Bogs have survived warmer, colder, wetter, drier, stormier and calmer periods and these climate changes have left their mark on the surface, writes Maynooth University's Dr Lisa Orme.

An everyday part of the Irish landscape, peat bogs are sweeping and featureless, and do not often catch the eye as we speed past in our cars.

To some people, these bogs are a wet, muddy area to be avoided. To others, they are a source of fuel for winter fires and a traditional part of living in the countryside.

Reading climate history of a location

Yet peat bogs are not all that they seem. To a palaeoclimatologist like myself, a bog provides a way of reading the climate history of a location.

The bog has survived periods that were warmer and colder, wetter and drier, stormy and calm, and these climate changes have left their mark on the bog surface.

If you know how, you can discover how the landscape and climate of the area have changed over thousands of years.

Peat bogs are climate archives because as they grow, and new peat forms at the surface of the bog, the sediment formed previously is preserved below.

Over hundreds and thousands of years, layer upon layer of peat accumulates and gradually builds the bog up until it is several metres thick.

Amount of rainfall and evaporation

In each layer of the bog is information about the past climate from the decades when that layer was at the bog surface. It tells us about the amount of rainfall and evaporation, the water table depth, the plants and forest cover that surrounded the bog and how stormy it was.

The first stage of reading the climate history of a peat bog is a mission to collect a core of sediment straight through the middle. This requires some heavy lifting of a metal corer and several metal poles.

The corer is a tube that is pushed into the bog using the poles to retrieve a one metre sample of peat. This step is repeated until you have a slice of sediment from the surface down to the base of the peat bog.

Back in the laboratory, the hard work begins. Peat cores can be analysed using a selection of methods to extract information about past climate changes, many of which involve long hours at the microscope.

For several decades, researchers have analysed changes in the fossilised pollen grains and from this understood how the landscape has changed over time, such as past variations in forest extent and the ancient development of agriculture.

Microscopic organisms called testate amoebae

Others analyse microscopic organisms called testate amoebae to learn about past water table changes. Some species of testate amoebae prefer wetter conditions and others drier, so by identifying the species fossilised in each sediment layer you can understand more about the amount of rainfall and evaporation in the past.

My own research on peat bogs has revealed past changes in the frequency and intensity of storms. Along the coast, sand from beaches or dunes is whipped up and carried inland by severe storms and some of it is deposited onto coastal bogs, where it is preserved as fine sand layers.

After chopping up, burning and sieving each layer of the peat core, you are left with tiny samples of sand. Layers with high sand content reflect periods of decades to centuries that had intense or more frequent storms.

Abrupt increases in storminess

Palaeoclimate records of storminess from Scotland and Wales have shown that there can be abrupt increases in storminess lasting hundreds of years.

Storm track changes can also be revealed: a long-term increase in storminess in Scotland 2000 years ago coincides with a decrease in storminess in Spain, which suggests that the storm track has moved northward during recent millennia.

As more and more palaeoclimate records are developed across the world (from bogs as well as other archives such as lake and ocean sediments, ice cores, tree rings etc), the picture of past climate change is becoming ever clearer.

You might be wondering why anyone is interested in how the climate changed thousands of years ago, when the changes in the next few decades are a more immediate concern.

One benefit of developing palaeoclimate records is that they tell us about the natural patterns and range of climate variability.

We do not know much about long-term changes, such as the timing of decadal or centennial fluctuations in climate, because instrumental weather measurements have only been taken over the last 100 to 150 years.

Palaeoclimate records would allow any changes in climate over the next few decades to be placed in a much longer context, so we can know if they are unusual.

Another benefit is that we can look at previous periods when the climate was warmer and see, for example, whether storminess or rainfall increased or decreased at that time. This can allow planners to get a better idea of the likely direction of climate change with warming.

This article first appeared in RTÉ's Brainstorm in November 2019.

Author: Dr Lisa Orme, NUI Maynooth. Dr Orme is part of the Irish Climate Research and Analysis Unit (ICARUS) and works on palaeoclimatology. Her research uses sedimentary archives to develop reconstructions of past storminess and atmospheric circulation. She also uses marine sediments to investigate past sea surface temperature.