Historical Climate Data - how can we trust its accuracy?
We need to trust the long term historical climate data, before we can accept global warming as a fact.
Meteorologists have kept detailed temperature records – recording the fluctuations of temperature of the atmosphere and the oceans – since 1850, using thermometer based records.
Since the 1950s, these historical climate records have been augmented with balloon temperature records to give an approximation of global coverage. Since the late 1970s, satellites have also been measuring the temperature of the troposphere by measuring radiances from which the temperature can be inferred.
Historical climate - The past 2000 years
Historical climate data before 1850 is not so accurate. Whilst some records exist from some countries, there is not enough global data to rely on to give a full enough picture.
In order to calculate our climate for the past 2000 years, temperatures can be estimated across the planet using a number of methods, including measuring tree rings and analysing ice core samples.
Indirect historical records are used too. Records of good or bad harvests; dates of spring blossom or lambing; extraordinary falls of rain or snow; unusual floods or droughts can be used alongside the more scientific information gleaned from tree rings and ice cores in order to give a more detailed picture.
Historical climate - From 2000 years to 1 billion years
Paleoclimatology is the study of studying the climate over a much longer time frame - over the past one billion years - using data gleaned from ice records and geologic evidence.
Much of the historical data for studying long term climate records come from ice cores made in Greenland and Antarctica. Ice cores provide us with accurate information which can be analysed for different seasons of the year going back several hundreds of thousands of years.
The European Project for Ice Coring in Antarctica provides us with climate information going back 780,000 years, whilst the North Greenland Ice Core Project gives accurate data going back 131,000 years, but has more recently recovered plant matter believed to be several millions of years old.
For longer term information, and for information from other parts of the world away from the ice caps, sediment cores provide us with records going back 5,000,000 years, whilst analysis of rock samples can give us information going back 100,000,000 (one billion) years.
Historical Climate - Beyond 1 billion years
Previous to this, evidence of climate change is too scattered and sporadic to draw firm conclusions from. Some evidence does exist going back 2-3 billion years ago which suggests that the earth was considerably cooler than today with significantly larger ice caps - possibly at one time covering the entire earth.
Evidence suggests that there were temperature fluctuations going back as far as 3½ billion years when it is thought there was liquid water at the surface of the planet, despite the intensity of the sun being 30% weaker than it is today.
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