Evidence for Climate Change
During the most recent history, scientists have been able to construct evidence of climate change from collected information on temperature, rainfall and other weather variables from measuring stations all over the world. The oldest time series of climate data is a temperature record from central England beginning in the 17th century. Most instrumental records however, date back only to the 19th century. Despite this relatively short period of data collection, by careful mathematical analysis scientists have been able to demonstrate that the Earth's surface has warmed on average by about 0.6°C during the 20th century. Such warming, it is believed, is related largely to mankind's pollution of the atmosphere by greenhouse gases.
Of course, it is now known that the Earth's climate has changed many times before mankind began altering the global environment, and before we began collecting information about the weather. Knowledge of palaeoclimate changes has come from the reconstruction of indirect or proxy sources of information. For climate changes that have occurred during the last 2,000 years or so, historical records may be used if they contain details of particular weather phenomena that occurred at the time the records were written. Other forms of proxy data come from the study of natural phenomena which are climate-dependent. The growth of tree rings, for example, is dependent on climate, and tree ring sizes and wood densities may be used to reconstruct climates at the time of growth. The oldest surviving trees are about 4,000 years old. Older wood from dead trees has been used to reconstruct past climates back about 10,000 years.
To reconstruct the climate of the last Ice Age, scientists have had to turn to the chemical and physical properties of ice, collected from ice cores drilled in Greenland and Antarctica. As snow is laid down and compacted it stores a record of the current climate. The longest cores recovered are several kilometres long. Such cores maintain a record of climate changes back over 100,000 years to the pervious interglacial warm period. Palaeoclimatologists have also been able to use sea sediments to reconstruct climate during the last Ice Age and many before it, stretching back millions of years. To investigate the oldest climate changes, some stretching back many hundreds of millions of years, geologists have to use clues from rocks, once laid down as sediment at the bottom of oceans, compressed and then uplifted to be exposed on the continents. Such geological evidence has been used to confirm ideas that continental drift and mountain building can change the Earth's climate over hundreds of millions of years.