Two of the most important questions asked by scientists studying global climate are: Is Earth's temperature rising because of human activities and, if so, to what degree? The answers may lie in temperature data from as far back as 300,000 years ago. Environmental information archived in ice cores provides data that enable scientists to compare recent observations to the reconstructed warming and cooling trends of the distant past. Such comparisons allow us to begin considering whether an observed warming trend in the last century can be explained solely by our models of natural processes, or whether human behavior may be a factor.

Climate studies focusing on the last hundred years show that Earth's average temperature has risen about one-half degree Celsius since the Industrial Revolution blossomed in the 1880s. Other studies reaching back much further in time suggest that global climates have seldom been stable for long. Ice samples collected as part of the North Greenland Ice Core Project, which provide a snapshot of temperatures from the last 300,000 years, tell a dramatic story. It seems Earth has been subject to recurring 10,000-year warm periods followed by 90,000-year cold spells — with most of the transitions between warm and cold periods being extremely abrupt.

The most difficult and, not surprisingly, the most politically charged question is whether this century's global temperature changes are consistent with natural climatic flux. The answer could indicate whether humans are responsible for global warming. Based on available data and computer-generated models, many scientists have concluded that some of the changes we're seeing now are the result of an increase in our reliance on fossil fuels for energy, combined with a dramatic rise in the world's population in the last century. The extent to which human actions and the resulting increase in greenhouse gas emissions ultimately affect global climate remains the focus of intensive scientific investigation.

To learn about the role greenhouse gases may play in global warming, check out Global Warming: The Physics of the Greenhouse Effect and CO2 Concentrations at Mauna Loa Observatory, Hawaiʻi.

To learn more about the role ocean currents have played in climate change, check out Great Ocean Conveyor Belt: Part I and Great Ocean Conveyor Belt: Part II.

To learn about the role orbital cycles have played in Earth's climate, check out Natural Climate Change in Djibouti, Africa.

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