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How often have you heard in a debate about climate change that the fundamental problem is so simple that only the mendacious or ignorant would take issue with it? If the debate is on TV the protagonist will allow themselves a condescending smile and perhaps an exhalation of exasperation. We have known about the Greenhouse effect for over a century (since the time of Jean Baptiste Joseph Fourier, 1768 – 1830) and carbon dioxide is a greenhouse gas. We are putting more and more carbon dioxide into our atmosphere, so by the simple laws of physics, the earth should get warmer. Lo and behold in the 20th century it got 0.7 deg C warmer. QED.

It is of course a debating trick for it oversimplifies the situation so that the audience can grasp it and see its logic whilst the rejoinder, being more complicated than the initial assertion, must fail to convince.

Sir David King often uses this argument. Talking to Greenpeace he said: “It was the British scientist Tindall who did the first measurements on how the atmosphere absorbs energy and discovered, much to everybody’s amazement, that oxygen and nitrogen don’t do any of the greenhouse gas effect. Most of the gas in the atmosphere doesn’t do it and we now understand in detail why that is the case. But what Tindall went on to show was that carbon dioxide and water vapour are the key greenhouse gases in the atmosphere causing the Fourier greenhouse effect. If you therefore extrapolate forward in time and say supposing we’re burning up too much in the way of fossil fuels and we generate more carbon dioxide in the atmosphere, what will that do to the global temperature? 1896, Arrenhius the great Swedish chemist who won the Nobel Prize, Arrenhius did the first calculation – real back of the envelope, pre-computer stuff – and he said what if we burn up so much fossil fuel that we double the carbon dioxide level in the atmosphere, what would the temperature of the earth do? And he concluded that it would rise by five degrees centigrade.”

I will leave to one side that the IPCC doesn’t believe that all the warming in the 20th century was due to mankind (whose influence according to them only came into effect in 1960-80). Tyndall’s 5 deg C is what is generally refereed to as climate sensitivity, how much would the Earth warm with a doubling of CO2.

But as one would expect when it comes to the earth and its climate, what might be simple physics becomes far from simple when operating in such a complex situation.

Firstly what exactly does climate sensitivity mean when expressed in such terms as a global change in temperature? When we look at how the earth is warming we see the limitations of the often-misleading metric of annual global average temperature that seemingly applies equally everywhere. It doesn’t of course. The Northern Hemisphere is warming more than the Southern Hemisphere and if you took away the world’s two main warming regions – the Arctic and the Antarctic Peninsular – then the temperature record for the rest of the world, most of it, would be far less impressive.

The greenhouse effect itself is not simple because of feedback complications, mostly from water vapour. Translating its effect into reality is one of the most complicated problems in climate science. The interplay between carbon dioxide, oceans, temperature, water vapour, solar irradiance, the cryosphere, clouds, etc. mean that any “back of the envelope” calculation has little relevance to the real world outside a debating chamber.

Wide Range

What does the IPCC say about climate sensitivity? Climate models exhibit a wide range of climate sensitivity estimates, Investigations of the computer models used to estimate climate sensitivity found that differences in feedbacks contribute almost three times more to the range in equilibrium climate sensitivity estimates than differences in the models’ radiative forcings.

Indeed, looking at the range of values of climate sensitivity in the models assessed by the IPCC  (23 of them) show the lowest climate sensitivity is 2.1, the highest 4.4. The IPCC concluded that “the likely range for equilibrium climate sensitivity was estimated in the TAR (Technical Summary, Section F.3; Cubasch et al., 2001) to be 1.5°C to 4.5°C.

Interestingly, the range is the same as in an early report of the National Research Council in 1979, as well as the two previous IPCC assessment reports. They all arrive in the same ballpark; 3.8°C ± 0.78°C, 3.5°C ± 0.92°C and in the IPCC AR4  assessment of 3.26°C ± 0.69°C (superfluous accuracy surely).

Subsequently the IPCC adjusted its estimates for climate sensitivity slightly saying, “we conclude that the global mean equilibrium warming for doubling CO2, or ‘equilibrium climate sensitivity’, is likely to lie in the range 2°C to 4.5°C, with a most likely value of about 3°C. Equilibrium climate sensitivity is very likely larger than 1.5°C. For fundamental physical reasons as well as data limitations, values substantially higher than 4.5°C still cannot be excluded, but agreement with observations and proxy data is generally worse for those high values than for values in the 2°C to 4.5°C range.”

But are all the estimates of climate sensitivity derived from computer models consistent with measurements and observations? A recent contribution in the form of a conference presentation says no.

Entitled, “How much CO2 really contributes to global warming? Spectroscopic studies and modelling of the influence of H2O, CO2 and CH4 on our climate.” It is authored by Hermann Harde of the Helmut-Schmidt-Universität Hamburg, Germany.

Although it is only a conference presentation and it would be interesting to see a complete write-up of the work, it is nonetheless thought provoking.

The research details spectroscopic studies on the absorbance of the greenhouse gases water, carbon dioxide and methane in the atmosphere. The objective was to examine and to quantify with newly available data the influence of these gases on our climate.

What the author is saying is that water vapour is a sufficiently strong greenhouse gas at the specific wavelengths where carbon dioxide is important that the earth’s atmosphere is saturated with absorption gases at those wavelengths. If this is so then the addition of more carbon dioxide won’t make any difference.

The IPCC on the other hand make the assumption that the wavelengths at which absorption takes place are far from saturated.

Prof Harde writes: “The simulations for the terrestrial and atmospheric warm-up show well attenuating and saturating progressions with increasing CO2-concentration, mainly caused by the strongly saturating absorption of the intensive CO2 bands and the interference with water lines. The climate sensitivity CS as a measure for the temperature increase found, when the actual CO2-concentration is doubled, assumes CS = 0.41 ̊C for the tropical zone, CS = 0.40 ̊C for the moderate zones and CS = 0.92 ̊C for the polar zones. The weighted average over all regions as the global climate sensitivity is found to be CS = 0.45 ̊C with an estimated uncertainty of 30%, which mostly results from the lack of more precise data for the convection between the ground and atmosphere as well as the atmospheric backscattering. The values for the global climate sensitivity published by the IPCC cover a range from 2.1 ̊C – 4.4 ̊C with an average value of 3.2 ̊C, which is seven times larger than that predicted here.

There are also some clues to the value of climate sensitivity from observations of carbon dioxide in the atmosphere and the global increase in temperature seen in the past century or so. Looking at the carbon dioxide increase along with temperature data would lead one to conclude that in the real world, away from computer models, the climate sensitivity was of the order of 1 deg C. Feedbacks are then postulated to increase this figure to 2.1 deg C.

The question of climate sensitivity is at the heart of the climate change debate. Estimates of it range from 0.4 deg C to 7.7 deg C ranging from the little to profound effect.

A detailed treatment by Professor Harde will be most interesting.