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Would putting all the climate scientists in a room solve global warming...

Skeptics meet Warmists at Cambridge

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A lot of hot air water

The most significant adjustment he'd made, said Jones, had actually had the effect of making late 20th Century warming less dramatic. Sea temperatures were once taken by thermometers in wooden buckets, then canvas buckets, but since 1940, they're sampled on ships' engine intakes. A scientist in the audience said they really should be taking air temperature at sea, not the temperature of the surface water – and disagreed with Jones' assertion that the man-made "signal" in the record is discernible.

Jones said he'd attempted to take the Pacific Decadal Oscillation out of the record but had been heavily criticised for it. "I doubt it's going to be time invariant," he said. "It's strong in some decades, and weak in others".

Jones was followed by Andrew Watson, the most active climate scientist throughout the day, who gave an overview of the case for man-made warming. A doubling of CO2 by itself theoretically adds, if the climate doesn't compensate for it. For example, the biosphere can respond to greater levels of CO2 by sucking more out – or making it worse. By itself, a doubling of CO2 (or the climate sensitivity) adds just 1.1C to global temperatures. Water vapour feedback adds more – around 2W/m2. Watson acknowledged many uncertainties before positing that climate sensitivity – a doubling of CO2 over pre-industrial levels – added 2 to 4.5C to global temperature. "This can be wrong – but it's hard to see how it can be a long way wrong," said Watson.

Other scientists disagree, with Lindzen putting sensitivity at 0.7C, which suggests we've had already had the manmade warming we're going to get. Clouds are poorly understood, and more low clouds means cooling.

The audience also challenged evidence of the causal relationship between CO2 and temperatures. Warmer temperatures mean more CO2 is released through outgassing, and the Antarctic ice core record shows temperature rising, then CO2 following closely behind. Watson said that with feedbacks, it was impossible to separate the two as cleanly as critics would like: more CO2 must surely have an amplification effect, he said.

"There is a good reason to believe ... that climate sensitivity is substantially changing the global climate. Such rapid global change is very rare in the earth's history," he concluded.

The telltale signature of greenhouse gas warming should be warming at the surface and in the troposphere, but not the upper stratosphere. It's even been suggested that carbon taxes should reflect the tropospheric temperature anomaly, rather than surface production. But while the models predict such telltale warming, the observational evidence shows it isn't there. More recently it has been cooling. This really shouldn't be happening.

"We can't explain it ... we have wide uncertainty estimates," acknowledged Watson. "Clouds is a very uncertain area." He again stated that CS was in the range of 1.1C-4C – with more no's from the skeptical side of the room.

Francis Farley, who helped invent microwave radar in World War II, then came on briefly to explain greenhouse gases and feedback effects, concluding "the system is unstable" and "a runaway chain reaction is very likely" - illustrated by a man pushing a large ball down a hill, which then rolled away. Solar physicist Mike Lockwood began with an odd observation: "The stewardship of the planet - and lifestyles - would be much easier if [climate change] was all about the sun." It was a rare example of the IPCC academics letting their intellectual prejudices slip out. Our lifestyles are surely up to us, and policies in response to climate change should be decided coolly and rationally, not handed down as instructions from academic priests. People can get carried away with their own importance at times - particularly scientists.

Lockwood's presentation was quite lucid, though – and surprisingly generous to the next speaker Henrik Svensmark.

"I think it's a lovely idea and I do think it happens, actually. It's very clever. But it happens slowly, and we think it happens in clean maritime air. Over land, there are already enough aerosols present for cloud formation".

(I'll cover Svensmark's work next).

Lockwood outlined the sun's influence: its irradiance (TSI), obviously the primary factor in climate change, and also changes in its magnetic field, which modulate to varying degrees its UV output, and its effectiveness in shielding us from cosmic rays. In a nutshell, Lockwood believes modulations in total solar irradiance contribute around 0.75W/m 2to temperature changes, but around 5Wm/2 is needed to explain them. He acknowledged that solar activity – like the value of your portfolio – can go up as well as down. Tracking the last 24 sunspot cycles he thinks there's an 8 per cent chance of arriving in a Maunder Minimum – a period of low sunspot activity and colder weather – in the next 40 years. The strongest argument, according to Lockwood, for the sun not being a driver in recent climatic activity is that "it has been going in the wrong direction for 30 years".

Lockwood was asked about the recent Shapiro paper (PDF/380KB), published in peer-reviewed journal Astronomy & Astrophysics, which suggested that solar irradiance was much more important than previously thought. Alexander Shapiro, at the World Radiation Centre in Switzerland, looked at magnetic fields as a proxy for TSI and concludes that historical reconstructions have underestimated both TSI and UV – by a factor of six.

Lockwood didn't think much of it. "It's based on the premise that there are small magnetic fields between the sunspots. There's no doubt that over 30 years the trend was downward."

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