The contribution of orbital forcing to the progressive intensification of Northern Hemisphere glaciation

In this study, we reconstruct the timing of the onset of Northern Hemisphere glaciation. This began in the late Miocene with a significant build-up of ice on Southern Greenland. However, progressive intensification of glaciation did not begin until 3.5-3 Ma, when the Greenland ice sheet expanded to include Northern Greenland. Following this stage we suggest that the Eurasian Arctic and Northeast Asia were glaciated at approximately 2.74 Ma, 40 ka before the glaciation of Alaska (2.70 Ma) and about 200 ka before significant glaciation of the North East American continent (2.54 Ma). We also review the suggested causes of Northern Hemisphere glaciation. Tectonic changes, such as the uplift of the Himalayan and Tibetan Plateau, the deepening of the Bering Strait and the emergence of the Panama Isthmus, are too gradual to account entirely for the speed of Northern Hemisphere glaciation. We, therefore, postulate that tectonic changes may have brought global climate to a critical threshold, but the relatively rapid variations in the Earth's orbital parameters and thus insolation, triggered the intensification of Northern Hemisphere glaciation. This theory is supported by computer simulations, which despite the relative simplicity of the model and the approximation of some factors (e.g. using a linear carbon dioxide scenario, neglecting the geographical difference between the Pliocene and the present) suggest that it is possible to build-up Northern Hemisphere ice sheets, between 2.75 and 2.55 Ma, by varying only the insolation controlled by the orbital parameters. (C) 1998 Elsevier Science Ltd. All rights reserved.