3.8 Ice cores and past CO2 levels
After a few years of measurement, Keeling was the first to discover that CO2 levels in the atmosphere were rising, rather than emissions being absorbed by the oceans. The problem, of course, with the Keeling CO2 data is that they extend back only to 1958. However, ice core researchers realised that the air bubbles trapped when the ice was formed would contain atmospheric gas samples. As well as giving a proxy record of past temperatures, ice cores can give the exact atmospheric CO2 concentration for the last 800 000 years.
Activity 8 Direct and proxy measurements
To understand how past atmospheric concentrations of greenhouse gases have changed, are measurements of gas concentrations from an air bubble in an ice core layer a direct or a proxy measurement?
Measurements of the greenhouse gas concentrations in a trapped gas bubble are direct data, not proxy data, because they are measurements of the actual quantities you wish to know about. It is perhaps surprising that it is direct, because it is a measurement of something that happened in the past! This is possible only because the actual quantity (gas) has been preserved (as bubbles trapped in the ice).
This can be contrasted with, for example, measurements of the ratio of oxygen isotopes in the water, which are proxy data because the ultimate aim is to know the temperature of the planet. Here, only the proxy variable (isotope ratio) has been preserved (in the form of ice), not the temperature itself.
It takes a certain period of time for the bubbles to be closed off and air to be isolated. As a result, it is not possible to measure the concentrations of gases until this has happened. In the case of the Dome C core, the most recent atmospheric CO2 concentration available is from around 100 years ago.