Figure 19.1 Core from Ocean Drilling Program hole 1220b (southeast of Hawaii) showing the boundary between the Paleocene and the Eocene (at 55.8 Ma). Marine life was decimated during the 100,000 years of the Paleocene-Eocene thermal maximum, and the dark part of the core represents the absence of carbonate sediment from planktonic organisms. The scale is in centimetres. [SE, after Ocean Drilling Program, used with permission]

If one thing has been constant about Earth’s climate over geological time, it is its constant change. In the geological record, we can see this in the evidence of glaciations in the distant past (see section 16.1 in Chapter 16), and we can also detect periods of extreme warmth by looking at the isotope composition of sea-floor sediments, such as those in the core shown in Figure 19.1. Not only has the climate changed frequently, the temperature fluctuations have been very significant. Today’s mean global temperature is about 15°C. During Snowball Earth times, the global mean was as cold as -50°C, while at various times during the Paleozoic and Mesozoic and during the Paleocene-Eocene thermal maximum, it was close to +30°C.

But in spite of these dramatic climate changes, Earth has been habitable from very early in its history — as soon as liquid water was present — right through to the present day. That continuous habitability is perhaps a little more surprising than you might think, as we’ll see below.

A significant part of this chapter is about the natural processes of climate change and how they work. It’s critically important to be aware of those natural climate change processes if we want to understand anthropogenic climate change. First, this awareness helps us to understand why our activities are causing the present-day climate to change, and second, it allows us to distinguish between natural and anthropogenic processes in the climate record of the past 250 years.