Today, we can measure the atmosphere using instruments on the ground, on boats, on ocean buoys, on aeroplanes, on satellites and with radar. But if we want to know what the weather was doing before we had these sorts of instruments, we have to look at other ways temperature and rainfall are recorded.
All sorts of things can be used for this, from diary accounts to the layers in ice cores, to the way that tiny sea creatures curl their shells - clockwise or anticlockwise.
Trees also respond to the weather - and they can live a long time, recording the weather over tens or even hundreds of years in their annual growth rings. What’s more, wood from trees which lived a long time ago can be found in old furniture, in houses and ships and also preserved in lakes, permafrost or bogs.
Look at the stump of a recently felled tree, and you’ll see the rings - one pair of light and dark rings for each year of the tree’s growth. The tree grew light coloured wood in spring and early summer, and dark coloured wood in late summer and autumn.
Luckily you don’t have to cut a tree down to be able to look at its rings. You can simply cut a pencil-sized cylinder of wood out of it. Oak trees are particularly useful - not only do they live long and were well used in houses and ships, they are widespread and they seem to record the weather well.
Trees can respond to how much water it is available during their growing season, as well as the temperature. Trees growing in different places respond to different conditions. Those growing in cold places such as the far north, or high up on mountains, grow more in a warmer summer. Trees growing in dry places, such as around the Mediterranean, grow more in a wetter summer. The more the tree grows, the wider the tree ring we see when we take a sample from the tree.
Unfortunately in most places, including the UK, the climate is too mild to have a really strong influence on the growth rate of trees.
So it’s lucky that it’s not just the tree ring width which records the weather. By looking at the wood in the ring, we can analyse the carbohydrates that the tree made using photosynthesis. The ratios of the different isotopes of carbon in the ring, for example, can tell us how fast photosynthesis was occurring. That relates to summer sunshine and temperature. In a similar way, the ratios of the different isotopes of oxygen in the ring can tell us how wet the summer was.
For teaching resources based on using tree ring data to tell us about past weather and climate, as well as an interactive game, visit www.metlink.or...