In Chapter 1, I defined climate change as a change in the statistics of the weather (changes in averages and extremes of temperature and precipitation and other meteorological parameters of interest) over a period of several decades. In this chapter, I address the question of whether the Earth's climate is currently changing and how it has changed in the past. We will see the overwhelming evidence that the climate is indeed changing – and that it has changed significantly over the Earth's entire history. In Chapter 7, I will discuss what causes climate change and whether humans are to blame for the present warming.

Although there are many statistics that we could examine, the discussion in this chapter focuses on temperature for two reasons. First, as I will discuss in Chapter 4, the most direct impact from the addition of greenhouse gases to the atmosphere is a warming planet. Changes in other variables, such as precipitation or sea level, arise as a response to the temperature change. Second, we have the best data for temperature. The technology for measuring it is centuries old, and people have been measuring and recording the temperature with reasonable global coverage since the middle of the 19th century. In addition to direct measurements of temperature, there are other techniques, such as studying the chemical composition of rocks, which allow us to indirectly infer the temperature of the Earth over nearly its entire 4.5-billion-year history.