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). [...] "Ocean sediments provide a longer term baseline [tens of millions of years] that allows you to compare the past with the present, giving you an idea of how variable ocean temperatures have been before we had thermometers," said de Menocal. "Over the last 2,000 years, there have been natural climate variations, but they were not especially large…the Medieval Warm period around 1,000 years ago, and the little ice age which was three separate cooling periods lasting a few decades each, beginning around 1300 to around the 1850s. It’s the warming after the 1850s that’s been really remarkable and unique over the last couple of millennia—you can see that in the sediment cores."