[Arragonis]

...what about reconstructions.

Reconstructions come from many source "proxies". Basically you find something which is captured and stored over time - ice cores, tree rings, lake sediments, even coral remains were used a few years ago.

The problem with these is that you get (at best) maybe a seasonal measurement, or an annual one, or maybe less than that.

Lake sediments can give you 2 measurements a year - a summer and a winter layer. These layers can be checked in terms of pollen and other samples to gauge what was happening at the time. Lake sediments are very delicate though - the top few layers will be disturbed by extraction and dating thereafter is an issue.

Ice cores tend to be once a year - you have a cycle of warm melt in spring summer, followed by snowfall and then freezing in winter, and then repeated. Lower ice cores can be compressed so much that the dating becomes more difficult the deeper (and further back) you go - so your deep sample is not one year but maybe at best one in 20, one in 100, one in 300, one in 500. Any extremes in that 20, 100, 300, 500 years is "averaged" out.

Tree rings are notoriously good and bad - every tree gets a new ring each year so cutting into one and measuring the rings and their widths should tell you (for example) temperature. And of course you can date this easily because the outermost ring is ~ 1 year old and you count in.

But there are problems. Firstly is your tree only affected by x (x being temp, or CO2, or something else) - or is something else affecting it's growth ? Did you pick the right tree - one not on the top of the tree line ? Did you sample from the correct side - trees have different growth patterns into the sun and away from it. And also did you isolate it from other plant life around, or maybe a tree bug or parasite - would you be able to detect this.

Even if you have what you might think is a perfect reconstruction you have to caveat it with doubts as outlined above, all are valid.

So lets see an example...