ANOTHER CASE OF THE MANN KEEPIN' US DOWN:

In the run-up to the Paris conference, said Curry, much ink has been spilled over whether the individual emissions pledges made so far by more than 150 countries -- their 'intentional nationally determined contributions', to borrow the jargon -- will be enough to stop the planet from crossing the 'dangerous' threshold of becoming 2°C hotter than in pre-industrial times. Much of the conference will consist of attempts to make these targets legally binding. This debate will be conducted on the basis that there is a known, mechanistic relationship between the concentration of carbon dioxide in the atmosphere and how world average temperatures will rise.

Unfortunately, as Curry has shown, there isn't. Any such projection is meaningless, unless it accounts for natural variability and gives a value for 'climate sensitivity' --i.e., how much hotter the world will get if the level of CO2 doubles. Until 2007, the UN Intergovernmental Panel on Climate Change (IPCC) gave a 'best estimate' of 3°C. But in its latest, 2013 report, the IPCC abandoned this, because the uncertainties are so great. Its 'likely' range is now vast -- 1.5°C to 4.5°C.

This isn't all. According to Curry, the claims being made by policymakers suggest they are still making new policy from the old, now discarded assumptions. Recent research suggests the climate sensitivity is significantly less than 3˚C. 'There's growing evidence that climate sensitivity is at the lower end of the spectrum, yet this has been totally ignored in the policy debate,' Curry told me. 'Even if the sensitivity is 2.5˚C, not 3˚C, that makes a substantial difference as to how fast we might get to a world that's 2˚C warmer. A sensitivity of 2.5˚C makes it much less likely we will see 2˚C warming during the 21st century. There are so many uncertainties, but the policy people say the target is fixed. And if you question this, you will be slagged off as a denier.'

Curry added that her own work, conducted with the British independent scientist Nic Lewis, suggests that the sensitivity value may still lower, in which case the date when the world would be 2˚C warmer would be even further into the future. On the other hand, the inherent uncertainties of climate projection mean that values of 4˚C cannot be ruled out -- but if that turns out to be the case, then the measures discussed at Paris and all the previous 20 UN climate conferences would be futile. In any event, 'the economists and policymakers seem unaware of the large uncertainties in climate sensitivity', despite its enormous implications.

Meanwhile, the obsessive focus on CO2 as the driver of climate change means other research on natural climate variability is being neglected. For example, solar experts believe we could be heading towards a 'grand solar minimum' -- a reduction in solar output (and, ergo, a period of global cooling) similar to that which once saw ice fairs on the Thames. 'The work to establish the solar-climate connection is lagging.'

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