How do tree rings and ice cores scientists understand how earth has changed over time

The study of tree rings is called dendrochronology. Trees produce a single growth ring each year and by studying the size and composition of each ring scientists can get information about the age of the tree, available nutrients in the area, and general climatic conditions. A relatively small ring may indicate a year when there was very little rainfall and a larger ring indicates favourable conditions for plant growth. 

Ice cores hold information on past global volcanic activity and past atmospheric conditions. Tiny bubbles in the ice cores give information on the concentration of gases in the atmosphere. Ice core data plays a key role in linking the increase in carbon dioxide concentrations to present day global warming.

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The answer is constant.