Future climate change may have considerable effects on the hydrologic cycle and temperature, with significant consequences for sea level, food production, world economy, health, and biodiversity. How and why does the natural climate system vary on decadal to millennial time-scales? Do we sufficiently understand natural climate change, and what is the relative importance of natural processes versus human activities in explaining the global warming of the last few decades? How much of the recent warming is induced by natural (solar, volcanic) rather than anthropogenic forcing?

In addition to historical documents produced by people observing environmental change, nature itself provides a wealth of information stored in various archives such as ice cores, sediments, tree rings, and speleothems. A growing proportion of this stored information can be deciphered thanks to impressive developments in analytic techniques (cf. Birks, this volume). An increasingly clear view of the past environment is emerging. We are still far away, however, from having a complete picture of all the physical, chemical, and biological processes involved. The main problem is identifying all the relevant processes and quantifying the forcing factors, and the corresponding response of the numerous interlinked components of the climate system (atmosphere, hydrosphere, cryosphere, biosphere). Therefore, any primary change will trigger a chain of secondary reactions, some of which amplify the primary change (positive feedback). These positive feedback mechanisms occur on very different time-scales, ranging from minutes (e.g. the ozone chemistry in the stratosphere) to millennia (e.g. the formation of continental ice sheets).

In this chapter we give an overview of the mechanisms causing natural climate change on decadal to millennial time-scales. Proxy climate records provide the basis for both a better understanding of the processes involved, and for testing climate models. Based on selected literature from high-resolution Holocene records of climate change, we evaluate the role of the various potential forcing factors.

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