Botkin Environmental Science Earth as Living Planet 8th txtbk

20.3 The Origin of the Global Warming Issue 433
Change in temperature (°C)
Eemian interglacial
2
Present interglacial
15°C = Global average temperature
0
–2
–4
–6
1000
(a)
800 600 400 200 0
Thousands of years before present (B.P.)
FIGURE 20.5 Changes in Earth’s temperature over varying time periods
during the past million years. Major changes correspond to glacial
(cool) and interglacial (warm) periods over the past 800,000 years.
(Sources: Modified from Marsh, W.M and Doties, J., 1981; and UCAR/
DIES, “Science Capsule, Changes in the Temperature of the Earth,”
Earth Quest 5, no. 1 [Spring 1991]; J.T. Houghton, G.L. Jenkins, and
J.J. Ephranns, eds., Climate Change, the Science of Climate Change
[Cambridge: Cambridge University Press, 1996]; Climate Change and
Its Impacts: A Global Perspective [U.K. Meteorological Office, 1997].
(b)
Change in temperature (°C)
2
15°C
0
–2
–4
–6
–8
–10
160 140 120 100 80 60 40 20 10
Thousands of years before present (B.P.)
(c)
Change in temperature (°C)
2
Medieval warm period
15°C
0
–2
Little Ice Age
–4
Younger Dryas
18 16 14 12 10 8 6 4 2 0
Thousands of years before present (B.P.)
Change in temperature (°C)
(d)
1
0.5
0
–0.5
–1
900
Medieval warm period
15°C
Renaissance
Little Ice Age
1100 1300 1500 1700 1900
Year (A.D.)
0.6
Change in temperature (°C)
0.4
0.2
15°C
0
–0.2
–0.4
20.3 The Origin of the
Global Warming Issue
(e)
That burning fossil fuels might enhance the levels of greenhouse
gases—gases that warm the Earth’s surface—was first
proposed in the early 19th century, about half a century
–0.6
1860 1880 1900 1920 1940 1960 1980 2000
Year (A.D.)
after the discovery of carbon dioxide, oxygen, and the other
gases that make up the atmosphere. But well into the 20th
century most scientists did not take the idea of global warming
seriously. It just seemed impossible that people could
be affecting the entire planet. For example, in 1938 the
scientist Gary Stewart Callendar studied measurements of
carbon dioxide concentration in the atmosphere taken in