The Greenhouse Effect and Global Warming - Air Pollution and Global Warming: History, Science, and Solutions - page 281

12.3.2.5. From 100 to 3 Million Years Ago
Between 120 and 90 m.y.a., in the mid- Cretaceous
period (145.5-65.5 m.y.a.), temperatures increased
above those today. This may have been the last period
of an icefree globe. Sea levels were high, covering
about 20 percent of continental areas (Barron et al.,
1980).
At the end of the Cretaceous period (65.5 m.y.a.)
(Figure 12.17d), a mass extinction of 75 percent of
all plant and animal species, including the dinosaurs,
occurred. This extinction is called the Cretaceous-
Tertiary (K-T) extinction and may have been due to
a 10-km-wide asteroid hitting the Earth (Alvarez et al.,
1980). Evidence for the asteroid theory includes a
worldwide layer of the element iridium (Ir) in clays at
depths below the Earth's surface corresponding to the
K-T transition. Although iridium has been measured
in a Hawaiian volcanic eruption plume (Zoller et al.,
1983), these plumes are not known to penetrate to the
stratosphere, which would be necessary for the iridium
to be distributed globally (Crowley and North, 1991).
Currently, the most accepted explanation for the iridium
layer at the K-T boundary is an extraterrestrial source,
such as an asteroid. The asteroid impact is believed to
have created a large dust cloud that blocked the sun
for a period of weeks to months, lowering the surface
temperature by tens of degrees (Toon et al., 1982). The
reduction in surface temperature may have been respon-
sible for the mass extinction. Because some extinctions
occurred before the K-T transition, the asteroid theory
is still open to debate.
Between 90 m.y.a. and the Paleocene epoch (65.5-
55.8 m.y.a.), global temperatures declined from their

mid-Cretaceous highs. Temperatures abruptly increased
between the late Paleocene to the early Eocene epoch
(55.8-33.9 m.y.a.). At the onset of the early Eocene
warming, the carbon content of deep sea bulk sed-
iments decreased, indicating an increase in atmo-
spheric CO 2 (g) (Berner et al., 1983; Shackleton, 1985).
Also, the Norwegian-Greenland Sea opened (Talwani
and Eldholm, 1977). This and other tectonic activity
may have resulted in enhanced volcanism, increasing
CO 2 (g).
Following the early Eocene temperature maximum,
agradual global cooling occurred from 50 to 3 m.y.a.
During this period, continents drifted toward higher lat-
itudes, cooling the land. A sharp cooling event occurred
near 40 to 38 m.y.a., in the late Eocene epoch, resulting
in one of the largest extinctions during the Cenozoic
era (65.5 m.y.a.-present).
Cenozoic era ice may have first appeared over the
Antarctic 40 m.y.a., forming the base of the present-

day Antarctic ice sheets .An ice sheet is a broad, thick
sheet of glacier ice that covers an extensive land area
for a long time. An ice sheet can also form on top of sea
ice if the sea ice is adjacent to land. A glacier is a large
mass of land ice formed by the compaction and recrys-
tallization of snow. Glaciers flow slowly downslope or
outward in all directions under their own weight. Sea ice
is ice formed by the freezing of sea water (Figure 12.18).
Today, the Antarctic is covered by two ice sheets, the
East and West ice sheets ,which are separated by the
Transantarctic Mountains. The West ice sheet, which
lies over the Ross Sea and over land (Figure 12.19), is
an order of magnitude smaller than is the East ice sheet,
which lies exclusively over land.

∼

(a) (b)

Figure 12.18. (a) Spring (1950) melt of sea ice in the Beaufort Sea, near Tigvariak Island, Alaska North Slope.
(b) Winter (1950) sea ice near the same location. Rear Admiral Harley D. Nygren, NOAA Corps (ret.), available
from NOAA Central Library, www.photolib.noaa.gov/.