Page 69 - Earth's Climate Past and Future

CHAPTER 3 • CO and Long-Term Climate 45
2

natural gas, while relatively little resides in the atmos- 0
No
phere. Combined with water vapor and other natural gases
greenhouse gases, the net greenhouse heating of Earth’s 1
atmosphere is relatively small—about 33°C (although
that difference keeps Earth from freezing solid). In com- 2
plete contrast, almost all the carbon on Venus resides in Byr ago
its atmosphere as CO and produces an enormous net
2 3
greenhouse warming (285°C) without any significant
contribution from water vapor. 4 With
This comparison shows how vital greenhouse gases greenhouse
can be to the climate of planets. It also highlights the gases
fact that Earth’s comfortably small greenhouse effect is 0.7 0.8 0.9 1 –50 –25 0 25
an important factor in its habitability. Solar luminosity Temperature (°C)
relative to present value
Faint Young Sun Paradox FIGURE 3-2 Faint young Sun paradox Astrophysical
models of the Sun’s evolution indicate that it was 25% to 30%
By studying the evolution of stars in the universe, weaker early in Earth’s history (left). Climate models show
astronomers have recreated the history of our own Sun that this situation would have produced a completely frozen
over the 4.55 Byr existence of our solar system. Through- Earth for more than half its early history if the atmosphere had
out this interval, the Sun’s interior has been the site of an had the same composition it does today (right). (Adapted
ongoing nuclear reaction that fuses nuclei of hydrogen from D. Merritts et al., Environmental Geology, ©1997 by W. H.
(H) together to form helium (He). Models developed by Freeman and Company.)
astronomers indicate that this process has caused our Sun
to expand and gradually become brighter. These models
indicate that the earliest Sun shone 25% to 30% more the end of this chapter, a debate is currently under way
faintly than today, and that its luminosity, or brightness, as to how close Earth’s climate came to a nearly frozen
then slowly increased to its current strength. condition during intervals between 850 and 550 Myr
This insight from the field of astronomy creates ago, but for most of Earth’s history the sedimentary evi-
an intriguing problem for climate scientists. A relatively dence leaves no doubt that most of the water on Earth
small decrease in our Sun’s present strength would has remained unfrozen.
cause all the water on Earth to freeze, despite the warm- This conclusion is supported by the continued pres-
ing effect from greenhouse gases. If all our oceans and ence of life on Earth. Primitive life-forms date back
lakes were to freeze, their bright snow and ice surfaces to at least 3.5 Byr ago, and their presence on Earth is
would reflect more solar radiation and they would be incompatible with a completely frozen planet at that
difficult to melt. One-dimensional numerical climate time. The succession of ever more complex life-forms
models that simulate the mean climate of the entire that have continuously occupied Earth ever since add
planet (Chapter 2) suggest that the combination of a further proof against extreme cold (or heat).
weak Sun and greenhouse gas levels at their present val- So we are confronted with a mystery: With so weak
ues would have kept Earth completely frozen for the a Sun, why wasn’t Earth frozen for the first two-thirds
first 3 billion years of its existence (Figure 3-2). of its history? This mystery has been named the faint
Yet evidence left in Earth’s sedimentary deposits young Sun paradox.
shows that Earth was not frozen for its first 3 billion Part of the answer to the faint young Sun paradox is
years. Although the first half-billion years of Earth’s exis- obvious: something kept the early Earth warm enough
tence left no record, evidence of Earth’s climatic history to offset the Sun’s weakness, but this easy answer only
gradually becomes more complete after that time and raises a more difficult problem. Whatever the process
toward the present. Most sedimentary rocks (Chapter 2) was that warmed the younger Earth, it must no longer
are made up of particles that were eroded from other be doing so today, or at least not as actively as it once
rocks, reworked by running water, and transported to a did. If this same warming process had continued work-
site of deposition. The prevalence of water-deposited ing at full strength right through the entire 4.55 Byr
sedimentary rocks throughout Earth’s history is direct of Earth’s history, it would have combined with the
evidence that Earth was not completely frozen. steadily increasing warmth from the strengthening Sun
The first evidence of ice-deposited sediments occurs (see Figure 3-2) to overheat Earth and make it uninhab-
in rocks dated to about 2.3 Byr ago, but these deposits itable. Yet that has not happened: somehow Earth has
were probably the result of glaciations in polar regions stayed within a moderate temperature range through-
similar to those on Earth today, and they are not evi- out the entire interval when the Sun’s brightness was
dence of a completely frozen planet. As summarized at increasing.