. â€œA normal person would conclude â€˜maybe we canâ€™t trust the modelsâ€™. But the models are simply taken as gospel for the future.” Rahmstorf believes the oceans will rise three times faster than the IPCCâ€™s highest projections. Assuming a 4Â°C temperature rise by 2100, that would also mean a rise in sea level of 1.2 metres.

Rahmstorf believes that unless emissions are under control by 2020, there will be no time to prevent warming going above 2Â°C. A member of the German governmentâ€™s Advisory Council on Global Change, he has issued a plan for how we can avoid such a rise. It gives every nation in the world a carbon allowance for the years up to 2050.

At current consumption levels, the US would exhaust its allowance by 2016. To give them time to adjust, rich countries can pay developing nations to share their allowance. Rahmstorf believes the payments will enable developing economies to â€œleapfrog the dirty model” practised by industrialised nations in the last century.

Personal stance: No air travel for holidays. Doesnâ€™t own a car and cycles to work. His house is insulated, cutting energy use by 60 per cent. His â€œsins” are buying bananas and having a long shower every morning.


Name: John Mitchell
Age: 61
Nationality: British
Position: Director of Climate Science, Met Office
Expertise: Climate modelling

After 36 years at the Met Office, including a spell writing the shipping forecast, John Mitchell has a weather-beaten mind. In person, there is no smug air of foreknowledge. Yet this softly spoken rugby fan from County Down in Northern Ireland is one of the most experienced climate modellers in the world.

Modelling is a contentious area. Thereâ€™s so much we donâ€™t fully understand about the climate â€“ clouds, hurricanes, ice melting, aerosols â€“ that questions remain over the ability of super-computers to model future climate. One experiment â€“, which generated thousands of versions of the Met Officeâ€™s climate model and got people to run them on their home PCs â€“ showed that models will produce radically different results if you make minor adjustments to their parameters. The week before I met Mitchell, the Met Office was forced to defend its â€œbarbecue summer” press release â€“ proving the difficulty of making even seasonal predictions.

Mitchell takes such criticism calmly. â€œEssentially, a model is based on Newtonâ€™s Laws of Motion. All weâ€™re trying to do with the models is quantify [the warming] better and say what the regional changes are.”

In practice, that means taking balloon and satellite measurements for temperature, winds and humidity in the atmosphere and using equations to give you a rate of change. It is always going to be an approximate picture but he insists there is a â€œstrong resemblance” with what happens in reality.

Big obstacles remain. â€œThereâ€™s so much variation out there,” he says, pointing through the window. â€œHow do you model that? Clouds are the â€˜Achillesâ€™ heelâ€™ of modelling.”

But modelling power is increasing all the time. This summer, the government gave people access to detailed regional climate projections for the coming decades. The modelling was done by the Met Office â€“ and though many scientists are uncomfortable with such â€œpostcode predictions”, saying they send out an overconfident message about our understanding of the future, Mitchell is proud that the US is now considering following suit.

Personal stance: Takes small steps â€“ turning down the thermostat and unplugging his mobile phone charger.


Name: Rajendra Pachauri
Age: 69
Nationality: Indian
Position: Chair, Intergovernmental Panel on Climate Change
Expertise: Energy efficiency

Two years ago Dr RK Pachauri shared with Al Gore the Nobel Peace Prize for their work on climate change. If Gore was the populist showman, Pachauri was the diligent committee man, accepting the prize on behalf of the IPCC (which he has chaired since 2002).

The IPCC is in effect a huge machine for reaching scientific consensus on global warming. From its creation in 1988, it has brought together thousands of the worldâ€™s climate experts to draft reports on the latest authoritative data and predictions. At the time of Pachauriâ€™s nomination for the post, Gore called him the â€œletâ€™s drag our feet candidate”. Yet while he is not strictly a â€œclimate scientist”, he plays a crucial role in creating consensus, particularly among the worldâ€™s developing nations.

Pachauri rejects the view that population is as big an issue as reducing per capita emissions. â€œIn North America,” he argues, â€œthey have emissions of over 20 tonnes per capita per year, while in Bangladesh itâ€™s half a tonne.”

But surely the point is that South Asiaâ€™s per capita emissions will catch up? â€œThatâ€™s precisely why a country like India should not allow its emissions to reach anywhere near [those of] the developed world.” He argues that India will have a very different pattern of development to the west with â€œlarge-scale mass transport systems” instead of cars, and energy-efficient buildings.

Meanwhile, industrialised nations will have to cut back fast. â€œMy big concern,” he says, â€œare the people living in several regions of the world where there is no infrastructure, no capability or financial strength to deal with the impact of climate change.”

Personal stance: Has given up meat, and only uses air-con on a moderate setting.


Name: Myles Allen
Age: 44
Nationality: British
Position: Head of Climate Dynamics, Department of Physics, University of Oxford
Expertise: Statistics and modelling

Myles Allenâ€™s young son John, who has been patiently reading in the corner, gets up and writes on the office whiteboard: â€œMyles is a penguin.” His father thanks him wryly, before launching back into his discourse on carbon sequestration. But there is something of the penguin about this freethinking statistician, who combines upper-crust vowels with the awkward charm of Griff Rhys-Jones.

Allen believes the current approach to tackling climate change is ill-conceived. His latest paper, published earlier this year in Nature, argues we should be looking at the total amount of carbon that humankind emits, not the rate at which we do so â€“ the measure the negotiators in Copenhagen will be focusing on.

He shows me a graph with three different curves representing the same amount of carbon being emitted over different timescales. â€œTemperature response is identical,” he says. â€œItâ€™s much easier to frame the problem if you say â€˜whatâ€™s the total amount of carbon we can afford to inject into the atmosphereâ€™ rather than what concentration should we be aiming for.” Yet because of the way the Rio Earth summit and Kyoto Protocol configured the problem, negotiations at Copenhagen are heading in the wrong direction.

Journalists are always asking Allen to say that unless emissions peak by 2015 we will pass a tipping point. He wonâ€™t, because he says such claims are nonsense. Worse, to pretend such arbitrary targets have a scientific basis will only disillusion the public once they find out the targets are going to be missed.

But isnâ€™t it necessary to tell white lies in order to get what you want? â€œThereâ€™s just no point,” he says, looking over at his son before adding: â€œTheyâ€™ll find you out!”

Personal stance: Cycles to work but loves Top Gear â€“ â€œfossil fuels are fun”. Believes focus on personal behaviour is a distraction.


Name: Tim Lenton
Age: 36
Nationality: British
Position: Professor of Earth Systems Science, University of East Anglia
Expertise: Earth systems science

Tim Lenton is James Lovelockâ€™s anointed successor. As an undergraduate he read Lovelockâ€™s The Ages of Gaia (a book which argues that the earth and its biosphere function as one complex system), and subsequently wrote to the author saying he wanted to do research in his field. It led to an â€œamazing” meeting â€“ and the two have stayed in touch.

Today the protÃ©gÃ© takes a different line to the mentor on climate change. Both are worried about positive feedbacks from the climate system â€“ a domino effect of warming â€“ but Lenton does not share Lovelockâ€™s apocalyptic vision. â€œWeâ€™ve agreed to disagree about the level of hope we should have for the future,” he says.

We are talking in the front room of his house in a quiet residential street in Norwich. Hardly the climate change frontline. And yet, in 2007, the freshwater Norfolk Broads were nearly flooded after storm surge conditions at sea. â€œAt some point â€“ maybe in the next 50 years â€“ weâ€™ll see the Broads revert towards a salt marsh ecosystem,” he says.

While others work in decades or centuries, Lenton thinks in millions of years. The book heâ€™s working on chronicles Gaiaâ€™s instability, focusing on abrupt changes such as a leap in the amount of oxygen in the atmosphere 2.5 billion years ago. â€œMy question is: â€˜are we at the start of another revolution?â€™ï¿½? He guesses that by the time his son James is 80, we will either be recycling energy and materials, or be on the way to a world of â€œunpleasantriesï¿½?.

Personal stance: Keen cyclist but also drives a Renault Clio. Holidays in the UK, but his wife is from New Zealand so they are using up a hefty dose of â€œlove miles” this Christmas to see family.


Name: Kevin Trenberth
Age: 65
Nationality: Dual New Zealand and US citizenship
Position: Head of climate analysis, National Centre for Atmospheric Research, Colorado
Expertise: Hurricanes

There is a big debate about whether climate change is causing more devastating storms. Some say the increase in hurricanes could simply be the result of natural variability.

New Zealander Kevin Trenberth, on the other hand, is sure of the impact that climate change is having on hurricanes: he says it is causing more devastating storms. But heâ€™s less sure what effect hurricanes have on global warming.

Hurricanes tend to cool things down. They leave a cold wake in oceans, and the extra rainfall they bring helps keep the land a bit cooler, too. Climate models do not currently have a good grip on this cooling effect â€“ so the projections for tropical sea temperatures in them may well be higher than they should be. Therefore, argues Trenberth, the number of projected future hurricanes is probably being overestimated.

That said, when the storms do occur, he is certain theyâ€™ll be of greater intensity. Water vapour in the atmosphere helps fuel storms â€“ and according to Trenberth it has increased by 4 per cent over the ocean since 1970. He traces that rise to the 0.6Â°C rise in global temperatures in that same period; as temperatures rise, the atmosphere is able to hold more water vapour.

Many scientists argue that his native New Zealand will be largely protected from climate changeâ€™s most severe effects â€“ thanks to its maritime climate. But Trenberth points out that energy price hikes will drive up shipping costs â€“ and if as a result other countries start growing their own food, exports will be hit hard.

Personal action: He has fitted solar panels, a new heat pump and water heater in his home.


Name: Chris Rapley
Age: 62
Nationality: British
Position: Director of the Science Museum
Expertise: Earth systems science

As a man who has moved between the different branches of climate science, Chris Rapley is well placed to judge his colleagues: â€œThereâ€™s a whole spectrum of views out there,” he says. â€œSome of the science is clear-cut,” but there are some big uncertainties. Will all the methane in the permafrost be released? What will be the rate of warming? Nobody knows what the world will look like in 50 years, but Rapley is confident it will be a warmer place facing a thousand years of rising oceans.

In 2005, he contributed to the Exeter conference on â€œavoiding climate change” commissioned by Tony Blair. His paper warned that the vast ice sheet on the western side of Antarctica might be starting to break up. As a result, he said, the IPCC would have to revise its prediction that the ice sheet was safe for 1,000 years. The following year he told the American Association for the Advancement of Science: â€œOnly five years ago, Antarctica was characterised as a slumbering giant in terms of climate change. I would argue that it is now an awakened giant, and we should take notice.”

Recently heâ€™s become more pessimistic about our ability to prevent serious warming. While there is no shortage of human ingenuity, rhetoric or Nobel Prizes, the emissions trajectory has not turned downwards. â€œUnless Copenhagen leads to deceleration in the emissions curve then I think weâ€™re going to have to be very pessimistic about the future,” he says.

He is not a deep green eco-warrior. Heâ€™s a Formula 1 fan and describes Jeremy Clarkson as a â€œreally intelligent guy and great supporter of the museum.” But he wishes the Top Gear presenter would tone down his views on climate change, which he fears encourage people to cock a snoop at â€œthat global warming shit.” The stakes are too high for such pantomime acts.

Hundreds of years from now Rapley believes London, New York and Amsterdam will be under water. But it is the short-term, rapid deadlines of the modern world that scare him most. â€œWe saw with the economic crash it takes hours for things to go wrong. And when [in 2000] the petrol tankers went on blockade, there was a very frightening timescale â€“ about a week for the supermarkets to run out of food, fear of food riots and civil disorder.”

He â€œhates being apocalyptic” but says his grandchildren will have to choose how to minimise their exposure to the risks. â€œYouâ€™ll find serious people talking about buying an AK47 and a small farm in northern Canada or Scandinavia, forming a commune to grow food and protect themselves in case the wheels come off society.”

Personal stance: Wanders around the museum switching off lights. But believes corporations hold the key to cutting emissions.


Name: Susan Solomon
Age: 53
Nationality: American
Position: Senior scientist, National Oceanic and Atmospheric Administration, Colorado
Expertise: Atmospheric chemistry

Susan Solomon was one of the first scientists to explain why the ozone hole was forming over Antarctica, and in 2007 she oversaw the historic Intergovernmental Panel on Climate Change report which said it was 90 per cent certain that the bulk of global warming that has occurred since 1950 had been man-made. Still, she believes in keeping the science â€œpureï¿½? and rejects the idea that her profession has a duty to speak out on global warming.

This conservative approach frustrates activists and some fellow scientists. In the reportâ€™s sea level projections, she excluded estimates for the impact of melting ice caps because modelling such a process is fraught with uncertainty. Neither would she discuss the policy response or make emotional appeals.

Almost three years on, Solomon believes sheâ€™s been vindicated: â€œThereâ€™s an enormous amount of emotion on this subject, influenced by both the left and right.ï¿½? The job of the IPCC is to present the science with the minimum of speculation, she says.

She is convinced that unless emissions are seriously reduced, the world in 2060 will be between 2Â°C and 3Â°C hotter. A deal will be tough she says, contrasting her own tribe with the political animal. â€œIf you sit 20 politicians in a room you wonâ€™t get them to agree on anything,ï¿½? she says. â€œBut bring scientists together, and at the end of a sensible discussion of the data youâ€™ll all agree.ï¿½?

But Solomon is hopeful that public pressure will bring about an eleventh-hour breakthrough at Copenhagen. â€œWhenever I hear a person say this or that is impossible, I think back to the ozone layer days when people said it would be impossible to phase out CFCs from deodorant.ï¿½?

Asked about the life her step-sonâ€™s generation can be expected to live, she flares up briefly, arguing that such emoting is unhelpful: â€œI donâ€™t know which way weâ€™re going to come out. But I want the scientific historians to be clear: scientists had primitive tools and limited information, but they played this one right.ï¿½?

Personal stance: Tries to eat vegetarian twice a week, cycles, drives a Prius.


Name: Carl Wunsch
Age: 68
Nationality: American
Position: Cecil and Ida Green Professor of Physical Oceanography, Massachusetts Institute of Technology
Expertise: Oceanography

To many scientists, Carl Wunsch is the worldâ€™s top oceanographer. Yet to the layman he is best known as the professor duped into appearing in Channel 4â€™s 2007 programme The Great Global Warming Swindle. Wunsch was horrified to see his nuanced statements taken out of context to back the programmeâ€™s dodgy premise. The broadcasting regulator Ofcom subsequently upheld his complaint and Channel 4 was obliged to broadcast a summary of its findings.

In reality, Wunsch accepts the evidence for climate change, though he worries about what he considers overconfident predictions â€“ and he may be unique amongst senior colleagues in having turned down the Intergovernmental Panel on Climate Changeâ€™s invitation to participate. He accepts the organisationâ€™s usefulness but believes it moves too fast for the available data and has played down some of the uncertainties to catch the eyes of politicians.

On the other hand, Stefan Rahmstorf (see above) says Wunschâ€™s comments on the show were not only irresponsible but wrong. â€œThey feed the message that the [film] makers wanted to portray.ï¿½? Perhaps understandably, Wunsch and Rahmstorf have little time for each other. Early in our conversation, Wunsch bemoans the fact that some of his colleagues are guilty of â€œoverstatingï¿½? what we know to fit the mediaâ€™s demand for simple answers.

â€œOften the right answer is â€˜we donâ€™t really know but weâ€™re worried about the possibilitiesâ€™.ï¿½? The ocean has a long memory, he says, yet our observations of it are â€œgrossly inadequateï¿½?. Unless you understand what is happening in the south Atlantic or northern Pacific, you are flying blind, he argues.

Accordingly, Wunsch takes a small-picture view. â€œWhy has Arctic sea ice recovered partially in the last two years?ï¿½? he asks. â€œWhat does this mean for the â€˜tipping pointâ€™ that people talk of?ï¿½?

In essence, he wishes people would stop making projections that read like strong probabilities and instead see climate change as â€œan insurance problemï¿½? â€“ where we donâ€™t know what will happen but acknowledge there is the potential for something serious to occur, and take precautions.

Like many scientists, he is less inhibited when straying from his patch. Stabilising world population should be the number-one priority and yet the politicians are â€œtoo spinelessï¿½? to talk about such a thorny issue. â€œIf the world goes to 9 and then 12 billion it wonâ€™t matter what the climate does,ï¿½? he sighs. â€œItâ€™ll be catastrophic.ï¿½?

Personal stance: Travels mainly by public transport but also drives a Prius. He has insulated his home.


Name: Isaac Held
Age: 61
Nationality: American
Position: Senior research scientist at the Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration. Professor at Princeton
Expertise: Climate dynamics

Isaac Held was born in a German refugee camp three years after the end of the second world war, to a mother who had survived Auschwitz. Several years later, the family secured their passage to the US and began a new life in Minnesota.

Held did a PhD in physics at New York State University (Stony Brook) but was lacking inspiration until he read a paper on carbon dioxide and climate by Syukuro Manabe, today regarded as one of the pioneers of climate modelling. Held moved to Princeton where he has remained for the past four decades. Today, he is one of the worldâ€™s foremost experts on predicting regional climate change. As the German oceanographer Mojib Latif says: â€œHeâ€™s not as visible to politicians as others, but heâ€™s one of the best scientists â€“ someone who really understands how the whole climate machine works.ï¿½?

He believes that changes in rainfall patterns will have the most direct impact on peopleâ€™s lives. In simple terms, he says, â€œthe wet places will get wetter and the dry drierï¿½?. In the Mediterranean â€“ one of the easiest regions to model â€“ a 3Â°C temperature rise would cut rainfall by 20 per cent. The real uncertainties lie in the tropics, although Held suspects that the region will be one of the biggest losers from climate change. His research on drought shows how interconnected the climate system is. The dramatic decline in rainfall over the Sahel â€“ a horizontal sub-Saharan strip that stretches from Senegal to Eritrea â€“ during the 1970s and 1980s has been commonly ascribed to overgrazing.

Heldâ€™s research suggests the real cause has been the warming of the South Atlantic, Indian and Pacific oceans during the latter half of the 20th century. Together with the hole in the ozone layer, the warmer oceans have had a similar impact on Australiaâ€™s rainfall. Unfortunately, itâ€™s not possible to know which is the dominant factor for Australia, he says. If itâ€™s the ozone layer, then the situation should gradually improve. Otherwise, the land will get drier still.

Held follows the maxim of the medieval philosopher William of Ockham â€“ forget complicated hypotheses if the simple explanation fits. â€œThe case has not been made for any specific tipping point,ï¿½? he says. â€œWe havenâ€™t seen that irreversibility. The climate system has so far behaved much as one would expect it to. Climate change means a steady warming with the impacts of warming growing bigger over time.ï¿½?

Others, such as Latif, argue that weâ€™re in for a decade of cooling before the warming trend reasserts itself. Quite plausible, Held replies, saying that natural variability may have been underestimated in the rapid warming of the last 30 years. But in the long term, global warming will continue.

Personal stance: His wife has set up a farmersâ€™ market, they eat locally produced food and drive a hybrid car. Says he could join a car pool but prizes his freedom.


The sceptic

Name: Richard Lindzen
Age: 69
Nationality: American
Position: Alfred P. Sloan Professor of Meteorology, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology
Expertise: Atmospheric dynamics

Richard Lindzen is by turns charming and cantankerous. â€œThat is the stupidest question Iâ€™ve ever heard!ï¿½? he yells when I ask whether global warming is occurring. Later, as I outline the laymanâ€™s notion of what warming means, he barks: â€œStop this bullshit!ï¿½? When I cite a spokesman for the Intergovernmental Panel on Climate Change saying the world warmed by 0.6Â°C over the 20th century, he puts it down to the natural variability of the climate.

Raised in the Bronx and educated at Harvard, Lindzen is the most respected of the climate change naysayers among other scientists. Arguing that CO2â€™s impact on climate has been overstated, he says that emissions can keep growing with no consequences. The pre-industrial level of CO2 was 280 parts per million, we are now at 380ppm and the negotiators at Copenhagen want to stabilise it at about 450ppm. But Lindzen says we can safely climb past 10,00ppm. Even if for some reason there is an impact, we will have ample time to see it coming, he claims.

He points out that, contrary to what the models predicted, the world has not warmed over the past decade. Weâ€™ve underestimated natural variability, he says. And in the context of â€œonlyï¿½? 0.6Â°C of warming, the argument that man has caused most of it is not tenable. His critics say his emphasis on the past 10 years is a red herring. Assertions of climate change were not based on a 10-year trend but on a 30- to 50-year period.

Lindzen is convinced that global warming will one day be exposed as a con. â€œI hope,ï¿½? he says, â€œitâ€™s in my lifetime.ï¿½?

Personal stance: Drives a small car, uses energy-saving lightbulbs and says he probably uses less energy than â€œthe climate change activists in Washington with their Mercedes.”

This article appeared in the FT Weekend magazine on 20 November 2009

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