Product has been added to the basket

The Relationship Between CO2 and Temperature

Planet Earth

Q: I have read your November climate change statement. You state that CO2 concentrations "follow closely the pattern of rising and falling temperature" (page 4, "How did levels of CO2 in the atmosphere change during the ice age?").

I would be interested in how you established this sequence because it is crucial evidence that CO2 is an effect of natural changes, and not a cause. This has also been illustrated in recent papers by Goldberg and MacRae. It seems to me that a sequence has to be established, otherwise your results are a correlation with no causality established, and so are ambiguous.

From Mr Peter Foster (November 2010)

Reply by Dr Colin Summerhayes

There is abundant evidence from bubbles of air trapped in ice cores in Antarctica that the greenhouse gas carbon dioxide (CO2) changed in parallel with temperature over the past 800,000 years, and that other greenhouses gases like methane (CH4) and nitrous oxide (N2O) did too, as shown in the diagram below (Figure 1). There is in geological terms a slight (a few centuries) lag between the beginning of each rise in temperature and the beginning of the associated rise in CO2. That is explained as follows: (i) the amount of solar radiation received at the Earth's surface varies regularly and predictably due to slight periodic changes in the Earth's orbit around the sun; (ii) as the oceans warm and sea ice melts at the end of cold periods, ocean circulation and the density structure of the ocean change; (iii) this process brings more CO2-rich deep water to the surface, causing the atmosphere to become richer in CO2; (iv) more CO2 in the atmosphere traps infrared radiation emitted from the Earth's surface, heating the atmosphere and rising the temperature further, and so on. Thus temperature and CO2 rise in lock step, until the periodic changes in Earth's orbit restore a cooling trend that eventually shuts off the oceanic source of the CO2. The CO2 gets into the deep ocean in the first place through the decomposition of sinking dead plankton. Of course, correlation is not causation, but the evidence is overwhelming that the lock-step behaviour of three independent gases with temperature throughout 800,000 years of Earth history reflects cause.

Furthermore, the rise in temperature at the start of each major interglacial is faster than would be expected from what we know of the way in which solar radiation was received by the Earth's surface, again suggesting a temperature-CO2 feedback. The association also fairly replicates what we would expect from greenhouse gas theory and experiment.

The declines in both CO2 and temperature following an interglacial reflect:
(a) lessening solar radiation, due to predictable orbital changes;
(b) weathering of mountain silicates, soaking up CO2 from the atmosphere;
(c) less emission of CO2 from a cooling ocean; and
(d) gradual increase in sea ice cover limiting exchange of CO2 between ocean and atmosphere.

In February 2010, the Royal Society held a meeting on "Greenhouse gases in the Earth system - setting the agenda to 2030" and the book of that title and containing the presentations from that meeting is about to be released as Phil. Trans. Roy. Soc. Part A. It contains a paper by Dr Eric Wolff of the British Antarctic Survey on "Greenhouse gases in the Earth system: a paleoclimate perspective", that provides an excellent summary on the use of ice cores to establish past changes in climate. Well worth a read!
Figure 1
Figure 1: Oxygen isotopes (grey) are from deep sea cores and show global change in temperature and ice volume from ocean bottom waters; deuterium isotopes (black), from ice in the EPICA Dome C ice core, are a proxy for Antarctic surface temperatures, which changed by 9 degrees C between glacial and interglacial periods, and which match the deep ocean record; CH4 (blue), CO2 (red), and N2O (green) are gases extracted from bubbles in the EPICA Dome C ice core (subsequently extended back with similar results to 800,000 years). The red, blue, and green stars at top right show where the modern levels of these gases lie, well above the natural envelope of the past 800,000 years (Source, IPCC 2007, Working Group 1, Figure 6.3, page 444).

To read the Geological society's statement on climate change, please go to