In the modern age today, we know a lot about the world as it was before. We know of the dinosaurs that roamed the Earth and ruled the lands. We know of the Cambrian Explosion event, wherein suddenly the diversity of biological life exploded. We even know of the five mass extinction events that have taken place in our planet’s history.
For a lot of these facts to be known today, we have our men and women in science to thank. Without them we would still be ignorant of our planet’s varied and fascinating past. One such information about our planet that we can easily access today is the environmental temperature. We can very accurately assume the temperature of any time period, in any location. How we do that though, is a very interesting talking point.
A proxy is a permanent or semi-permanent record that responds in a known manner to certain environmental variables, and this is what is used by scientists to reconstruct information about the environment (such as temperature) in the past. A huge number of proxies are used by scientists to infer things about the past – which one you use depends on what timespans you are interested in, what environmental variables you are trying to reconstruct, where the region of interest is geographically, and so on.
The question is about thousands of years ago, which is actually very young geologically speaking. The key proxies of interest for these timescales are ice records and tree rings (although others such as corals exist).
Ice cores can be used to reconstruct temperature because the ratio between the two main oxygen isotopes (atoms that are chemically identical but have slightly different masses) depends on a number of factors, including sea surface temperature. If you can control for non-temperature related factors and then use a calibration curve (which we can use known instrumental records over the past few centuries and lab experiments for), then we can use these oxygen isotope ratios to infer temperature.
Tree rings can also be used to reconstruct temperature because the growth of a tree depends on several parameters such as temperature and rainfall in a known fashion. If you know how to calibrate growth patterns for a specific tree species (which you can do by comparing tree ring patterns to known, modern temperature records) then you can in turn infer temperature changes in the past.
Apart from the environmental study conducted by scientists, we also have past records to thank if we’re not looking that far back. So the last few centuries are easily covered, as we have testimonies from that time regarding the temperature. And other than ice cores and tree rings, we can study yearly banding in fossilized corals. We also have fossilized pollen grains to examine, which give us a good idea of the kinds of plants growing back then.
Just the type of flora found can give us an indication of the temperature of that time. And then we have marine sediment cores. Analyzing these and the fossilized marine creatures contained within, we can get a picture of the temperature and how it was millions of years ago.