The Setting

I t was not until the mid-1980s that scientists became aware that our planet's climate system was capable of taking abrupt jumps from one state of operation to another. These jumps are the subject of this book. Before introducing them, however, we need to explore their context, namely, the stately progression of glaciations and interglaciations that are paced by cyclic changes in the configuration of the Earth's orbit (figure 1-1a). Although, early on, physicists identified the precessing (that is, the slow gyration) and wobbling of our planet's spin axis as the likely drivers of the ice ages, geologists dragged their feet. In order to convince them that orbital changes were indeed the cause, during the 1920s and 1930s Milutin Milankovitch, a Ser-bian mathematician, made elaborate calculations elucidating the time sequence of seasonal changes in the amount of sunlight (i.e., solar insolation) reaching high northern latitudes. He reasoned that ice caps in North America and Europe likely grew during times of reduced summer insolation (that is, delivery of solar radiation) and retreated during times of enhanced summer insolation. In so doing, he hoped to provide geologists with a chronology to be compared with that for past glacia-tions. The problem was that, prior to World War II, geologic age