From Tree Rings to Ice Cores

Organised records of the weather and climate have been kept since the middle of the nineteenth century. In addition to this the weather may be mentioned in contemporary writings.

When Dickens wrote 'A Christmas Carol' in 1843, Britain was in the grip of the 'Little Ice Age' with severe and persistent winters. Had the novel been written now, it is unlikely that snow would have been featured quite so heavily!

Records of weather and climate are locked away by nature around the world - in tree rings, the sediment at the bottom of lakes and oceans, and ice cores from mountain glaciers and polar regions.

Dendrochronology
Every spring, when moisture is plentiful, a tree's energy is spent in creating new growth, which continues until the autumn. Each ring of growth is a permanent record of the climate and weather conditions for the growing year - of the precipitation, sun, wind, soil properties, temperature and snow accumulation.

The bristlecone forests of California are among the oldest living trees. They have been known to live almost 5,000 years and even those that have died have remained standing for the same length of time. These trees can therefore provide a vast memory bank of local and global climate data.

Variation in the size of the tree rings, known as sensitivity, shows when the growth of the tree has been limited by hostile environmental conditions such as drought or even volcanic eruptions - when dust and debris in the atmosphere reduces the amount of sunlight reaching the earth.

Records of the climate from tree rings extends back almost as far as the end of the last Ice Age, which ended 10,000 years ago. The uses of these records are varied, from dating Stradivari violins and viking longboats to the dates of volcanic eruptions. The records have even been used to recalibrate dates by previous radioactive carbon dating.

Sediment cores

Ice cores
The current geographical location of glaciers and ice sheets is more restricted than that of vast ancient forests, but they have provided records of climate that reach back much further in time. They are also the records most threatened by global warming, with the icecaps in the tropics such as Kilimanjaro in Tanzania and Quelccaya in Peru not expected to last beyond 2020.

As snow accumulates and is compressed, either as glaciers or ice sheets, it encodes a record of the climate at that time. Ice cores provide precise records of temperature and precipitation over centuries and millennia, through dissolved acids and salts, natural variations in gases and liquids and trapped bubbles of air.

In Peru, samples from the Quelccaya ice cap showed that the collapse of the Tiwanaku civilisation around Lake Titicaca coincided with a drought in the area.

The ice sheet in Greenland covers 80% of the country and holds 8% of the Earth's freshwater supply. Core samples taken from the ice sheet show information on previous climates in extreme detail as the snow doesn't melt and the differences between Summer and winter snow is visible even after thousands of years.

The cause of controversy
The use of knowledge of previous climates in dating archaeological finds can sometimes prove controversial. Probably the most well-know, ongoing, argument involves the dating of the Sphinx in Egypt.

The traditionally accepted view is that the building of the Sphinx started around 2500 B.C. during the reign of the pharaoh Khafre. However, in October 1991, the now Professor Robert Schoch, a geologist from Boston University, presented evidence to the Geological Society of America that the Sphinx was significantly older.

His research suggested that the erosion patterns seen on the Sphinx, were the deep weather patterns produced by water erosion. Schoch points out that as the last wet period in Egypt ended around 6000 B.C., this puts the building of the Sphinx at a date much earlier than currently accepted.

Egyptologist, Dr Mark Lehner disagrees. In his view, the pattern of weathering is consistent with that expected of the soft bands of limestone the Sphinx is built of, with some of the weathering having taken place since the Sphinx was excavated some 70 years ago.

The future
For those around to remember, and even some of those who weren't, the Summer of 1976 is still remembered as one of long drought in Britain, and the autumn of 2000 is remembered as one of floods. While we only remember significant weather events, we can rest assured that around us, nature is slowly but surely laying records of our climate for generations to come.