In this chapter, we describe and explain some of the patterns observed in the behaviour of Earth’s climate system. We explain some of the causes of the climate’s natural variability, setting contemporary climate change in its longer-term context. We describe the various lines of evidence about climate forcing and the feedbacks that determine the responses to perturbations, and the way in which reconstructions of past climates can be used in combination with models and contemporary observations of change.

Introduction

Human activity is creating a major perturbation to the Earth, directly affecting the composition of the atmosphere, and the nature of the land surface . These direct effects are expected in turn to cause impacts on numerous aspects of the Earth: regional climates , the distribution of ice and vegetation types, and perhaps the circulation of the oceans. Numerous interactions within the Earth system must be understood to enable prediction of the effects of the imposed changes. Models used for prediction are underpinned by a physical understanding of the climate. Aspects of these models are generally tuned to the Earth we experience today, but it is their representation of Earth’s response to change that really interests us.

By observing the Earth, both directly in the present and indirectly in the past, we learn about processes and feedbacks that models need to represent; and we can test whether the real Earth has responded to perturbations with the speed and magnitude that our models display. The ultimate goal is to use such observations to test models quantitatively, and to calibrate some of their less-constrained parameters. This goal cannot be fully realized unless we have knowledge of both the perturbation and the spatial pattern and magnitude of the response. This chapter concentrates on observations of how the Earth’s climate has responded to perturbations in the past.