Friday, 13 March 2015

Review of BBC4's 'Climate Change by Numbers', March 2015

Presented by three mathematicians - Dr Hannah Fry, Prof Norman Fenton (Statstiician) and Prof David Spiegelhalter - it hones in on just three key numbers that clarify all the important questions around climate change:-
·         0.85 degrees (the amount of warming the planet has undergone since 1880)
·         95 per cent (the degree of certainty climate scientists have that at least half the recent warming is man-made)
·         1 trillion tonnes (the total amount of carbon we can afford to burn - ever - in order to stay below 'dangerous levels' of climate change)

I studied Mathematical Physics very many years ago and had a career in computer engineering. I'm well aware that statistics and numerical computational solutions can give misleading answers, therefore require testing in the real world (which we can't do with climate) so was very interested to view this programme. 

It covered lots of uncertainties and issues that need working around. The history of how the differing techniques were developed illuminated the problems well.  This is good because far too often we get told simplified analogies about the science which numerate and educated people can be deeply suspicious of – like I am.

My review covers a few issues about the presentation for each number, and follows with a few more numbers that I would like investigated.
·         200-300% Amplification by water vapour of the carbon-dioxide heating
·         0.2% of the total Carbon Flux in the atmosphere
·         the average ammount that Minimum & Maximum Temperatures have increased

The First Number:- 0.85 degrees Centigrade Warming Since 1880
Question:-       Is 0.85 deg. Centigrade unusual?
Answer:-         No. The previous 170 years also warmed at about the same rate. (I applaud the presentation in explaining that, as we go back in time, the accuracy of known temperatures declines so that we have talk in approximations.) Unfortunately, noone knows why the warming of 300 years ago started, and continued in fits and starts. Thus there's no way to show that the same natural warming isn't happening now.

The Second Number – 95% Certainty that Human Carbon-Dioxide Emissions Contribute at least Half of Recent Warming
As a statistician, Prof. Fenton should have explained that just because 2 things happen at more or less the same time, (i.e. in his example, winning and football club wage bill; for climatology, carbon dioxide and temperature) doesn't mean one causes the other or even that they have the same cause.  In fact, in the case of carbon dioxide, it's a 'confounding' variable which is expected to increase due to warmth. As the oceans get hotter, carbon-dioxide is transferred to the atmosphere (and the sea holds correspondingly less carbon-dioxide, becoming more acidic).  Another number that would be really interesting to look at is the % of the carbon flux that is due to humans.  I understand it's tiny, ~0.04%
Prof Fenton's presentation used a Bayesian Statistics approach which showed that the computer models depended on human carbon dioxide emissions creating more than half of the recent warming. The impression given was that observational data from the real world supports this conclusion.  There was no investigations into how good the models were at modelling past temperatures, nor of the methods of model 'tuning' that take place.
A Computer Model output of the troposphere was shown indicating additional heat over the tropics is expected from human carbon dioxide warming but not from natural variability.  If observed for real, it would be decisive. However, it's not present in the real atmosphere!
… I'm really disappointed that it was unclear that the model results hadn't been checked against reality.

The Third Number – Total Amount of Carbon we can burn to stay below 'dangerous levels' of climate change

First, you have to show that it could be dangerous.  A really important number was omitted – an expected 200-300% amplification by water vapour of the basic carbon-dioxide heating.  This is unproven and is probably the most contentious thing in climate science because, without it there's no catastrophe.

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