Page 68 - Earth's Climate Past and Future

44 PART II • Tectonic-Scale Climate Change

Greenhouse Worlds

The first clue that a factor other than distance to the Sun
is involved in Earth’s habitability comes from comparing
it to Venus, another “terrestrial” planet with a similar Incoming Reflected back to space
overall chemical composition (Figure 3-1). Venus is a solar radiation 515 W/m 2
very hot planet with a mean surface temperature of 645 W/m 2
460°C, and it lies 72% as far from the Sun as Earth does.
The average amount of solar radiation sent to each 285˚C greenhouse effect
460˚ at surface
planet varies inversely with the square of its distance
2
from the Sun (1/d ). Based on this relationship, Venus
receives almost twice (1.93 times) as much solar radia- W/m 2 absorbed
tion as Earth does: 96% CO 2
1
2
(1)
Earth –––––– = –––––– = 1.93 130
Venus (0.72) 2 0.518
At first, this calculation might seem to confirm that cli-
mate depends entirely on distance from the Sun: because
Venus is closer to the Sun, its surface is hotter. In fact,
however, this is not the real answer, because most of the
Sun’s radiation never arrives at the surface. The upper
atmosphere of Venus is shrouded in a thick cover of sulfu-
ric acid clouds that reflect 80% of the incoming radiation A Venus
and allow only 20% to reach the surface of the planet. In
contrast, clouds on Earth reflect just 26% of the incom-
ing radiation, allowing the other 74% to reach its surface.
This large difference in average albedo (the per- Reflected back to space
100 W/m 2
centage of incoming radiation reflected back to space) Incoming
solar radiation
between the atmospheres of the two planets almost 342 W/m 2 33˚C greenhouse effect
15˚ at surface
exactly reverses the relative amounts of solar energy that
actually reach their surfaces. Even though Venus receives
almost twice as much incoming solar energy at the top of
its atmosphere, its higher albedo reduces the amount that W/m 2 absorbed .02% CO 2
reaches its surface to just over half that received on Earth:
0.20
1.93 × –––– = 0.52
0.74 242
With less incoming solar radiation, how can Venus be so
much hotter? The answer is that Venus has an atmos-
phere 90 times as dense as that of Earth, and 96% of its
atmosphere is composed of carbon dioxide (CO ), a
2
greenhouse gas that is very effective in trapping radiation.
Some sunlight does penetrate the thick atmosphere and B Earth
heat the surface, which causes Venus to emit radiation,
just as Earth does. This kind of back radiation from its FIGURE 3-1 Why is Venus hot? (A) Venus receives almost
heated surface, called longwave radiation, is analogous twice as much solar radiation as (B) Earth, but its dense cloud
to the heat emitted by a radiator. But most of the long- cover permits less radiation to penetrate to its surface. Yet
wave back radiation never leaves the atmosphere of Venus Venus is much hotter than Earth because its CO -enriched
because the CO gas traps it and retains it as internal heat. 2
2 atmosphere creates a much stronger greenhouse effect that
In contrast, much less of the energy radiated back from traps much more heat. (NASA photos.)
Earth’s surface is trapped by water vapor, CO , and other
2
greenhouse gases (companion Web site, pp. 2–3).
Because Venus and Earth both formed as rocky plan-
In summary, the main reason Venus is so hot com- ets in the inner part of our solar system, they contain
pared to Earth is not its closer proximity to the Sun nearly equal amounts of carbon. Yet the two planets store
but its far greater concentrations of heat-trapping their carbon in very different reservoirs. Most of Earth’s
greenhouse gases. carbon is tied up in its rocks, some as coal, oil, and