SNOWLINE INSTABILITY IN A GENERAL-CIRCULATION MODEL - APPLICATION TO CARBONIFEROUS GLACIATION

For over twenty years it has been known that energy balance models (EB Ms) with snow-albedo feedback are characterized by unstable behavior i n some areas of parameter space. This behavior leads to rapid changes in snow area due to small changes in forcing, and has been termed the small ice cap instability (SICI). It has never been clarified whether this behaviour reflects a real feature of the climate system or a limi tation in EBMs. In this study we demonstrate that evidence for similar unstable behavior can also be found in an atmospheric general circula tion model (GCM), using a realistic set of boundary conditions for the Carboniferous (300 Ma), one of the most extensive periods of glaciati on in Earth history. When solar luminosity is sequentially lowered to near values appropriate for the Carboniferous, there is a discontinuou s increase in summer snow area. The instability occurs in approximatel y the same area of parameter space as one previously found in an EBM. Analysis of selected fields indicates that the circulation is primaril y affected in the area of snow increase; far-field effects are minimal . There is good agreement between model-generated summer snowcover and one reconstruction of Carboniferous ice cover. Although more work is required on this topic, our results provide increased support for the possibility that the snowline instability represents a real feature of the climate system, and that it may help explain some cases of glacia l inception and abrupt transitions in Earth history.