A low-resolution general circulation model is used to examine the separate and combined effects on high-latitude climate of large changes in Earth's obliquity (3¿ tilt angle increase) and precession (summer versus winter perihelion in conjunction with large eccentricity (0.04). The orbital changes, encompassing the extremes for the past 150,000 years, produced significantly increased (decreased) northern summer (winter) radiation. Increased tilt caused a much greater reduction in sea ice than the precessional change. The decreases in annual mean sea ice thickness and fall sea ice coverage are respectively 50% and 41% (21% and 0%) for the tilt increase (precessional change). The larger obliquity effect is a consequence enhanced annual mean radiation with tilt increase; the modest response of sea ice in the precessional experiments is attributed to seasonal interaction between processes governing model sea ice growth and melt and heat transfer through the ice. The reduced winter insolation in the experiments with increased tilt and summer versus winter perihelion produced colder conditions and up to 15% greater areal coverage of snow over the northern continents from fall to early spring.

The average depth of snow on the continents increased (by 20%) only with the change from winter to summer perihelion because of a greater length of the snow accumulation season. During late spring and summer, enhanced radiation (in the experiments with increased tilt and summer perihelion) rapidly melted the snowpack so that it did not persist as long into the summer season as in the experiments with reduced summer insolation (winter perihelion and decreased tilt). The experiments indicate that orbital reduction of spring-summer radiation could be an important factor in initiating glaciation over northeastern Canada. The duration of the snow cover--free season was found to decrease nonlinearly with decreased spring-summer insolation. The change from modern to 115,000 year B.P. orbital conditions reduced the snow cover--free season over northeastern Canada by 3--4 weeks. For a more exaggerated tilt (20¿) and eccentricity (0.06), snowfields occasionally survived the summer season. Overall, the model results suggest that the orbital conditions at 115,000 years B.P. greatly enhanced the likelihood of perennial snowfields over northeastern Canada. ¿ American Geophysical Union 1995