9:28am Wednesday 6th January 2010
By Keith Thomson
Recognising that the climate is changing depends on knowing past temperatures, and this could be a problem as collecting the data using mercury-based thermometers only began seriously after 1850, and was barely worldwide until the middle of the last century with the spread of aviation. Indeed, satellite measurements throughout the lower atmosphere didn’t begin until 1979.
Despite this, climate scientists make very confident statements about temperature and rainfall for the last few thousand years and, while written accounts give evidence for the recent past, it needs special scientific methods of investigation to provide the information.
Layers of sediment in old lake beds trap pollen and indicate what was growing nearby, and cores through ice caps show the balance of gases trapped in the ice. However, another useful proxy technique is the study of tree rings, or dendochronology, that indicates the weather conditions throughout the life of the tree. It’s often supported by carbon dating.
It’s well-known that the cross-section of a tree trunk shows a series of concentric rings of wood of different colours and density. They reflect the annual growth of the tree as it matures, and they are best represented in temperate and cold climates where there’s a winter when growth shuts down. The temperature and the rainfall determine the nature of each ring.
In the UK, some of the older trees go back at least 300 years, so it is possible to work out the climate for that period, and then link it back for hundreds more years to wood that is in old buildings and churches. Significant events at these dates may be cross-referenced to written records and more recently to the mercury thermometer figures.
The oldest living trees are the bristle cone pines in California, almost 4,000 years old, and the only year when no annual ring was recorded in British oak trees was 1816 – the year with no summer because of the low solar activity and the amount of dust in the atmosphere from five volcanic eruptions culminating in Tambora, the largest in 1,600 years.
The tree record, the written statements and the thermometer readings match well for the last thousand years-or-so, until the last 60 years, when the trees deviate from the amount of warming that is being measured.
Because man-made acid rain and air pollution make recent interpretation difficult, the East Anglia Climate Research Unit used thermometer readings for the most recent years on their tree ring graph. It was this valid technique, known as a ‘trick’, that was referred to in the hacked e-mails that misled the climate-change sceptics.
© Copyright 2001-2012 Newsquest Media Group
http://www.thetelegraphandargus.co.uk/trade_directory/