Introduction

This book addresses one of the key questions facing climate scientists today: how important is natural variability in explaining global warming? The book aims to place the past few decades of warming in the context of longer term climate variability and considers the causes of such variability on different time-scales through the Holocene, the period of Earth history covering approximately the past 11 500 years that has elapsed since the last major Ice Age. In particular it reviews the evidence for past climate change based on the analysis of data from naturally occurring climate archives (such as tree rings, peat bogs, corals, and lake and marine sediments) and describes progress being made in developing the climate models needed to simulate and explain past climate variability. It also considers how people in the past have changed the environment and responded to climate change.

Over the past decade it has become increasingly clear that there is now a human contribution to global warming (IPCC 2007). Antarctic ice-core records (e.g. Petit 1999; EPICA Community Members 2004) show that greenhouse-gas concentrations are already higher than at any time in the past 750 000 years, temperatures in the Northern Hemisphere are now on average probably higher than the previous 1000 years (Mann et al. 1998) and climate models can only simulate temperatures accurately over the past 150 years if greenhouse gases are included as a forcing mechanism (Stott et al. 2001).

Evidence is also accumulating to suggest that changes in natural ecosystems that can be unambiguously attributed to rising temperatures are also occurring. In particular most mountain glaciers across the world are receding (Oerlemans 2005) and unprecedented changes in the ecology of remote arctic lake ecosystems have been recorded by lake sediments (Smol et al. 2005).

The evidence for human impact on the climate system is thought now to be so compelling that Crutzen has argued that the recent period of Earth history dating from the late 18th century increase in atmospheric CO2 should be given a new geologic name, the Anthropocene (Crutzen and Stoermer 2000). Indeed Ruddiman has even argued that human activity may have affected atmospheric greenhouse-gas concentrations much earlier in the Holocene as a result of deforestation and land-cover change associated with early agriculture (Ruddiman 2003).

Yet despite the strength of the evidence for human-induced change, climate-change sceptics still remain, arguing that the role of natural variability is being underestimated. It can indeed be maintained that recent changes in climate, exemplified by ice-cover loss on lakes (Magnuson et al. 2000) or earlier spring flowering (Menzel et al. 2006) are still within the long-term natural range of the climate system, if viewed on centennial time-scales. In Europe, for example, historians can point to the more northerly cultivation of vines in Medieval and Roman times and in Africa major periods of very low lake-levels in previous centuries are well documented (e.g. Verschuren 2004).

This debate, about the relative importance of natural variability and pollutant greenhouse gases in explaining recent warming, is therefore still very much alive. In this book we consider this issue in a Holocene perspective. We present evidence for climate change on different time-scales using both paleoclimate reconstructions and modeling, and we include the results of recent research from both high-and low-latitude environments.

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