Another favourite argument of global warming sceptics is that increases of carbon dioxide (CO2) lagged increases of temperature during the ice ages, so CO2 can’t be the cause of global warming. At first sight this seems like a very reasonable argument. However climate scientists are not as stupid as some make them out to be.
Climate scientists have not been arguing that the ice ages were driven just by CO2. They argue that the triggers for ice age fluctuations are small changes in the heat from the sun, due to small changes in the Earth’s orbit, called Milankovic cycles. By themselves, the small changes in solar heating would not cause very large changes in temperature. However CO2 is released into the atmosphere from the oceans and from soils as the Earth warms. The greenhouse effect of this extra CO2 then adds to the increase in solar heating and amplifies it – a lot.
In other words, CO2 was not the driver of the ice ages, but it was an important amplifier. This does not mean CO2 could not also drive global warming, if the level of CO2 in the atmosphere were to change for some other reason – such as from people burning fossil fuels.
This has been understood by climate scientists for some time, but that understanding tends to be buried in complex scientific papers about complicated climate models. Recently my colleague Andy Hogg at ANU published some simplified modelling that left out a lot of the detail in order to highlight the amplifying effect of CO2. He also did a calculation (unpublished) of the present situation, i.e. CO2 increasing for an extraneous reason. The calculations show CO2 can be both the follower in the ice ages and the driver in the present global warming. There is no contradiction.
Therefore the fact that CO2 lagged temperature in the ice ages does not “prove” climate scientists are wrong and people are not causing global warming. On the contrary, it shows that our understanding of the effect of CO2 can explain its role during the ice ages as well as its present role in global warming.
That is the story in words. Now I’ll show it in graphs, for those who would like to see it in more detail. You’ll need to follow carefully, there are quite a few details.
Ice Age Calculation
Here is a graph of changes of temperature (blue) and CO2 (red). There have been about 6 ice ages over the past 600,000 years. Typically an ice age comes on slowly, with a lot of fluctuations, but it ends “suddenly”, in only a thousand years or so (much more quickly than the 50,000 – 100,000 years it took to develop).
You can see that CO2 also went up and down in the same way and around the same times – slowly down, with fluctuations, and then suddenly up. You can barely see the contentious lag in this plot. If you look closely around 130,000 years ago you can see the CO2 seems to lag the temperature rise a little bit. You have to be careful though, because the vertical scales on the two superimposed plots are a bit arbitrary. You have to use statistical correlation methods to do a rigorous comparison. Such careful comparisons show that the rise in CO2 tends to lag the rise in temperature by as much as 800 years (a very small amount on the scale of this plot).
Now here is Andy’s calculation, replicating the main features of the observations. Both the temperature (black) and the CO2 (red) go through the same kind of variations – a slow, fluctuating decline and a sudden rise. There aren’t as many wiggles in his plot because he’s left out minor details to make the main pattern clearer. (The plot is from A. M. Hogg, Glacial cycles and carbon dioxide, a conceptual model, Geophysical Research Letters, Vol. 35, doi:10.1029/2007GL032071, 5 pp, 2008)
(The scale on Andy’s plot goes from 0 to 500,000 years because he’s calculating forward in time. The scale on the observations, above, goes from 450,000 years ago up to the present – in other words it shows years before now, or age.)
The blue curve is the solar heating. You can see that mostly the temperature isn’t affected very much. However the big jumps in temperature always occur when the solar heating is rising. This occurs when a temperature rise happens to trigger the CO2 feedback.
In the bottom panel the changes between 405,000 years and 413,000 years are shown on an expanded scale. You may be able to see that the CO2 goes up just slightly later than the temperature. This is as it should be, because in this calculation the initial rise in temperature is driven by the increase in solar heating (blue), which starts well before the temperature actually starts to go up. The temperature rise takes less than a thousand years.
This calculation was done in order to understand the ice ages better. (The point was to highlight how the “asymmetry” of the temperature fluctuations (meaning the slow declines and sudden rises) can be explained by the strong amplifying effect of CO2)
Fossil Fuel Calculation
Now let’s look at a calculation that simulates the present situation. In the plot below, the CO2 was suddenly increased at about 284,000 years, which is near the end of a simulated ice age, as the temperature is slowly rising. In the top plot, the CO2 (red) spikes up to double its value (from about 280 ppm to about 560 ppm, a good simulation of what we are doing now). The CO2 drops back quickly and then declines more slowly over the next 50,000 years.
The whole thing is clearer in the expanded scale of the lower panel. There you see the CO2 rise rapidly within a few hundred years, then drop back over the next thousand years or so. In response, the temperature rises by more than 2 degrees, but it does so only after the CO2 has jumped. Thus in this case the CO2 leads and the temperature follows. However essentially the same physics is operating as in the ice age simulation shown first.
People who claim to be serious sceptics need to do their homework. This example is a little more complicated than interpreting the recent global temperature record (see earlier post), and there are traps for the unwary. When you look carefully at what climate scientists have been saying (ice ages are triggered by solar heating, not by CO2) and what they have come to understand (CO2 can still cause global warming), you see there is no contradiction.
Miranda Devine quotes Ralph Alexander, scientist and author of Global Warming, False Alarm. Alexander has not done his homework.
Miranda Devine is not interested in what scientists really understand, she just wants to rouse the rabble, which she does very effectively. I don’t know if she believes what she claims, but I do know her proprietor makes money from her rabble rousing. I also know she takes a highly partisan position in a complicated subject in which she has no qualifications at all. Why does she choose to believe a minority of scientists rather than the majority? It’s not because she understands the science.
18 Jan 10: Here’s an excellent YouTube link that explains it all very clearly, including taking you back to original sources.