Page 106 - Earth's Climate Past and Future

82 PART II • Tectonic-Scale Climate Change

What Explains the Warmth 100 Myr Ago? and crocodiles, all existing north of the Arctic Circle
(Figure 5-2). This evidence contrasts markedly with the
Around 175 Myr ago, the giant single continent of cold climatic conditions at such latitudes today.
Pangaea began to break apart. By 100 Myr ago, most of The rest of Earth’s surface also seems to have been
today’s continents had separated from one another, pro- warmer than today. Dinosaurs lived even on those parts
ducing a very different-looking Earth consisting of a of Australia and Antarctica lying south of the Antarctic
half-dozen smaller continents. In addition, global sea Circle. At lower latitudes, coral reefs indicative of warm
level stood at least 100 m higher than it does today, and tropical ocean temperatures extended some 10º of lati-
a shallow layer of ocean water flooded continental mar- tude farther from the equator than they do today
gins and low-lying interior areas. (40º versus 30º).
The geographic effect of this flooding was to frag-
ment the existing continents into even smaller areas of 5-1 Model Simulations of the Cretaceous
land (Figure 5-1), making the geography of this green- Greenhouse
house world even more unlike the single Pangaean land-
mass. Geologists call this interval the middle Cretaceous, To explore the reasons for the warmth 100 Myr ago, cli-
a word meaning “abundance of chalk,” because marine mate modeler Eric Barron and his colleagues used con-
limestones deposited by these high seas are common straints provided by plate tectonic reconstructions of
around the world. This interval is important to climate Earth’s geography to run general circulation model
scientists because geologic records contain no evidence of simulations of the climate of the Cretaceous world. The
permanent ice anywhere on Earth, even on the parts of surface temperature estimates from the resulting simu-
the Antarctic continent situated right over the South lations were plotted in the form of zonal mean trends
Pole. Much of this interval seems to have been a warm averaged around lines of latitude (Figure 5-3). These
(greenhouse?) world. latitude-averaged plots represent the complex spatial
In the northern hemisphere, evidence for unusual pattern of temperature changes across Earth’s surface in
warmth comes from several fossil indicators, including a shorthand form. The latitudes shown in this and sub-
warm-adapted vegetation such as broad-leafed evergreen sequent figures are plotted so that they correct for the
trees that hold their leaves throughout the year except for much larger area of Earth’s surface at low latitudes than
a brief interval of leaf fall and regrowth. Other evidence in polar regions. These plots show all latitudes in
includes warm-adapted animals such as dinosaurs, turtles, proportion to the area they actually cover.

60°

30°

High land areas
Low land areas 0°
Inundated areas
Oceans

30°

60°

FIGURE 5-1 The world 100 Myr ago By 100 Myr ago, plate tectonic processes had broken the
giant Pangaean continent into separate smaller continents that were flooded by shallow seas.
(D. Merritts et al., Environmental Geology, 1997 by W. H. Freeman and Company.)