The Earth's climate has changed throughout history. From glacial periods (or "ice ages") where ice covered significant portions of the Earth to interglacial periods where ice retreated to the poles or melted entirely - the climate has continuously changed.

Scientists have been able to piece together a picture of the Earth's climate dating back decades to millions of years ago by analyzing a number of surrogate, or "proxy," measures of climate such as ice cores, boreholes, tree rings, glacier lengths, pollen remains, and ocean sediments, and by studying changes in the Earth's orbit around the sun.

This page contains information about the causes of climate change throughout the Earth's history, the rates at which the climate has changed, as well as information about climate change during the last 2,000 years
Causes of Change
Known causes or “drivers” of past climate change include:

Changes in the Earth's orbit: Changes in the shape of the Earth's orbit (or eccentricity) as well as the Earth's tilt and precession affect the amount of sunlight received on the Earth's surface. These orbital processes -- which function in cycles of 100,000 (eccentricity), 41,000 (tilt), and 19,000 to 23,000 (precession) years -- are thought to be the most significant drivers of ice ages according to the theory of Mulitin Milankovitch, a Serbian mathematician (1879-1958). The National Aeronautics and Space Administration's (NASA) Earth Observatory offers additional information about orbital variations and the Milankovitch Theory.
Changes in the sun's intensity: Changes occurring within (or inside) the sun can affect the intensity of the sunlight that reaches the Earth's surface. The intensity of the sunlight can cause either warming (for stronger solar intensity) or cooling (for weaker solar intensity). According to NASA research, reduced solar activity from the 1400s to the 1700s was likely a key factor in the “Little Ice Age” which resulted in a slight cooling of North America, Europe and probably other areas around the globe. (See additional discussion under The Last 2,000 Years.)
Volcanic eruptions: Volcanoes can affect the climate because they can emit aerosols and carbon dioxide into the atmosphere.
Aerosol emissions: Volcanic aerosols tend to block sunlight and contribute to short term cooling. Aerosols do not produce long-term change because they leave the atmosphere not long after they are emitted. According to the United States Geological Survey (USGS), the eruption of the Tambora Volcano in Indonesia in 1815 lowered global temperatures by as much as 5ºF and historical accounts in New England describe 1815 as “the year without a summer.”
Carbon dioxide emissions: Volcanoes also emit carbon dioxide (CO2), a greenhouse gas, which has a warming effect. For about two-thirds of the last 400 million years, geologic evidence suggests CO2 levels and temperatures were considerably higher than present. One theory is that volcanic eruptions from rapid sea floor spreading elevated CO2 concentrations, enhancing the greenhouse effect and raising temperatures. However, the evidence for this theory is not conclusive and there are alternative explanations for historic CO2 levels (NRC, 2005). While volcanoes may have raised pre-historic CO2 levels and temperatures, according to the USGS Volcano Hazards Program, human activities now emit 150 times as much CO2 as volcanoes (whose emissions are relatively modest compared to some earlier times).
These climate change “drivers” often trigger additional changes or “feedbacks” within the climate system that can amplify or dampen the climate's initial response to them (whether the response is warming or cooling). For example:

Changes in greenhouse gas concentrations: The heating or cooling of the Earth's surface can cause changes in greenhouse gas concentrations. For example, when global temperatures become warmer, carbon dioxide is released from the oceans. When changes in the Earth's orbit trigger a warm (or interglacial) period, increasing concentrations of carbon dioxide may amplify the warming by enhancing the greenhouse effect. When temperatures become cooler, CO2 enters the ocean and contributes to additional cooling. During at least the last 420,000 years, CO2 levels have tended to track the glacial cycles (IPCC, 2001). That is, during warm interglacial periods, CO2 levels have been high and during cool glacial periods, CO2 levels have been low (see Figure 1).

Figure 1: Fluctuations in temperature (blue) and in the atmospheric concentration of carbon dioxide (red) over the past 400,000 years as inferred from Antarctic ice-core records. The vertical red bar is the increase in atmospheric carbon dioxide levels over the past two centuries and before 2006. From A. V. Fedorov et al. Science 312, 1485 (2006). . Reprinted with permission of AAAS* . this text Copy to GlobalWarming Site.....