 Since 2000 various scientific investigations looked at the behaviour of glaciers in relation to climate and found they were excellent indicators of temperatures, especially in summer.Glacial evidence of climate is matched by evidence of variations in tree ring growth and studies of ice core and ocean sediment samples indicating the types of microorganisms (some flourish in warm temperatures, other in cooler temperatures) and historical carbon dioxide ratios.Evidence found that the climate was more strongly influenced by the impact of tropical air masses in warm periods and polar air masses in cool periods.