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Orbital cycles

Orbital forcing from cycles in the Earth's orbit around the Sun has for the past 2,000 years caused a long-term northern hemisphere cooling trend, which continued through the Middle Ages and the Little Ice Age. The rate of Arctic cooling is roughly 0.02 °C per century. That trend could be extrapolated to continue into the future and possibly lead to a full ice age, but the 20th-century instrumental temperature record shows a sudden reversal of that trend, with a rise in global temperatures attributed to greenhouse gas emissions.