My first post on HuffPost UK blogs. On Climate Sensitivity

Wahey. Today is my first day as a HuffPost UK blogger.  It is a happy day for me.  My first blog on HuffPost UK is over here, but for all those who cannot tear yourself away from the Mabinogogiblog, here it is:

Can Popper Resolve the Global Warming Debate?

While about 97% of climate scientists accept that man-made global warming (AGW) is a real threat to humanity, the public is in two minds as to whether climate change is a real threat. This is the result of a highly effective campaign by the climate change "sceptics", who use mainstream and social media to attack each and every point made by climate scientists. This has caused journalists and broadcasters to bracket climate change as "controversial", and therefore to feel obliged to balance any climate scientist with a speaker for the sceptics. This in turn confuses Joe Public, who tends to think, "Well, the truth must be somewhere in between".

Democratic politicians are nervous of introducing radical measures when there is no overwhelming public enthusiasm, and so the uncertainty in the mind of the public translates into inadequate action from politicians in matters like energy conservation and transition to renewable energy.

This journalistic balancing act struck between science and sceptics would be right and proper if there were an equal balance of scientific evidence for either point of view, but it is not right to balance science with ideologically driven attacks from a point of view which lacks any real scientific basis. Journalists do not feel the need to balance every scientific statement about evolution by giving equal airtime or column inches to creationists.

Are climate sceptics in the same category as creationists? They could be. We are going to have to get technical here - but the only alternative is to take the scientists at their word, and this the sceptics are unwilling to do. Therefore we have to understand the science, starting with what we know.

There is a common misconception that science "proves" things. It does not. The great philosopher of science, Karl Popper, showed that the best status that any scientific statement can achieve is not "proven", but "not yet disproven". Refutation is central to science. If a statement is not capable of refutation, it cannot be a scientific statement.

The central claim of the climate sceptic is that CO2 does not seriously affect the global climate. In scientific terms, this translates into the hypothesis that climate sensitivity is low - that is, the global temperature does not rise significantly when more heat is put into the system.
Let's start from what we know:
· We know from textbook physics that CO2 is a greenhouse gas - that is, it retains heat within the atmosphere.
· We know from measurement that since the industrial revolution (taken as 1750), the levels of CO2 in the atmosphere have increased from 280ppm to 394 ppm.
· We know from radioisotope studies that it is our fossil fuels that are responsible for this increase.
· We know from basic textbook physics that this increase in CO2 alone is sufficient to push up temperatures by about 1ºC.

OK? All straightforward, uncontroversial physics so far - albeit intensely simplified.
Now we come to Climate Sensitivity (CS).

CS is the amount by which the average temperature of the atmosphere will eventually increase from a doubling of CO2. The extra warming comes from positive feedbacks - factors that are affected by the initial warming, which in response will produce further changes.

· Warmer air holds more water vapour, which itself is a greenhouse gas, so further warming takes place. That is a positive feedback.
· Cloudiness will increase, which has a complex effect. High clouds produce cooling, by reflecting incoming solar heat, while lower clouds hold heat in by a blanket effect. The net result is a small net warming.
· Further down the line, as warming takes place, melting ice loss means less reflected heat, another positive feedback.
· Methane, another greenhouse gas that is far more potent than CO2, will escape from unfrozen tundra, which is a long term positive feedback.

All of these factors - and more - have to be entered into computer models to find how they interact.

Climate science has applied a great deal of effort to the question of CS. Multiple lines of evidence - from models, observations of known temperature changes, and proxy records from ice cores - show remarkable convergence towards a value of 3ºC (+/- 1.5º) C for a doubling of CO2 in the atmosphere. In particular, it looks as if a value less than 1.5ºC is very unlikely.

In science, when there is this kind of convergence onto a value from disparate lines of enquiry, the value should be accepted unless very strong evidence is brought against it.

There are a small number of papers by sceptics which claim a low value around 0.5ºC. These have been comprehensively criticised on various grounds by the climate science community - indeed, they would say "refuted" - but because of the technical nature of the argument, and the media-shy nature of scientists generally, this part of the debate rarely emerges into the public domain.

It is time for the CS debate to be brought out into the open in order to show journalists, commentators and opinion formers that the position of the sceptics lacks any scientific credibility. Instead of defending their position against an endless series of attacks by the sceptics, the scientists should take the fight to the sceptics, and will be able to show that their case is disproven.

That way, we will be able to move on from the debate, and will be able to make the changes necessary to ensure that our grandchildren can live secure lives.