Page 30 - Earth's Climate Past and Future

6 PART I • Framework of Climate Science

focuses on a fraction of Earth’s history too small even to years (Figure 1-3A). This gradual cooling led to the
be shown on this simple linear scale. One way to over- appearance of massive northern hemisphere ice sheets
come this problem is to again start with a plot of Earth’s that have advanced and retreated many times during the
full age and then progressively expand out and magnify last 3 million years at cycles of several tens of thousands
successively shorter intervals to show how they fit into of years (Figure 1-3B). Superimposed on these climatic
the whole (Figure 1-2, center). The other method is to cycles were shorter oscillations that lasted a few thou-
plot time on a logarithmic scale that increases by succes- sand years and were largest during times when climate
sive jumps of a factor of 10 (Figure 1-2, right). This kind was colder (Figure 1-3C). The last 1000 years has been
of plot compresses the longer parts of the time scale and a time of relatively warm and stable climate, with much
expands the shorter ones so that they all fit onto one plot. smaller oscillations (Figure 1-3D).
Each of these successive time scales reveals short
oscillations embedded within longer ones, just as cycles
1-2 How This Book Is Organized
of daily heating and nighttime cooling are embedded in
Within the focus on the most recent 10% of Earth’s age, the longer seasonal cycle of summer warmth and winter
this book is organized by time scale. Part II mainly cov- cold. To understand the extreme heat reached during a
ers climatic changes during the last several hundred mil- specific afternoon in July in the northern hemisphere, it
lion years, an interval during which mammals evolved first makes sense to consider that such an afternoon
from primitive to diverse forms. Part III looks at the occurs in the larger context of the hottest season of the
last 3 million years, a time span when our primitive year and then to factor in the additional contribution
ancestors were evolving. Part IV explores changes over from daytime heating. For a similar reason, it makes
the last 50,000 years, an interval during which our fully sense to follow time’s arrow and trace climate changes
human ancestors initially lived a primitive hunting- from older to younger eras and from the larger cycles to
and-gathering life, then developed agriculture, and later the smaller ones superimposed on them.
created the first recorded human civilizations. Part V As the book progresses from older to younger time
examines the last 1000 years, most of the historical era. scales, you will notice a change in the kind of informa-
This progression from longer to shorter time scales tion about past climate changes. In part this develop-
is a natural one because faster changes in climate at ment reflects a change in the amount of detail that can
the shorter time scales are embedded in and superim- be retrieved from climatic records, called the degree of
posed on the slower changes at the longer time scales resolution. Because older records tend to have less res-
(Figure 1-3). At the longest time scale, a slow warming olution, much of the focus of Part II of the book is on
between 300 and 100 million years (Myr) ago was the longer-term average climatic states over millions of
followed by a gradual cooling in the last 100 million years and the way they differ from our climate today. By

Linear scale Expanded linear scales Log scale
III-V 1
II II V
Section II
10
III IV
100
years ago 1000
1 1 (10 )
2
III IV V
3
(10 )
10,000 II
4
2 2 (10 )
100,000
Byr ago Byr ago 1 million
5
(10 )
6
(10 )
3 3
3,000,000 50,000 1000 10 million FIGURE 1-2 Earth history Earth’s
years ago years ago years ago (10 ) age is 4.55 billion years. Most of this
7
book focuses on a very small fraction
100 million of this immense interval and can be
8
(10 )
4 4 represented only by a series of
1 billion magnifications or by plotting time
9
(10 )
Earth’s age on a log scale that increases by
factors of 10.