My first correspondent thought I should be worried about being associated with the Climate Change activists. My second correspondent wrote to me privately because he didn’t want to be associated with Climate Sceptics! But he did have a question that had puzzled him. In other words he was a sceptic in the dictionary sense of the word, rather than the ‘tribal’ sense.
This correspondent said that he had understood that the ‘CO2 bands were saturated‘ and so he couldn’t see how increasing the concentration of carbon dioxide could further affect the cooling of the Earth’s surface. This is a subtle point which I had worried about a few years ago. Let me explain.
Averaged over all times of year and locations around the globe, each square metre of the Earth’s surface is warmed by approximately 240 watts of sunlight. The surface cools by radiating infrared light with wavelengths ranging from around 5 micrometres to around 30 micrometres with the most intense radiation at wavelengths around 10 micrometres. CO2 molecules undergo internal vibrations at just the right frequency to absorb infrared light with wavelengths in small ranges around 5 micrometres, 10 micrometres and 15 micrometres: these are called the ‘CO2 absorption bands’.
There is already enough carbon dioxide in the atmosphere that any infrared light with a wavelength within the range of a carbon dioxide absorption band will be trapped in the atmosphere. My correspondent was asking “If all the radiation leaving the Earth’s surface is already trapped on Earth, how can adding more CO2 make any difference?“.
To understand why the CO2 concentration does matter, one has to think what happens to the light after it has been absorbed by a vibrating molecule of CO2. The vibrating molecule re-emits the light in a random direction – a process known as scattering. Roughly half the light will travel upwards through the thinning atmosphere, and half will travel back downwards, warming the Earth.
Thinking about the upward-travelling radiation, the atmosphere grows less dense with height and so as the light travels upwards, it travels further before being absorbed again. And eventually it reaches a height where upward-travelling radiation will leave the Earth for ever – taking with it a fraction of the radiation which left the Earth’s surface. This is how the Earth cools itself. When the intensity of radiation leaving the top of the atmosphere balances the sunlight reaching the Earth’s surface, then the Earth’s surface temperature will be stable.
What fraction of radiation leaving the Earth’s surface makes it out of the atmosphere? As the concentration of CO2 in the atmosphere increases, the fraction of radiation which makes it to the top of the atmosphere gets smaller. The Earth’s surface will then warm (from the downward-scattered radiation) until it can achieve radiation balance again: this is the process of Global Warming about which ‘Climate Activists’ (and me!) are concerned.
Some numbers and a reminder about water. Over the course of the twentieth Century, we think the Earth’s surface radiated roughly 390 watts per square metre, and around 61% of that reached sufficiently high in the atmosphere to drive 240 watts per square of infrared light into space. Around 150 watts per square metre of the radiated infrared light returned to the Earth – this is called radiative forcing.
We estimate that around 144 watts per square metre of the radiative forcing was caused by water vapour (H2O) in the atmosphere, and around 6 watts per square metre of the radiative forcing was due to CO2. We think the effect increasing the atmospheric concentration of CO2 by 30% has been to increase the amount of radiative forcing from around 6 watts per square metre to around 8 watts per square metre.