ScienceDaily (Oct. 21, 2011) — A common argument against global warming is that the climate has always varied. "Temperatures rise sometimes and this is perfectly natural," is the usual line. However, Svante Björck, a climate researcher at Lund University in Sweden, has now shown that global warming, i.e. simultaneous warming events in the northern and southern hemispheres, have not occurred in the past 20 000 years, which is as far back as it is possible to analyse with sufficient precision to compare with modern developments.
http://www.sciencedaily.com/releases/2011/10/111021074532.htm