Past rates of climate change in the Arctic

Climate is continually changing on numerous time scales, driven by a range of factors. In general, longer-lived changes are somewhat larger, but much slower to occur, than shorter-lived changes. Processes linked with continental drift have affected atmospheric circulation, oceanic currents, and the composition of the atmosphere over tens of millions of years. A global cooling trend over the last 60 million years has altered conditions near sea level in the Arctic from ice-free year-round to completely ice covered. Variations in arctic insolation over tens of thousands of years in response to orbital forcing have caused regular cycles of warming and cooling that were roughly half the size of the continental-drift-linked changes. This \textquotedblleftglacial-interglacial\textquotedblright cycling was amplified by the reduced greenhouse gases in colder times and by greater surface albedo from more-extensive ice cover. Glacial-interglacial cycling was punctuated by abrupt millennial oscillations, which near the North Atlantic were roughly half as large as the glacial-interglacial cycles, but which were much smaller Arctic-wide and beyond. The current interglaciation, the Holocene, has been influenced by brief cooling events from single volcanic eruptions, slower but longer lasting changes from random fluctuations in the frequency of volcanic eruptions, from weak solar variability, and perhaps by other classes of events. Human-forced climate changes appear similar in size and duration to the fastest natural changes of the past, but future changes may have no natural analog.