Weather vs. Climate The records show a cyclic pattern of brief warm spells called interglacials (the peaks in the graph) separated by long cold spells, the ice ages . The pattern repeats with a period of approximately 100,000 years. Because there were no thermometers way back then, temperature reconstructions must use measurements of isotope abundances such as deuterium (hydrogen-2) or oxygen-18 to create temperature data. The isotope measurement methods use the fact that water made of oxygen-18 and/or deuterium will evaporate and condense (then precipitate) differently than ordinary water (made of hydrogen-1 and oxygen-16) because of differences in the masses of the isotopes. Water made of the lighter isotopes evaporates more easily and water made from the heavier isotopes condenses and precipitates out more readily. When the climate near the poles is colder than usual, the separation of the isotopes from the water vapor in the air is more pronounced. The result is that ice sheets are isotopically light and the oceans are
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