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). Scientists can track these earlier natural changes in climate by examining ice cores drilled from Greenland and Antarctica, which provide evidence about conditions as far back as 800,000 years ago. The ice cores have shown that rising CO2 levels and rising temperatures are closely linked. [...] Here is how scientists know that the climate change we are experiencing is mainly due to human activity and not a result of natural phenomenon.