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Scientists can get a grasp of long term climate by studying sedimentary rock going back billions of years.
The division of earth history into separate periods is largely based on visible changes in sedimentary rock layers that demarcate major changes in conditions. Coral "rings" are similar to tree rings except that they respond to different things, such as the water temperature, freshwater influx, p H changes, and wave action.
The international ice core community has, under the auspices of International Partnerships in Ice Core Sciences (IPICS), defined a priority project to obtain the oldest possible ice core record from Antarctica, an ice core record reaching back to or towards 1.5 million years ago.
The first atmosphere would have consisted of gases in the solar nebula, primarily hydrogen.
Sediments from nearby locations can show significant differences in chemical and biological composition.
On a longer time scale, the rock record may show signs of sea level rise and fall, and features such as "fossilised" sand dunes can be identified.
Older wood not connected to a contemporary record can be dated generally with radiocarbon techniques.
The first observations which had a real scientific basis were probably those by John Hardcastle in New Zealand, in the 1880s.In addition, there would probably have been simple hydrides such as those now found in gas giants like Jupiter and Saturn, notably water vapor, methane, and ammonia.As the solar nebula dissipated, the gases would have escaped, partly driven off by the solar wind.He noted that the loess deposits at Timaru in the South Island recorded changes in climate; he called the loess a 'climate register'.
Paleoclimatologists employ a wide variety of techniques to deduce ancient climates.From there, certain equipment can be used to derive the sea surface temperature and water salinity from the past few centuries.The δ A multinational consortium, the European Project for Ice Coring in Antarctica (EPICA), has drilled an ice core in Dome C on the East Antarctic ice sheet and retrieved ice from roughly 800,000 years ago.Mountain glaciers and the polar ice caps/ice sheets provide much data in paleoclimatology.