During the past 40 million years, and particularly during the past 15 million years,
this warm, wet climate largely disappeared. Colder climates and much greater
regional extremes of precipitation have developed. What caused this cooling and
diversification of climate and vegetation into a complex mosaic of many regionally
distinctive types?
One school of thought focuses on the changing positions of the earth’s
continents and oceans. The Atlantic Ocean has expanded at the expense of the
Pacific Ocean, whereas an ancient equatorial sea that extended across much of
Eurasia (called the Tethys Sea) has shrunk to become the modern, much smaller
Mediterranean Sea
. In addition, the fraction of continents flooded by shallow inland seas has slowly
decreased, exposing large amounts of land and creating climates less moderated
by the temperature-stabilizing effects of oceans. Computer model simulations
show that changes in the arrangement of the continents and the size of inland seas
can have important effects on global climate over very long intervals of geologic
time. But they are significantly less convincing as sole explanations for the dramatic
changes of the past 40 million years.
Another possibility is a long-term decline in the concentration of carbon dioxide in
the atmosphere, which would lessen the amount of heat trapped by the
atmosphere and lead to “greenhouse cooling.” The amount of carbon dioxide in
the earth’s atmosphere over million-year timescales is controlled by two major
processes. Chemical weathering of continental rocks removes carbon dioxide from
the atmosphere and carries it in dissolved chemical from to the ocean, where it is
taken in by marine biota and deposited in sediments on the seafloor.Tectonic
activity eventually frees this trapped carbon dioxide, in the earth’s lithospheric
plates transports the seafloor to ocean trenches, where subduction carries old crust
and sediments down toward the earth’s hot interior. At great depths, the
sediments melt, releasing carbon dioxide, which emerges from the volcanic islands
that overlie the buried curst and rejoins the atmosphere, completing the cycle.
If the pace of seafloor spreading (and hence of subduction) slowed significantly,
less carbon dioxide would be vented to the atmosphere, the atmosphere would
become relatively depleted of carbon dioxide and temperatures would fall. In fact,
globally averaged seafloor spreading rates slow little or no net change in the past
40 million years. Subduction and volcanism eventually return the carbon dioxide to
the atmosphere, but this process requires a long time (tens to hundreds of millions
of years) to complete.
Plateau uplift may alter climate by increasing chemical weathering of rocks, thereby
reducing atmospheric carbon dioxide concentrations. Carbon dioxide combines
with rainwater and ground water to form carbonic acid, which reacts with silicate
minerals in rocks during weathering. The resulting bicarbonate ions drain into the
oceans, where they are taken up by marine animals such as plankton and corals and
eventually deposited on the seafloor. The net effect is that chemical weathering
removes carbon dioxide from the atmosphere and locks it away at the bottom of
the oceans.
Maureen Raymo proposed that uplift of plateaus and mountain ranges has
increased the rate of chemical erosion of continental rock on the globally averaged
basis. Uplift could enhance chemical weathering in several ways. Heavy monsoons,
which develop at the margins of plateaus, unleash particularly intense rainfall. In
these regions, uplift-related faulting and folding also expose fresh rock to the
weathering process. Moreover, the steeper slopes created by plateau uplift cause
faster runoff, which removes erosion products and intensifies the chemical attack
on the rock. Raymo suggests that long-term uplift in Tibet and other regions may
have increased the rate at which carbon dioxide is removed from the atmosphere.
In this way, concentrations would have fallen even though the amount of carbon
dioxide exhaled by volcanoes (as inferred from seafloor spreading rates) remained
nearly constant. Falling carbon dioxide levels would reduce the ability of the
atmosphere to retain heat, thereby amplifying the global cooling