Weather cycles: Real or imaginary?

Journal of Armospheric end Terrcvrrol

Ph.wrs.

Vol. 55, No. 9, pp. 131 I-1316.

OOZI-9169193 %6.W+ .W PergamonPressLtd

1993

Printedin Great Britain.

Book reviews Weather Cycles: Real or Imaginary?, BURROUGHS W. J., 1992,201 pp., Cambridge University Press, f24.95 hb, ISBN O-521 38178 9. The unresolved debate on the existence of weather cycles is explored in detail within this book. The evidence for and against the existence of cycles in the weather is examined and the difficulties in establishing the arguments involve the natural variability of the climate, the influence of sunspots and the variations in the Earth’s orbit. A wide range of events from the ice ages to the El Niiio and many examples of apparently cyclic behaviour are presented, with evidence from both instrumental observations and other records such as tree rings, ice cores and ocean sediments. These data are set in the context of statistical analysis, global climatology and the predictability of complex non-linear systems (Chaos Theory). The general conclusion is that, with few exceptions. the case for cycles is not proven, but that further work on the origin of long term fluctuations in the weather is essential to the understanding of the current changes in the Earth’s climate. The complex analysis needed to look at the evidence for cycles is examined with the minimum of mathematics, so that this book will appeal to readers with an interest in how the weather works. as well as to the workers in this important and controversial field.

The above, on the inside front cover and first page of this well-written and well-produced book, clearly explains its rationale, purpose and content. An in-depth study confirms these worthy, and necessary, objectives. To obtain the flavour of this book, it is worthwhile to quote further. here from the Preface. Only by understanding the true nature ofclimatic fluctuations wdl it be possible to reach an early conclusion on what proporlion of current global warming is due to natural causes. This book sifts through the huge amount of work that has been published on weather cycles. The aim is to clear the air by identifying the consistenl features in the climate records and providing a basis for addressing the continuing stream of new evidence of periodicities in the weather.

Each chapter starts with an apt quotation, whether it be from Genesis, Disraeli (there are three kinds of lies: lies, damn lies and statistics). Shakespeare, Milton, or others. So Chapter I starts with the search for cycles of weather throughout history. Chapter 2 covers the fundamental statistical properties of time series, problems of sampling, smoothing using running means and filters, the statistics of series of random numbers, and power spectral density analysis. Chapter 3 is on records such as Professor Gordon Manley’s monthly series of the temperature in Central England since 1659; this exhibits the quasibiennial oscillation (QBO), containing 10% of the variance and significant at the 95% level, a 23 yr (double sunspot) cycle, containing 8% of the variance and significant at the 99.9% level, and a 76 yr cycle.

When combined

with the failure in 1988/89 and a successful winter forecast for 1991/92 based on computer models of the El Nifio it suggests that at best the QBO sunspot effects are a minor factor to winter weather patterns and that events in the tropical Pacific, plus the natural variability ofthe global weather system. are more important (p. 58).

Chapter 4 considers proxy data-tree rings, varves (‘pairs of thin layers of clay and silt of contrasting colour and texture’), ice cores, the extent of glaciers, ice ages and ocean sediments. It also covers economic time series (wheat prices, wine harvest dates or immigration to the U.S.A.)-but does not stoop to mention the height of mini skirts. So while there can be no dispute about the fact that weather extremes have immediate economic effects, these are combined with many other factors in even the most basic economic series. What is more significant is that some of these series, like many other econonnc series, do show such a propensity to quasi-cyclic behaviour. This may tell us more about complex non-linear systems than it does about the links between the weather and economic activity (p. 92).

The global climate is the theme of Chapter

5.

But to understand what insight, if any, these observed fluctuations provide about the overall cyclic properties of the Weather. we need 10 look more closely at how the global climate

functions (p. 94). Blocking cesses of patterns, discussed.

patterns of global atmospheric circulation, proradiative balance, prolonged abnormal weather and the El Nifio atmosphereeocean interaction are indicating

that in terms ofclimatic autovariance understanding iour of the tropics may hold the key to explaining regular variations in the weather (p. 119).

the behavappareotly

Chapter 6 is on extraterrestrial influences, 11yr temporal variations of the solar ‘constant’ solar and lunar tidal influences, where physical mechanisms are discussed and periodic changes of the Earth’s orbital parameters (Milankovitch theory). If, however, it is possible to postulate a realistic and physical plausible link between the two processes then the importance of the observed weather cycles is much greater (p. 147). Before considering any particular models or explanations, we must address the matter which has lurked beneath the surface of all aspects of weather cycles. This is the question of nonlinearity (p. 147). This is the topic of chapters

processes,

7 and 8 which

trigger mechanisms

also cover

feedback

and chaos theory.

The computer models used to estimate the size of the Greenhouse Effect can neither reproduce the QBO nor deal with external perturbations in a way that reflects observed climatic variability. This means we cannot estimate what proportion of observed climatic variations is due to autovariance and how much is due to external perturbations. It is therefore also impossible to reach an unequivocal conclusion about what proportion of the current warming is due to the build-up of greenhouse gases in the atmosphere. Faced with such doubt, caution about embarking on massive and expensive economic readjustments, which are not currently cost-effective, is understandable until we know more about the causes of current warming. In this context, a better understanding of a quasi-periodic weather fluctuations is an essential ingredient (p. 168).

There is no shortage of significant cycles. What is missing is evidence of the same periodicity showing up at all times of the year and throughout Ihe entire record (p. 37).

examples are rainfall records, which often exhibit the QBO, in the U.K., China, U.S.A., or the Nile valley, pressure patterns around the globe such as the Southern oscillation, and signatures of the QBO and solar cycle over the North polar region.

Other

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Book reviews

Against this uncerlain background there is one remaining question : canweuseanyof these conclusions about cycles to forecast future weather? (p. 169). The answer which this fascinating book presents is no, but we should not give up trying to understand the physical mechanisms which account for the Earth’s climate, and their various interactions. This book of thought-provoking discussions ends with a useful (for all students) appendix on the mathematical background to searches for weather cycles, an annotated bibliography, references for the figures, a glossary and index. M. J. RYCROFT Cranfield Institute of Technology

Meteorology of Air Pollution : Implications for the Environment and its Future, SCORER R. S., 1990, 160 pp., Ellis Horwood, Chichester, f22.50 hb, ISBN 13-577230-3. Air pollution meteorology has traditionally been concerned with the dispersal of pollutants from point and area sources and, more recently, with the long-range transport of materials by the atmosphere. Professor Scorer covers this conventional subject matter in part I of his book, while part II deals with a series of environmental problems-the enhanced ‘greenhouse effect’. stratospheric ozone depletion and forest damage by acid rain-which are outside the traditional scope of the subject. The book is based on part of a series of lectures on pollution control and part I contains much of the material one might expect to cover in such a course. However, the treatment of the atmospheric physics involved is, of necessity in,such a short text, somewhat cursory. Students using the book would be well advised to refer to Afmospheric D#kion by Pasquill and Smith, and to Scorer’s own Enoironmental Arrodynamics (both published in the same series by Ellis Horwood) for further background material. The fundamental message conveyed in part I is that all models are oversimplifications and, in a given situation, a seemingly sophisticated model may perform no better (or even worse) than a simpler model which encapsulates the underlying physics. The author illustrates the dangers of misunderstanding the physics by considering fallacies. such as that surrounding the ‘tall stack’ policy practised in the design of British power stations. This policy has been blamed by uninformed critics for ‘dumping’ British pollution in Scandinavia. A simple consideration of the mixing processes involved shows that concentrations far downwind must be independent of stack height. while a tall stack makes life considerably more pleasant for those close to the source. The real issue is whether money should be spent on cleaning up the emissions in the first place. Part I contains copious photographs illustrating the phenomena described in the text. While some of these have not reproduced well (in black and white, one foggy day in a London town tends to look much like another) the majority provide a vivid depiction of the processes involved in atmospheric dispersion. Those who have attended Royal Meteorological Society meetings in recent years will be aware of Professor Scorer’s enthusiastic promotion of satellite imagery as a tool for studying atmospheric processes. so it is not surprising to see a large number of illuminating examples included. The second part of the book, which deals with global issues, is more controversial. Chapter 8, on radiation and the

‘greenhouse’ effect, considers how changes in atmospheric CO, concentration may affect climate. Existing General Circulation Models (GCMs) are dismissed as inadequate because they cannot represent all the complexities of the climate system. While it is refreshing to hear a voice of caution, no indication is given of how GCM predictions of global warming have developed as models have been refined. The reader is thus unable to make an informed judgement on the likely success of current modelling techniques. Indeed, the author goes so far as to consider future climate scenarios based on intuition, which are best described as highly speculative. Clearly, Professor Scorer does not believe that these outcomes are any more likely than those predicted by GCMs and he includes them merely to provoke debate. However, in a text aimed at undergraduate and early postgraduate level, such speculation needs to be balanced by a proper review of the successes and failures of more conventional modelling techniques. Chapter 9 on the ozone layer opens with the sentence ‘The story so far is, of course. incomplete.’ So is this chapter! The observations of polar (particularly Antarctic) ozone depletion which led to the current concern are not even presented, let alone discussed, although they are alluded to. Does Professor Scorer think that all of his readers will already know this background? Or does he dispute the role of chlorine compounds in creating the ‘ozone hole’? The statement on p. I28 that ‘. it has not been possible to demonstrate any reductions in ozone which might be attributable to the presence of pollution’ suggests the latter, although remarks on the following page appear to contradict this. I suspect that anyone reading this chapter without any background knowledge would end up rather confused. Would I recommend this book as a text to accompany a course on air pollution? Certainly there is much useful and informative material in part I, while the ideas put forward in part II could stimulate lively debate. However, it must be recognized that these ideas are but a small sample of the spectrum of opinion on a subject in which there are no straightforward answers. Students need to be made aware of the full range of opinion if they are to arrive at a reasoned viewpoint. J. C. KING British Antarctic Survey

Theoretical Concepts and Observational Implications in Meteorology and Geophysics, SCHRGDER W. and TREDER H. J., 1993, 206 pp,, Interdivisional Commission on History of the IAGA. Bremen-Rbnnebeck, DM20.0 pb. During the 1991 IAGA meeting in Vienna, a special symposium was organized by the editors of this book commemorating the 50th anniversary of Hans Ertel’s celebrated paper in which he introduced his hydrodynamic theorem of potential vorticity (PV). The potential vorticity is the product of the specific volume, the absolute vorticity (which is the curl of the Eulerian fluid velocity in a nonrotating inertial frame of reference), and the gradient of some scalar field such as, for example, the potential temperature. The potential vortlcity of a moving fluid remains constant within a layer bounded by two constant surfaces of the scalar field as long as the flow is inviscid and adiabatic. The book contains reprints of Ertel’s two seminal papers from 1942, the first mention of Ertel’s theorem in the literature by Moran (1942) (in Spain), some papers presented during the symposium, and additional reprints of selected