Abstract
Through the past few million years large ice sheets have repeatedly grown and disappeared on the Northern hemisphere. These are the Pleistocene glaciations. They are related to the changing solar heating of the Earth due to changes in Earth's orbit and axis of rotation. The climate response to these changes is highly non-trivial and non-linear, expressing the complex nature of the climate system. Many aspects of glacial cycles still need a convincing explanation, one particular mystery being the change from approximately 40 kyr (kilo year) glacial cycles to approximately 100 kyr cycles around 1 million years ago. This transition is called the middle Pleistocene transition (MPT). Here we review some conceptual models to explain the dynamics of glacial cycles and possible dynamical causes of the MPT. We especially focus on the well studied van del Pol oscillator as a conceptual model for the glacial cycles and propose that the MPT is a result of changes in frequency locking of the climate system to the astronomical forcing. This is compared to a recently presented model that relates the MPT to a transcritical bifurcation in the structure of a generic critical/slow manifold for a fast-slow dynamical system.
Highlights
The climate of the planet is governed by the radiative energy balance between heating by short wave radiation from the Sun and cooling by the long wave radiation back to space
We have presented some simple nonlinear models of the Pleistocene glacial cycles and the transition at the middle Pleistocene transition (MPT) from a 41 kyr to a 100 kyr response to the astronomical forcing
The MPT is described by an internal reorganization of the response to the forcing, as there is no noticeable difference in the forcing across the MPT
Summary
The climate of the planet is governed by the radiative energy balance between heating by short wave radiation from the Sun and cooling by the long wave radiation back to space. Many internal dynamical factors play a role in determining the climate: for example, the amount of snow and ice influences the fraction of sunlight being reflected back into space without heating the planet, the interchange of CO2 between ocean and atmosphere influences the greenhouse trapping of heat in the atmosphere and the Atlantic overturning circulation influences the oceanic heat transport to the polar regions. The strength of these factors depends in turn on the state of the system, constituting non-linear feedbacks for determining the equilibrium state. In order for this paper to be self-contained for non-expert readers, we review a few theoretical aspects
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