evidence: Earth’s climate has changed naturally over the past 650,000 years, moving in and out of ice ages and warm periods. Changes in climate occur because of alterations in Earth’s energy balance, which result from some kind of external factor or "forcing"—an environmental factor that influences the climate. The ice ages and shifting climate were caused by a combination of changes in solar output, Earth’s orbit, ocean circulation, albedo (the reflectivity of the Earth’s surface) and makeup of the atmosphere (the amounts of carbon dioxide and other greenhouse gases such as water vapor, methane, nitrous oxide and ozone that are present). [...] Global temperatures have risen an average of 1.4˚ F since 1880. Sea ice in the Arctic has thinned and decreased in the last few decades; the Greenland and Antarctic ice sheets are decreasing in mass. The North and South Poles are warming faster than anywhere else on Earth. Glaciers are retreating on mountains all over the world. Spring snow cover in the Northern Hemisphere has decreased over the last 50 years. [...] Evidence from ocean sediments, ice cores, tree rings, sedimentary rocks and coral reefs show that the current warming is occurring 10 times faster than it did in the past when Earth emerged from the ice ages, at a rate unprecedented in the last 1,300 years.