How coal cooled the climate 300 million years ago

Reconstruction of the levee of a river that flowed through the tropical wetlands 300 million years ago. The plants growing on these levees are often found as fossils in the rocks associated with coals in Wales. Painting: Annette Townsend.

A comparison of how the area of coverage of the Coal Forests varied with time with evidence of changing climate in late Carboniferous and early Permian times.

Reconstruction of giant lycophytes growing in tropical wetlands of Wales, about 300 million years ago. Note that there are plants in different stages of their life-cycle. Painting by Annette Townsend.

A map of the tropical lands about 300 million years ago, showing mountains (dark brown), lowlands (light brown) and wetlands where peat was being deposited (green).

Bark from the trunk of a Late Carboniferous giant lycophyte, found at the Risca Colliery in south Wales. The diamond-shaped structures on the surface, which are about 1cm long and 0.5cm wide, are where the leaves were originally attached.

A cone from a Late Carboniferous giant lycophyte, found in an ironstone nodule in south Wales. These cones produced spores. The scale is marked in centimetres.

The leafy shoot of a giant lycophyte from the Upper Carboniferous Llantwit Beds of Beddau, south Wales.

What is coal?

Coal is what is left of peat when it has been compressed and heated, so that virtually all that remains is carbon.

The coalfields in Wales are the remains of part of a wetland forest that extended over large areas of the tropics, about 300 million years ago (the Late Carboniferous Period). These are known as the Coal Forests.

There is also evidence of extensive ice cover over much of the land around the southern pole at this time. This is in fact the only other time in the geological past, other than the last 2 million years or so, when there has been this combination of extensive tropical forests and polar ice. Looking at the Late Carboniferous world therefore provides valuable insights into how plants, climate and atmosphere might be interacting in our present-day world.

Plants of the Coal Forests

The Coal Forests were quite different from anything growing today. The main plants were tree-like lycophytes ('club mosses') that could grow up to 50m tall.

Unlike a modern tree, most of the trunk of these giant lycophytes did not consist of wood, but of soft cork-like tissue (periderm). This allowed the plants to grow to their full size in as little 10 years.

Also unlike modern trees, when these lycophytes had reached their full size, they reproduced by producing cones, and then died.

Life, death and carbon

Because these plants grew so quickly and then died, vast quantities of peat accumulated on the forest floor. This eventually formed the coal found in the coalfields of Wales and other parts of Europe, as well as North America and China.

All plants obtain carbon for growth from the atmosphere. These forests are thought to have been responsible for extracting nearly a hundred thousand-million tonnes (100 gigatonnes) of carbon from the atmosphere every year, and would have had a profound influence on the composition of the atmosphere during Carboniferous times.

The contraction of coal forests and global warming

The Coal Forests habitats remained essentially stable for about 10 million years. Then they contracted in size, probably due to changes in drainage patterns in the wetlands where they grew.

This coincided with a marked increase in global temperatures. Most notable was a significant contraction of the ice sheet in the southern polar regions, which has been recognized in the rocks of both Australia and Argentina.

It seems that the contraction of the Coal Forests caused the amount of carbon (as CO2) to build up in the atmosphere, and that this caused temperatures to increase through a greenhouse effect.

South Wales Coalfield

There are Late Carboniferous coalfields across Europe, North America and China. However, the South Wales Coalfield is particularly important as it shows one of the most complete successions of rocks in which the remains of the Coal Forests are preserved. It has also yielded an excellent fossil record of plants, as well as of animals including insects, spiders and freshwater molluscs.

It is also one of the few places in Europe where these rocks are exposed at the surface. In most other places, the geology of these coal deposits has to be investigated in underground mines — an increasingly difficult thing to do as mines are progressively closing.

The geological record of the South Wales Coalfield has therefore played an important role in developing our understanding of the evolution of the Coal Forests, especially through the work of Welsh geologists such as Emily Dix and David Davies in the 1920s and 1930s.

More recently, scientists at Amgueddfa Cymru have been investigating how the south Wales forests changed in composition with time. This has been done by looking at changes in species diversity in the plant fossil record, and at the evidence from pollen and spores extracted from the rocks.

This suggests that the Coal Forests were remarkably stable habitats for most of the time they existed in south Wales, at least until they contracted and caused the increase in global temperatures.

Further reading

• Cleal, C. J. & Thomas, B. A. 1994. Plant fossils of the British Coal Measures. Palaeontological Association, London.
• Cleal, C. J. & Thomas, B. A. 2005. Palaeozoic tropical rainforests and their effect on global climates: is the past the key to the present? Geobiology, 3, 13-31.
• Thomas, B. A. & Cleal, C. J. 1993. The Coal Measures forests. National Museum of Wales, Cardiff.