What Climate Sceptics are Sceptical About: Part 3

Following my article in Chemical Industry on climate change, I have received another long missive. Whereas my first correspondent saw the entire issue as being essentially political, and my second correspondent required an answer to a technical question, my third correspondent has brought together four arguments which they feel together make a rounded case for a sceptical position. But four weak arguments are still just four weak arguments whether taken collectively or individually. Summarising the arguments are:

  1. James Lovelock says ‘everyone got it wrong’
  2. CO2 by itself can never cause more than 0.3 °C of Global Warming
  3. There is correlation between CO2 concentration and Global Mean Temperature
  4. Global Mean Temperature is no longer rising.

Let’s take a look at them one by one.

1. James Lovelock is not a climate scientist of any note, and I really don’t think that his personal opinions count for anything. I don’t believe things just because other people say they believe things, and neither should anyone else. I listen to people, reflect on what I have heard, and consider how it affects my own views. Lovelock has changed his mind, ‘Great’. When a person has changed their mind – that means they have learned something’. I discussed his comments on my blog here.

2. Regarding the warming one might expect from CO2, I do not recognise the figure of 0.3 °C. The idea of a maximum limiting amount of warming is based on imagining that when the atmospheric CO2 absorbs all the infrared light emitted directly from the Earth’s surface then the warming effect is saturated. That is not how it works. This post describes the process more fully, and one expects a pretty much linear increase in climate forcing with CO2 concentration.

As an analogy consider a torch shining upwards through a tank containing a coloured solution. How is the radiation which emerges from the top affected by the concentration of the solution?

Light through a tank

An analogy to the absorption of infrared light in the atmosphere. Light is shone through a tank containing a coloured solution. In the left case, the solution is weak and the light travels straight through, and a weakened beam emerges at the end of the tank. In the right case, the concentration of the solution has been increased so that all the light is absorbed. No more light can be absorbed no matter how strong the solution. This analogy is of limited value when considering the transmission of infrared light through the atmosphere. When infrared light is absorbed in the atmosphere, the Earth begins to warm until exactly the same amount of infrared light leaves the atmosphere as did before. This is equivalent to increasing the brightness of the torch until the same amount of light leaves the far side of the tank as did in the left-hand case above.

If the solution is dense enough, then all the light will be absorbed, and this is the basis of the point being made. But in the case of the Earth, when all the infrared light is absorbed it creates a warming (which is equivalent to turning up the brightness of the torch) until exactly the same amount of light emerges as did before the change in the solution. So increasing the carbon dioxide concentration does temporarily reduce the amount of infrared light that leaves the atmosphere. But this then warms the Earth and the climate system only attains equilibrium when the radiated infrared light leaving the top of the atmosphere returns to its previous value.

3. The correlation between CO2 and global mean temperature is astonishing. But in order to see this correlation it is necessary for the global climate system to be relatively stable, which takes decades to centuries. The figure below shows the data over the last 400,000 years and there is a strong correlation between global mean temperature (inferred from the the relative amounts of oxygen isotopes found in water in ice cores) and atmospheric CO2 concentration (deduced form the residual concentration of CO2 in bubbles in the ice). Very roughly 100 ppm CO2 corresponds to around 10 °C of warming – a truly terrifying statistic if it holds for warming as well as cooling. Looking over a shorter time-scale is frankly meaningless.

ICe Core Data

Graph of atmospheric concentration of CO2 (Green graph) and Global Mean temperature (Blue graph), deduced from the Vostok, Antarctica ice core as reported by Petit et al., 1999.The data are plotted at ‘thousands of years before the present, so ‘now’ is on the left of the graphs and the past runs over to the right. The main oscillations arise from changes in Earth’s orbital eccentricity, tilt, and precession called Milankovitch cycles. The correlation between CO2 concentration and global mean temperature is very clear. Picture taken from wikipedia, but other versions are available.

A good deal has been made in sceptic circles as to whether the CO2 concentration rise preceded the temperature rise (and may thus be said to ’caused’ or contributed to it) or followed the temperature rise (in which case sceptics assert that it cannot have ’caused’ or contributed to the temperature rise). Detailed analysis indicates that changes in CO2 concentration usually follow temperature changes, but that they sometimes precede them. The reason that the link works in both directions is because increases in temperature cause increases in atmospheric CO2 concentration by enhancing the release of dissolved CO2 from the oceans. And increases in atmospheric CO2 concentration then cause further rises in temperature because CO2 is an infrared active gas. In short, the quantities are highly-correlated.

4. Has the warming stopped? The high sceptical Berkley group discussed the possibility  under the heading ‘Has the warming stopped’ in this pdf slide presentation. It could be that for some reason this graph has peaked and that in the future it will stabilise or fall. Time will tell, but the trend looks quite clear.

Annualised data for the air temperature above the land surface of the Earth. Do you think this shows a graph in which the warming trend has stopped. Data From Berkeley Earth Group

Annualised data for the air temperature above the land surface of the Earth. Do you think this shows a graph in which the warming trend has stopped? Data From Berkeley Earth Group

The temperature oscillations on the above graph arise from a number of oscillations in the global climate system, most notably the El Niño-La Niña cycle (called ENSO – El Niño Southern Oscillation), the Atlantic Multi-decadal Oscillation (AMO),  North Atlantic Oscillation (NAO) and a Pacific Decadal Oscillation (PDO). The teams take these oscillations out of the data in order to reveal the underlying trend. The correlations of the data with 4 separate indicices of the largest climate system oscillations are shown below. Notice in the top-left graph that the correlation coefficient for the AMO reaches 0.65 with zero lag. In other words the strength of the AMO directly affects the average temperature above the land surface of the Earth – or at least it has done for the last 62 years.

Berkeley Correlations

Correlations between the air temperature above the land surface of the Earth and 4 different indicators of climate oscillations. These oscillations are removed from the data to reveal the underlying trend. The oscillations have periods of a few years to around a decade. Data From Berkeley Earth Group

Saying that the above data shows that the temperature rise has stopped is liking asking someone if a car indicator light is working and receiving the answer “No…. Oh, Yes it is!……Oh, No, now it isn’t…”

The final argument of my third correspondent was that taking action to reduce emissions would damage our economy. This is a red herring. First we need to be as clear as we can be about what is happening. Although we still don’t understand every single aspect of the effect of the CO2, we would expect effects something like what we observe. But this doesn’t mean that we all have to agree on what to do. Some people think that we should halt emissions in a way which devastate the economy, and others think we should just enjoy the benefits of burning fossil fuels and blow the consequences.

As the science becomes clearer year on year, the contrasts between different policy choices should become clearer too. But it is important not to conflate scepticism about the science, with reluctance to take action.

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