We are told that in the past the planet’s atmosphere has been colder and warmer than it is today but how do scientists actually know this?
A variety of methods are used but one uses some straight forward (to understand – not necessarily carry out) chemistry.
Oxygen has two stable isotopes, 16O and 18O.
16O evaporates faster than 18O so seawater will contain more 18O than 16O due to evaporation. The water will eventually make its way into rivers and back into the sea but at the poles water will precipitate as snow or ice and becomes locked away. If the climate is cold, the ice sheets grow and more 16O gets locked away so seawater contains more 18O.
The carbonate shells of sea creatures will also reflect the 16O : 18O ratio. These will make their way into sediments. If we obtain these sediments we can look at the 16O : 18O isotope ratio and deduce the amount of ice coverage at the time.
Please not, this is not recommend for an IA investigation 😉
The ideas behind this article were taken from ‘The Elements’ by Philip Ball ISBN 0-19-284099-1