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Elementary Climate Physics Elementary Climate Physics F.W. TAYLOR Department of Physics, University of Oxford, UK UNIVERSITY PRESS OXFORD OXFORD UNIVERSITY PRESS Great Clarendon Street, Oxford OX2 6DP Oxford University Press is a department of the University of Oxford. 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No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, or under terms agreed with the appropriate reprographics rights organization. Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department, Oxford University Press, at the address above You must not circulate this book in any other binding or cover and you must impose the same condition on any acquirer BritishL ibrary Cataloguing in Publication Data Data available Library of Congress Cataloging in Publication Data Data available Typeset by Newgen Imaging Systems (P) Ltd, Chennai, India Printed in Great Britain on acid-free paper by CPI Group (UK)L td, Croydon, CRO 4YY ISBN 978 0 19 856733 2 (Hbk) ISBN 978 0 19 856734 9 (Pbk) 10 9 8 7 6 in © Understanding the Earth's climate and predicting its future behaviour is, first and foremost, a problem in Physics. Climate Physics is a new subject on many university syllabuses, but one that has strong echoes of the traditional precursor of modem Physics, Experimental Philosophy, with its emphasis on understand ing the natural world around us. During the twentieth century, much of the emphasis had swung towards the physics of the very small, with the advent of quantum theory and elementary particle physics, and the very large, with the growth of extra-galactic astronomy and cosmology. Today, the pendulum is swinging back, and physics on the scale of direct human experience is reclaim ing its place as a topic of compelling interest, in schools and universities, and in everyday life as well. The reason for the renaissance is clear: at last we are in a position to begin to answer some of the most fundamental questions facing humankind. How did our Earth develop, and come to be the way it is? How is its observed behaviour to be explained in terms of scientific theory, and in terms of the parallel behaviour of the neighbouring planets of the Solar System and beyond? How did life evolve? What makes a planet support life, and is habitability sustainable in the face of climate change? How accurately and at what range can we forecast weather and climate? And so forth. Even at a very basic level, the key to the answer of questions such as these lies in modem studies of the physical world, of which the climate system is now seen to be a key part. Climate Physics is a modem subject in that the relevant data bases on the physical states of the atmosphere and ocean, their planetary-scale history and evolution, the global measurement systems, and the sophisticated computer models, which collectively make quantitative studies and predictions possible, are all recent innovations. At the same time, interest in understanding the climate has received an enormous boost from the concern generated by the realization that rapid climate change, much of it forced by the relentless increase in popu lation and industrialization, is potentially a serious threat to the quality of life on Earth. Our ability to resist and overcome any such threat depends directly on our ability to understand what physical effects are involved and to predict how trends may develop. In an introductory course, we want to clarify the basics, topic by topic, and see how far we can get by applying relatively simple Physics to the climate problem. This provides a foundation for more advanced work, which we can identify and appreciate at this level although of course a full treatment requires more advanced books, of which there are many. In January 2004 a new unit entitled Climate Physics was introduced into the mandatory part of the undergraduate Physics syllabus at Oxford University. The topics covered, under the banner of Physics ofA tmosphere and Ocean, had previously been a final-year option taken by students who had some expectation Preface Preface of continuing in the field, either as a graduate student or in some other capacity, or at least some special interest in the subject. Now we had to cater for younger, less-experienced students coming to the subject for the first time, and leaving it a few weeks later, most of them for ever. In preparing the lectures for this course, it rapidly became clear that, despite a burgeoning interest worldwide in the basic science of climate and a corres ponding increase in the literature, there was no suitable textbook in existence for students at this relatively low level. A number of excellent books deal with the physics of the atmosphere and/or ocean, but these are invariably aimed at graduate students and researchers, or possibly fmal-year undergraduates spe cializing in the subject. A student coming to the subject as part of a general Physics course needs a very basic introduction to what is important in the cli mate field and how it relates to the rest of Physics that he or she is learning at the same time. The treatment has to be elementary, quantitative, and empirical so that it can be applied to simple numerical problems in tutorial classes and examinations. I have also found that the majority of books in climate-related fields, at least those that address the physics of climate problems, are oriented towards geophysical fluid dynamics. This is a complex and specialized topic, which forms a separate module at Oxford, with its own textbooks. The few books that do deal with thermodynamics, radiative energy balance, modelling, and measurements systems, and that are not too advanced, tend to be written for interdisciplinary students and do not relate well to the fundamental physics. This new book was therefore conceived and aimed squarely at undergradu ates taking physical sciences courses who require an elementary treatment, but with the expectation that it could also provide a useful introduction to the sub ject for first-year graduate students and others entering the field for the first time. The style is deliberately didactical, with a certain amount of repetition of key definitions and concepts between chapters both for emphasis and to aid the student who uses the text selectively or for revision. References to some appro priate further reading have been provided that will allow the student who wants to delve further into the physics or applications of any of the areas treated in a basic way here. Further reading on all of the topics covered in this introduction will be found in the chapters that follow. At the end of each chapter is a guide to more advanced books on each subject, and a set of questions. The latter are provided as samples of what might be found in examinations at the undergradu ate, non-specialist level, and can also be used by the student to test his or her comprehension of the material if they are attempted after the chapter has been studied, but without referring back to the text until the answer is required. Thanks are expressed to my Oxford colleagues who have taught similar topics in the past and have been generous with their material, especially Jim Williamson, Clive Rodgers, and David Andrews. They also provided many useful comments on the manuscript, and so significantly improved it. The fig ures were specially drawn by Dr DJ. Taylor, to whom I am extremely grateful for excellent work. The attribution of ideas and data contained in the text and figures is often difficult in a book as basic as this; precedence is difficult to establish and in many cases an attempt to make an attribution would be con fusing. I have sought therefore to limit references to cases where information useful to the student is conveyed, and hope a blanket acknowledgement will be Preface accepted by the many researchers universities, institutes, government depart in ments, and space agencies, worldwide, past and present, who contributed to the material covered by this tutorial introduction to the field. Finally, I thank the Senior Physical Sciences Editor of Oxford University Press, Sonke Adlung, who has been most helpful and supportive. F.W. Taylor Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, Oxford. January 2005 VII The climate system 1 1 1.1 Introduction: A definition of climate 1.2 Solar radiation and the energy budget of the Earth 3 1.3 Atmosphere and climate 6 1.3.1 Evolution of the atmosphere 6 1.3.2 Temperature structure 6 1.3.3 Pressure, composition, and temperature variations and dynamics 7 1.4 Ocean and climate 9 1.4.1 Heat storage and transport 9 1.4.2 Hydrological cycle 10 1.4.3 Carbon dioxide exchange with the oceans 11 1.4.4 Dynamical coupling between the atmosphere and the ocean 12 1.5 Radiative transfer in the atmosphere 12 1.6 The greenhouse effect 14 1.7 The ozone layer and ozone depletion 17 1.8 Climate observations 19 1.9 The stability of the climate 21 1.9.1 Data on past fluctuations 21 1.9.2 Origin of the observed fluctuations 22 1.10 Climate modelling 23 1.11 Climate on other planets 26 28 Further reading 28 Questions Solar radiation and the energy budget of 2 the earth 29 2.1 Sun and climate 29 2.2 Solar physics 30 2.3 Source of the Sun's energy 30 2.4 The radiation laws 31 2.5 The solar constant 32 2.6 The solar spectrum 33 2.7 Solar observations 34 2.8 Absorption of solar radiation in the atmosphere 35 2.9 The balance between incoming solar and outgoing thermal radiation 36 39 Further reading 39 Questions Contents