evidence: There is abundant evidence from bubbles of air trapped in ice cores in Antarctica that the greenhouse gas carbon dioxide (CO 2) changed in parallel with temperature over the past 800,000 years, and that other greenhouses gases like methane (CH 4) and nitrous oxide (N 2O) 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 CO 2. 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 CO 2-rich deep water to the surface, causing the atmosphere to become richer in CO 2; (iv) more CO 2 in the atmosphere traps infrared radiation emitted from the Earth's surface, heating the atmosphere and rising the temperature further, and so on.