An ancient Irish bog oak challenges the view that the sun is to blame for climate change, writes Dr Marina Murphy
Claims that the sun is to blame for the world's warming climate are being challenged by the results of a study of Irish bog oak. The study shows that although there are cyclical changes in both climate and the activity of the sun, there is no obvious link between the two.
Irish tree populations are good indicators of climate change, according to researcher Chris Turney from the University of Wollongong in New South Wales, Australia. "Basically, when the Atlantic waters get cooler, Ireland gets wetter. So when the North Atlantic sneezes, Ireland gets a cold," he says.
Previous studies have claimed to have established a link between solar activity and climate. But these studies used ice cores to establish solar activity and data from other sources to determine how climate changed. One of the strengths of this new study is that data on both aspects are obtained from the trees, which means there is no difficulty matching up the dates of climate change to the dates of changes in solar activity.
"We looked at cycles in the tree-ring record which seem to depend on climate," says researcher Mike Ballie of Queen's University in Belfast. "Then we looked at cycles in the radiocarbon record, which is linked to solar activity. The two records do not have the same cycles, QED the sun is not driving the climate in any straightforward way," he said.
Ballie and Turney used an archive of more than 750 trees excavated from bogs across Northern Ireland and dating back nearly 8,000 years to study changes in climate over thousands of years. These trees were preserved growing on bog surfaces in the past when the surface was dry enough to colonise. They now form the basis of an international radiocarbon calibration curve, the gold standard for determining solar activity.
The climate determines the pattern of tree rings. In good growing periods, certain tree species will produce wider tree rings. In poor growing periods, the tree rings tend to be much narrower. A sequence of tree rings can be build up using timber of different ages, and this can be used to date each tree-ring exactly.
The pattern of tree rings in the bog oak archive can be used to determine tree cover over thousands of years, giving an indication of what the climate was like. In good growing periods (warm and dry), there will be more trees. In poor growing periods (cold and wet), there will be fewer trees. A high water table prevents saplings from becoming established, hence fewer trees.
The tree rings also provide information on changes in solar activity. Each tree ring represents a growing season during which the tree lays down a new layer of wood on its trunk and branches. Growth requires carbon dioxide, which the tree absorbs from the air. A certain amount of this carbon dioxide will contain carbon-14, a radioactive isotope of carbon produced in the upper atmosphere by radiation from the sun. So changes in the amount of carbon-14 in the trees reflect changes in solar activity. By comparing carbon-14 levels between rings, it is therefore possible to determine how carbon-14 levels changed year on year and how solar activity changed year on year.
The tree rings therefore provide information on how good the growing season was, which depends on climate, as well as information on solar activity.
Paul Mayewski, a climate expert at the University of Maine, says the Irish study is interesting. But, he says, "One of their main points is the fact that many of the records used to compare climate to solar activity are dated using radiocarbon, but this is not true for ice cores."