Today, rainforests cover much of the tropics and there are large icecaps at the poles. An essentially similar arrangement has existed for the last 3 or 4 million years, but back in time, conditions were quite different to what we find today.
3-4 million years may seem a long time but, in the context of the 4700 million years of Earth's total history, it is not. If we look deeper into geological time, such as when dinosaurs roamed the Earth, conditions were quite different to what we find today.
In only one other time in our geological past have conditions been similar to today's, with extensive polar ice and tropical rainforests &mdsh; what geologists refer to as Late Carboniferous, 300 years ago.
We are of course no longer in the tropics, having drifted north to temperate latitudes. However, 300 millions years ago Wales was positioned right on the equator and was largely covered in lowland tropical swamp-forests.
The dominant plants of these ancient swamps were giant club mosses. Club mosses still live today, as very small plants (hence the 'moss' part of their name), but these ancient forms were up to 40 metres high.
Despite their size, they were not strictly trees, as their trunks were made up of soft cork-like tissue, not wood. This allowed the plants to grow extremely quickly, growing to their full size in as little as 10 years.
The club mosses were not long-lived plants: they would grow to their mature size, reproduce (by spores, not by seeds as in most of today's trees) and then die.
The colossal amount of dead plant-debris produced meant that the mud and silt in which they grew became very acidic, hindered the rotting of the plant tissue. The result was the build-up of thick peat deposits, which subsequently changed into the coal that has been mined in the coalfields of both north and south Wales.
The Carboniferous tropical forests were one of the most powerful terrestrial 'sponges' in geological history for drawing carbon out of the atmosphere and burying it underground.
By looking at how these forests changed in size (and thus how much carbon they extracted from the atmosphere) and comparing it with changes in the size of the polar icecaps, we can get a much better idea of how atmospheric carbon and global temperatures match up.
One particular marked reduction in the size of the forests appears to have coincided with a shrinking of the icecap.
To understand these global changes properly, we need to understand the causes and exact timing of the changes to the forests. To do this, we need to look carefully the changes in the composition of the vegetation as preserved in the fossil record and the changes in the geographical extent of the forests.
The pioneering 19th century geologist Charles Lyell coined the expression, 'the present is the key to the past'. However, the message that the Late Carboniferous geological record is telling us is that the past may in fact be the key to understanding the present.