I still have some science to cover – but I’d like to take an aside and write something about the history of our understanding of the climate science.

Joseph Fourier, in 1820, was the first person to realise what the very simple calculation that I described in Climate Lesson 4 that calculates that the temperature of the Earth “should be” much colder than it is. Blackbody radiation would not be fully understood for another 80 years, so his calculation was based on somewhat different premises, and you can read those for yourself (in old-fashioned French) in his paper. He recognised that somehow the incoming radiation must make it through the atmosphere easily, but that the outgoing radiation from the Earth would be blocked in some way by the atmosphere.

In the 1850s, John Tyndall was able to measure the amount of heat absorbed by different atmospheric gases and he concluded that the “Greenhouse effect” that Joseph Fourier had surmised was dominated by water vapour absorption and that carbon dioxide had a smaller, but observable heating effect too.

Svante Arrhenius, in 1896, published a significant paper “On the influence of Carbonic Acid in the Air upon the Temperature of the Ground” (available in full here – this one is in English). In this he calculated that doubling the amount of carbon dioxide in the atmosphere would lead to a temperature rise of around 4 ºC.  I’m amused by how he starts his discussion section with “I should not have undertaken these tedious calculations if extraordinary interest had not been connected with them…” The extraordinary interest was to understand the causes and effects of natural climate variations during and between ice ages, but he already realised:

“The following calculation is also very instructive for the appreciation of the relation between the quantity of carbonic acid in the air and quantities that are transformed. The world’s present production of coal reaches in round numbers 500 millions of tons per annum … Transformed into carbonic acid, this quantity would correspond to about a thousandth part of carbonic acid in the atmosphere …

In a later book he would go on to say that burning coal would have a positive effect on the planet as it would stop the next ice age and would allow more crops to grow (I assume as he was living in Sweden, that he could only imagine warming in a positive way). He did, however, think it would take a 1000 years for humanity to double the carbon dioxide in the atmosphere – he assumed a linear, rather than exponential, increase in our burning of coal (we are on track to have doubled it in 150 years).

[The IPCC AR5 report (see page 82 in the Technical Summary) in 2013 stated that the “Equilibrium Climate Sensitivity” (impact of a step doubling of CO₂ in the atmosphere and the planet going into equilibrium thereafter) is “likely between 1.5 ºC to 4.5 ºC”.]

But Arrhenius’s paper was met by a strong criticism by Knut Ångström. Ångström, and his assistant “Herr J Koch”, were doing absorption experiments with carbon dioxide and realised two things that seemed to suggest problems in Arrhenius’s work. First, they changed the amount of carbon dioxide in glass tubes and measured how much infrared radiation was absorbed. Their measurements suggested that carbon dioxide absorption saturated very quickly, meaning that very quickly all the infrared was absorbed and increasing the amount of carbon dioxide made no difference beyond this point.

Even more convincingly, they also showed that water vapour had absorption bands that overlapped the carbon dioxide bands – meaning that those wavelengths were already completely absorbed by water vapour.

This time – around the turn of the 20th Century – was a time when there was a real “greenhouse gas debate”. These two excellent scientists were arguing about confusing evidence and an incomplete and necessarily highly simplified conceptual model of the Earth system.

The assistant Koch’s observations actually didn’t show that there was no difference in absorption as the carbon dioxide was increased – he saw a 0.4 % decrease, which Ångström dismissed as trivial. (Modern calculations suggest he should have seen a 1 % decrease, and this suggests that Koch and Ångström underestimated their uncertainties). 

Arrhenius published a long response (this time in German) to explain why Ångström was wrong – he apparently (I haven’t been able to access the full text) correctly realises that Ångström was oversimplifying his analysis – the spectral bands of water vapour and carbon dioxide do not fully overlap (we also now know carbon dioxide absorption is not fully saturated), but most importantly, the atmosphere is not like a single thin sheet of glass – it has layers, and while the lower layers may mostly absorb the infrared, the outer layers are drier (less water vapour) and the atmosphere itself emits thermal infrared radiation.

Other scientists seem not to have noticed, or understood, Arrhenius’s 1901 paper, and the assumption that Ångström had proven Arrhenius wrong limited research in this area for many decades. Furthermore, there was growing recognition that the Earth itself could, and would, regulate any increase in carbon dioxide by absorbing it mostly in the ocean, and, with any that the oceans didn’t absorb, in increased growth of trees, peat bogs and so forth. The Earth would sort itself out, there wasn’t that much coal anyway and we weren’t (then) burning it fast enough for there to be a problem. (We now know that there are limits to that absorption too – I’ll come back to that).

It was Guy Stewart Callendar who, in the 1940s and 1950s, revitalised the Arrhenius ideas. He, as a hobby, started compiling temperature measurements since the 19th century and started to see an upward temperature trend (we now know that trend was not based on the relatively low increase in carbon dioxide, but on natural effects). To understand this he re-investigated the absorption of carbon dioxide and newer observations that provided more detailed spectroscopy of carbon dioxide absorption; he started to make a coherent model of the atmospheric effect. His papers influenced scientists to start systematic measurements of carbon dioxide in the atmosphere (although he also got a lot of criticism). Charles Keeling started taking Mauna Loa observatory measurements in 1958 as a response (see my earlier blog on that).

Now my opinion on all this: I’ve been reading climate sceptic blogs and webpages and many of them gleefully say that “the first climate alarmist Arrhenius, who was an amateur scientist, was proven wrong by the much better scientist Ångström…” In this they are misunderstanding the whole scientific method (and confusing Ångström with his father). Both Arrhenius and Ångström were good scientists who were working on limited information, poor models and experiments that were in their very early days. Both made mistakes of understanding – but both also showed new concepts that were essential pieces of the jigsaw that more recent scientists have put together. Most importantly – this argument is over – we now understand what neither of those scientists understood, we have better observations of everything from the absorption spectra of carbon dioxide and water (using similar  experiments to those of Ångström and Koch, but with more sophisticated analyses) to the atmospheric composition and we have models that split the atmosphere into far finer levels than Arrhenius imagined, and which also include clouds and atmospheric circulation (that he couldn’t include).

Oh, and as a personal note, when I was at Imperial College in the mid 1990s, I won both the Tyndall and the Callendar prizes. It’s nice to be building on their work!