For the last 150 years, scientists have been searching for the cause of the cyclic appearance and disappearance of the great continental ice sheets of recent geologic time. This book is part of that search. It has been written for readers with a general interest in climate and Earth science as well as for scientists working in this field, and it is hoped that the conceptual model and ideas pre-ented here will contribute significantly to our knowledge of the cause of ice-age climate change. And they should, because the new model is much more consistent with the physics of climate and the geological record than is the classical Milankovitch model. Customarily, the new concepts would be published as papers in archival journals, but these journals are not readily available to the ordinary reader. Furthermore, some of the climate-change events described in the book have not yet been exhaustively verified by conventional research. Ideas based on such events are seldom accepted by the journals' peer reviewers, even when supported by good evidence and sound inferences.

A generation ago exploratory Earth-science ideas had a better chance to be seen in print. But data trump ideas, and today, the pressures to publish data acquired in funded research leave little journal space for concepts that push the envelope of conventional wisdom. Investigation of such concepts is not favored in the allocation of available research funds, and the unconventional proposal is scorned by peer reviewers. However, new ideas are needed because the Milankovitch model of direct thermal control of glaciation is woefully inadequate, and, although much progress has been made, a half-century of intensive data acquisition has not yet yielded a satisfactory understanding of the most important features of Pleistocene climate change. The variations of Pleistocene climate and the variations of glacial-ice volume are inextricably coupled together, and much of the ice volume change occurs as a result of internal factors within the earth's climate system. However, climate proxies in the geological record do correlate with orbital insolation variations to some extent, and the correlations require an explanation.

Hence this book, which goes beyond the limitations of peer review to examine the role of the Mediterranean Sea in climate change. Among oceanographers and other Earth scientists the possibility of a significant Mediterranean influence on climate is a contentious matter, because it implies that a little of the conventional understanding of North Atlantic oceanic circulation is incorrect. Most peer reviewers favor only ideas and data that do not conflict with conventional facts. But facts are merely accepted ideas, and as Francis Crick of double helix fame has said: "Any theory that agrees with all the facts will be wrong because some of the facts will be wrong." Conversely, a theory may be quite correct but not agree with all the "facts."

The book is therefore an opportunity to present to a wide audience facts and arguments that support the importance of the Mediterranean Sea, and which might not otherwise appear in print. In doing so, it is necessary to examine past climates. However, there are no time machines to enable us to go back a hundred thousand years and observe the climates of the past, and we only "know" indirectly what has happened from clues in the geological records. Some important facts are consequently beyond the reach of observation and must be inferred if we are to make sense of climate history.

The central issue in Pleistocene climate change is the effect of long-term variations of incoming solar energy (insolation) on ice-age cycles. These variations occur because the tilt of Earth's polar axis is not constant and because orbital precession causes the summer season to occur when Earth is at varying distances from the sun. In the model advocated by Croll and Milankovitch, the ice-age cycles are supposed to be caused by the direct effect of received solar energy variations on climate warming and ice-sheet melting. This idea has been known loosely for some time as the Milankovitch hypothesis. Although many correlations between incident solar energy variations and ice-age changes are consistent with this hypothesis, the direct warming and melting effects cannot be the cause, and a satisfactory physical connection between the ice-age cycles and orbital variations has not been identified and accepted by the scientific community.

In this book, the Mediterranean Sea is the proposed link between variations of orbital parameter values and changes in the volume of glacial ice. In this very indirect and imperfect way, orbital variations have modulated the volume of glacial ice over the last three million years. The Mediterranean role and the associated new ideas discussed in the book are the result of disciplined analysis and are supported by considerable evidence. The wider acceptance of these ideas will not depend on any single new research result. In the earth sciences there is no "proof" of a disputed concept. Acceptance of any new idea depends, first of all, on thoughtful consideration of the idea itself, followed by acquisition of supporting data, if possible. Eventually the idea may fit into a broad web of related facts, and the skeptics - or most of them - become convinced.

The book is like a legal brief in which the evidence is outlined and the inferences are made to make the case for Mediterranean influence on ice-age cycles. It is not a review of climate-change research, however, and no attempt is made to mention all the significant contributions made by the many workers in this active field. In another decade or two the science will have advanced and some of the theoretical ideas discussed here may then have been validated - or replaced, for even the best hypotheses are but good approximations. I have said very little about the role of modern global warming. That topic is covered well by others. However, warming does introduce an element of uncertainty into a timely ice-age prediction. But the arguments for the past and the predictions for the future are laid out here for all to see, and that great analog super-computer that we call the natural world will provide correct answers in due time.

— Robert Glenn Johnson