3.3 Proxy data and past climates

The process of analysis and checking for plausible mechanisms using proxy data has revolutionised the study of past climates. This is because many parts of the environment respond to climate: they change if the climate becomes warmer, or wetter, and so on. Wherever these changes are preserved, they serve as a record of the past climates.

For example, the thickness of annual layers in an ice core is a simple proxy for moisture in the atmosphere at the time snow fell. This is because more snow forms and falls when the air is more moist. A thicker layer means more snow fell, so the atmosphere must have been wetter to form and hold the increased snow before it fell. A thinner annual snow layer would imply the opposite.

Another type of proxy data from ice cores is the chemical composition of the water itself. Past Antarctic temperatures can be deduced from ice cores. Past temperature records have been constructed entirely from the relative amounts of oxygen-16 and oxygen-18 isotopes (see Study note: the central part of an atom). Water molecules in the ice have a proportion of all three isotopes of oxygen in them, and it has been shown that the relative amounts of the different isotopes vary depending on the temperature of the oceans at the time the snow fell. So scientists can measure the amount of oxygen-16 compared with the amount of oxygen-18 in an annual layer of an ice core to derive the temperature at that time. The ratio of the oxygen isotopes is a proxy for the temperature of the planet.

You now know that proxy data measured in ice cores include:

• the concentration of lead, as a proxy for global lead production
• the thickness of annual layers, as a proxy for atmospheric moisture
• the ratio of oxygen isotopes, as a proxy for temperature.

There are also many other types of proxy than those found in ice cores. For example, the types of pollen found in ancient lake and ocean sediments are a proxy for the temperature and rainfall in the area at the time the plants grew.

The great advantage of proxies is that they form a historical record of the planet, surviving from the past and giving information about things that cannot be observed directly. An important disadvantage is that proxy data are less accurate than direct measurements. This is because as well as measuring the proxy variable, scientists need to know the relationship between this and the variable of interest, which is an extra source of error.

The following video sees poet Nick Drake read another of his poems from The Farewell Glacier. This one is about ice cores and how they can give a picture of the past. Nick is filmed reading in the ice core laboratory at the British Antarctic Survey, which is kept at a temperature of around minus 20°C.