The D/H and ^(18)O/^(16)O ratios in meteoric waters reflect the climatic temperature of the areas where these waters are found. For example, these isotopic ratios are higher for Lake Michigan water than for waters found in lakes in northern Canada. The isotopic composition of unexchangeable oxygen and hydrogen in cellulose extracted from woody plants are related to the isotopic composition of the meteoric waters the plants utilize in their synthesis of cellulose. Thus, the isotopic analyses of oxygen and hydrogen of modern and ancient woods can be used to study the climatic history of the sites where the plants grew over the past thousands of years. The ^(18)O/^(16)O ratios in the cellulose are especially sensitive to the humidity of the plant environment. The above relationships will be applied to studies pertaining to the effect of solar sunspot activity on climate and to the determination of the inferred climatic temperatures during the glacial periods of North America.