How much natural variation has there been in climate in the past?
Climate’s natural variation has been large. Paleoclimatologists, who attempt to reconstruct the climate’s of past eras, have evidence that at times the Earth’s average temperature has been 10 degrees cooler than today and at other times 20 degrees warmer than today. Many natural factors (operating on very different time scales) are known to change climate: subtle variations in the Earth’s orbit round the sun; the positions of the continents as the tectonic plates move; the output of the sun’s light, the eruption of volcanoes, ocean patterns etc.
