One of the Milankovic cycles (Section 2.2), the precession of the equinoxes, affects the distance between sun and Earth in various seasons. Earlier in the Holocene, 6,000 years BP, the Earth was close to the aphelion during the northern winter. Currently the Earth is in the perihelion during the boreal winter (on 3 January to be exact). In this configuration one would expect the northern hemisphere, especially its continents, to be have warmer summers 6,000 years ago, and colder winters. On the other hand, the seasonal cycle would have been attenuated in the southern hemisphere. Two computer GCM simulations, differing only in the seasonal cycles of radiation, one corresponding to present-day conditions, and the other to the Earth’s orbit around the Sun 6000 years ago, confirm this speculation (1).
The GCM simulations indicate that the jet stream was stronger in winter and weaker in summer compared to present conditions, which is consistent with the temperature changes (Note 12.F). Also, the jet stream traveled further south around the northern continents in winter, and the ITCZ further north in summer. The simulated precipitation changes can be understood in terms of the circulation changes, e.g. more winter rain in the Mediterranean, and more summer rain in the Sahel region of Africa. The simulated differences are also confirmed by the geologic record, e.g. less rain in California and northern Europe.
(1) Hall, N.M.J. and P.J. Valdes 1997: A GCM simulation of the climate 6000 years ago. J. Climate, 10, 3-17.