- Ole Humlum
Evidence for past climatic changes
Outlining a few past geological events is useful to clarify the overall importance of climatic variations, and to place our perception of modern changes in a broader context.
During an interglacial-glacial cycle, lasting for about 100 ka (kilo-years) years and occurring with fairly regular periodicity since mid-Quaternary about 800 ka ago (Lowe and Walker, 1997), polar regions have acted as ice-sheet nucleation areas during glaciations. The formation of high latitude ice sheets during the Quaternary (last 2.8 million years) has repeatedly led to fundamental shifts in the global climate system, caused by increased planetary albedo and globally altered planetary wave structure (see, e.g., Forman et al., 2002). In turn, 120-130 m global sea-level changes, caused by water exchange between ice sheets and oceans, affected both global ocean circulation and the bathymetry of the vast continental shelf seas that border northern Eurasia and Beringia (Weaver, 1995). Land bridges formed across sounds and between islands, in turn affecting ocean surface currents, shallow-sea life and productivity, and opening and closing routes of migration for plants and animals. The Bering land bridge, existing due to lowering of sea level during the last glaciation, made possible the spread of humans from Asia to North America, as well as spread of large mammals like the woolly mammoth to the Siberian islands (Mandryk et al., 2001). The Quaternary Eurasian ice sheets terminated in the Russian-Siberian lowlands and on the adjacent shallow continental shelf, modulating freshwater flux into the global ocean through blockage of two of the ten largest river systems in the world, the Ob and Yenisei rivers (Rudoy and Baker, 1993). Continental-scale proglacial lakes that may have formed with ice sheet damming of drainages in northern Siberia (Mangerud et al., 2001), re-routed fresh water from the Arctic Ocean to lower latitude seas, potentially impacting North Atlantic deep-water formation (Weaver, 1995), affecting temperatures and precipitation at lower latitudes.