When I wrote the preface to the first edition of this book over 10 years ago, I noted that the history of meteorology was littered with whitened bones of claims to have demonstrated the existence of reliable cycles in the weather. These failures had led many to conclude that the search for cycles is a pointless exercise. Not much has changed over the intervening years, but the pace of effort to provide better answers about the periodic behaviour of the climate has quickened.

The reasons for what might be regarded a Sisyphean task are not hard to find. Now more than ever, we need to understand why the climate changes and to what extent human activities are producing effects which are comparable to or exceed the natural variability of the global climate. The record-breaking warmth of global temperature in the 1990s has made it even more obvious that only by understanding the true nature of climatic fluctuations will it be possible to reach an early conclusion on what proportion of current global warming is due to natural causes. This sense of urgency was built on the emerging realisation during the 1980s that quasi-cyclic fluctuations in the tropical Pacific (El Niño) had global implications. When the record-breaking El Niño of 1982/83 was followed by a comparable event in 1997/98 there was widespread speculation that these events were in some way linked to global warming and hence to anthropogenic activities.