If you want tomorrow's weather talk to Met Eireann. If you want the local weather report from 8000 BC talk to Dr Frank McDermott.
The University College Dublin lecturer in the Department of Geology is uncovering what the Irish climate was like 10,000 years ago by analysing stalagmites from Crag Cave in Co Kerry. The advanced techniques, which rely on laser technology, are providing an exceptional view of our weather as it was thousands of years ago.
"The object is to try to reconstruct as much as possible the climate for the past 10,000 years," Dr McDermott explained. He is interested in the Holocene, the era which covers this period, and uses the Irish cave data to augment climate information available in the Greenland ice cores and sediment cores recovered from the north Atlantic.
He analyses stalagmites from Crag, located about two kilometres north-west of Castleisland, Co Kerry. The rock hard coneshaped structures form on the floor of caves over thousands of years by the slow persistence of dripping water. These structures contain a wealth of information that provide a very accurate chronology of past temperatures. Stalagmites are made of calcium carbonate, calcite, and usually have a matching icicle-shaped stalactite overhead. Groundwater trickling through cracks in the roofs of caverns carries along dissolved calcium bicarbonate which, when exposed to air partially precipitates to calcite.
The calcite is laid down layer upon layer over the centuries to form the stalagmite. They grow extremely slowly, Dr McDermott said, about 50 millimetres in 1,000 years, so a 10,000-year-old cone might be no more than half a metre high.
"The reason we look at caves is because they are a unique environment," he explained. Any calcite formed on the surface is exposed to weather and contamination, but inside a cave the calcite and the chemicals it holds remain beautifully preserved.
"Caves have the ability to average the temperature of a region," Dr McDermott said, levelling off seasonal extremes. The calcite evidence holds this temperature information concealed in the ratio of two oxygen isotopes, Oxygen16 and Oxygen-18. "The isotope variation is a measure or proxy for the mean annual temperature at the site."
This oxygen is delivered via the rainwater, so there is also hidden information about where this water came from, whether from over the ocean or from the Continent. "Effectively you are seeing the signature of the rainfall."
Calcite is also useful for dating purposes through an analysis of the ratio of Thorium-230 to Uranium-238. Calcite cannot hold thorium as it forms so the stalagmite initially traps only uranium. This decays to thorium at a known rate and the thorium-uranium ratio changes over time. "We can use that to date climate between the present day and 600,000 years ago."
The carbon trapped in calcite can also provide information about vegetation at the surface, but this is much less relevant in an Irish context, he said. The carbon isotope changes depending on the volume of material overhead, but this has remained remarkably constant in Ireland over the whole of the Holocene.
"Most of the Irish caves have a lot of Holocene material," Dr McDermott said. Crag Cave was chosen because of its close proximity to the coast. "We believe the ocean is very important in driving climate change over this period."
The data resolution is very fine, with temperature variability good to about 0.2 of a degree C. The time line information is also very accurate and correlates well with the Greenland ice cores, he added.
One of the first things Dr McDermott wanted to find was evidence for a sudden, very strong 40 year long cooling period that occurred about 8,200 years ago. This was seen in the ice cores, but there was no explanation for it.
"Our intention was to see if we could find this in the stalagmite and low and behold we did." He could not provide a temperature for the change because the chemical signature was too abrupt, and suggested such low temperatures that water seepage from the surface would have stopped. Yet seepage did continue given the evidence in the calcite so some other change, such as the direction where the storms and weather originated must be at play.
"What is new is previously we would never have been able to detect this because we wouldn't have been able to see a 40 year event." Only 18 months ago he was sampling the calcite using a dentist's drill, but now a laser is used to shave off layers of calcite just 250 millionths of a metre thick.
The technology was developed by Dr Dave Mattey of Royal Holloway, University of London, and the two scientists are involved in ongoing work using the laser. "This is one of the first applications of the technology," Dr McDermott said.