A history of climate is recorded in glacial ice. Glaciers form as layers of snow accumulate on top of each other. Over time, the buried snow compresses under the weight of the snow above it, forming ice. Particulates that were captured by falling snow become a part of the ice, as do bubbles of trapped air. Layers of ice accumulate over seasons and years, storing information about the climate conditions at the time of formation (including precipitation, temperature, chemical composition of the atmosphere, volcanic eruptions, and solar activity). Scientists can study cores of glacial ice to learn about Earth's climate history. For example, scientists can measure the concentration of atmospheric gases in the bubbles of air and infer average temperatures through analysis of the ratio of oxygen isotopes.

Ice cores are cylinders of ice that are extracted from glaciers and ice sheets using hollow drill bits. Layers in the ice correspond to seasons or years, with the oldest ice at the bottom of the core and the youngest ice at the top. The ice-core record provides a natural time line of climate history. The National Ice Core Laboratory (NICL), a joint facility operated by the U.S. Geological Survey and the National Science Foundation, is the primary repository of ice cores in the United States. Located in Lakewood, Colorado, it houses over 45,000 feet (14,000 meters) of ice cores taken from locations in Greenland, Antarctica, and North America. The samples of ice are stored in a carefully temperature-controlled environment to ensure the integrity of the cores for current and future scientific investigations.

Using ice cores, scientists have constructed a history of climate extending back about 800,000 years. Ice cores show evidence of long cold periods interspersed with shorter warm periods; these natural climate variations are likely related to changes in Earth's orbit around the Sun. However, Earth's average temperature has risen at an alarming rate of about 1.4 degrees Fahrenheit (0.8 degrees Celsius) over the past century. The ice-core record shows that Earth's climate varies naturally and that it has gone through a number of warming and cooling trends in the past; however, it also indicates that the current warming is human induced. Scientists have found that there is a positive correlation between rising levels of greenhouse gases (such as carbon dioxide and methane) and global temperatures. The magnitude and rate of the abrupt recent increase in greenhouse gas levels, as well as the geochemistry of the carbon in the ice-core record, suggest it is caused by human activity—in particular, the burning of fossil fuels, which has greatly increased since the Industrial Revolution. The rapid temperature increase over the past century could cause portions of Greenland's ice sheet to melt faster than they have during previous warming events.

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