How far back in time can scientists measure CO2 levels?
The level of CO2 was measured directly by a monitor on top of a volcano in Hawaii starting in 1957 but we have other, very clever ways of measuring it from a hundred, a thousand, and 100,000 and 700,000 years ago. Scientists go to the centre of Greenland and Antarctica, and drill with a machine that looks like an oil rig. They drill down thousands of feet, pull up huge sections of ice, and segment that ice in laboratories. Ice from these cores looks just like ice in an ice cube in your refrigerator. It has air bubbles trapped in it, which are samples of the atmosphere as it was when the ice formed, as I said, as many as 700,000 years ago. So, we have a very, very solid record of carbon dioxide levels going back a very long time. Over that period, the Earth moved in and out of ice ages. We have other ways of measuring what the Earth's temperature was at the time, and there's a very, very close correlation. Every time that carbon dioxide was high for a sustained period, the Earth's temperature was high. When carbon dioxide was low for a sustained period, the Earth's temperature was low throughout that whole 700,000 years. That's very strong evidence that, as we continue to pump up carbon dioxide and the other greenhouse gases, the Earth's temperatures are going to warm significantly.
