Introduction

Of all the changes resulting from human activities over the past two to three centuries, the possible effects on climate have rightly received most attention both in scientific literature and in the media. This chapter seeks to outline some of the accumulating body of evidence for changes in climate that have occurred over the last few decades. It also considers the question – to what extent can these be attributed to human activities?

The analyses of northern-hemisphere temperatures over the last thousand years summarised in Section 7.9 (Figure 7.11) have set the scene. They highlight the outstandingly warm conditions during the 1990s, culminating in 1998, the warmest year of the millennium, even when the uncertainties associated with the proxy-based temperature reconstructions are taken into account. In all of the reconstructions, as well as in the globally integrated observations (Figure 12.1), the strong warming trend begins in the mid nineteenth century, slows down, even reverses, between c. 1946 and 1970, then resumes, continuing into the 1990s. As it stands, all that this tells us is that unusually warm conditions have prevailed in recent times. It does not answer the key question – does this reflect natural, low-frequency variability (recovery from the ‘Little Ice Age’ perhaps) or is it, in whole or part, a consequence of increased atmospheric concentrations of greenhouse gases?