3Figure 1.1 shows that Earth has undergone vast changes in climate as well as the
abundance of atmospheric CO 2. A logarithmic scale is used to represent the mixing
ratio of atmospheric CO 2 , because the radiative forcing of climate (see Sect. 1.2.1)
is proportional to the logarithm of CO 2. Figure 1.1 shows, clearly and beyond
debate, the strong association of Earth’s climate and atmospheric CO 2. Looking
backwards in time, as CO 2 rises, Earth warms.
8 6 4 2 0 2 4 6 8101214500400300200(^100) Dinosaurs
Era 6
60
50
40
30
20
10
Rise of
Mammals
Era 5
5
4
3
2
1
Lucy
Era 4
200 400 1000 2000
Rise of
Forests
Pangea
Breaks
Apart
Era 6
Rise of
Himalayas
Era 5
Greenland
Glaciation
Era 4
700
500
300
100
Homo sapiens
Era 3
15
10
5
Agriculture
Begins
Era 2
8 6 4 2 0 2 4 6 8101214
8
6
4
2 Anthropocene
Era 1
Ice Ages
Era 3
Last Glacial
Maximum
Era 2
200 400 1000 2000
Anthropocene
Era 1
T (°C) from preindustrialCO 2 (ppm)
Climate History, 500 Million ybp to Present
Hundred ybp
Thousand ybp
Million ybp
Fig. 1.1 Earth’s climate history, past 500 million years. Historical evolution of global mean sur-
face temperature anomaly (ΔT) relative to a pre-industrial baseline (i.e., mean value of global
temperate over 1850–1900) (left) and the atmospheric mixing ratio of CO 2 (right). Major events in
the evolution of life on Earth as well as either changes in climate of the global carbon cycle are
denoted. The vertical line on the ΔT panel at zero marks the pre-industrial baseline; the vertical
lines on the CO 2 panel denote mixing ratios of 280 ppm (pre-industrial), 400 ppm (current level)
and 560 ppm (twice pre- industrial level). These time series are based on hundreds of studies; see
Methods for further information
1.1 Earth’s Climate History