The Solar System

CHAPTER 20 | EARTH: THE STANDARD OF COMPARATIVE PLANETOLOGY 437

greenhouse eff ect (■ Figure 20-10a). When sunlight shines

through the glass roof of a greenhouse, it heats the benches and

plants inside. Th e warmed interior radiates infrared radiation,

but the glass is opaque to infrared. Warm air in the greenhouse

cannot mix with cooler air outside, so heat is trapped within the

greenhouse, and the temperature climbs until the glass itself

grows warm enough to radiate heat away as fast as the sunlight

enters. Th is is the same process that heats a car when it is parked

in the sun with the windows rolled up.

Earth’s atmosphere is transparent to sunlight, and when the

ground absorbs the sunlight, it grows warmer and radiates at

infrared wavelengths. However, CO 2 makes the atmosphere less

transparent to infrared radiation, so infrared radiation from the

warm surface is absorbed by the atmosphere and cannot escape

back into space. Th at traps heat and makes Earth warmer

(Figure 20-10b).

It is a Common Misconception that the greenhouse

eff ect is only bad. Evidence indicates that Earth has had a green-

house eff ect for its entire history. Without the greenhouse eff ect,

Earth presently would be at least 30°C (54°F) colder and unin-

habitable for humans. Th e problem is that human civilization is

adding CO 2 to the atmosphere more rapidly than it can be natu-

rally removed, thereby increasing the intensity of the greenhouse

eff ect.

CO 2 is not the only greenhouse gas. Water vapor, meth-

ane, and other gases also help warm Earth, but CO 2 is the

most important. For 4 billion years, natural processes on Earth

have removed CO 2 from the atmosphere and buried the car-

bon in the form of limestone, coal, oil, and natural gas. Since

the beginning of the Industrial Revolution in the late 18th^

century, humans have been digging up lots of carbon-rich

fuels, burning them to get energy, and releasing CO 2 back into

the atmosphere. It is a Common Misconception that

human output of CO 2 is minor compared to natural sources

such as volcanoes. Careful measurements of carbon isotope

ratios and relative amounts of CO 2 versus O 2 in the atmo-

sphere show that the CO 2 added to the atmosphere since the

year 1800 is mostly or entirely due to human burning of fossil

fuels. Estimates are that the amount of CO 2 in Earth’s atmo-

sphere could double during the 21st century. Th e increased

concentration of CO 2 is increasing the greenhouse eff ect and

warming Earth in what is known as global warming. Studies

of the growth rings in very old trees show that the average

Earth climate had been cooling for most of the last 1000 years,

but the 20th century reversed that trend with a rise of 0.56 to

0.92°C (1.01 to 1.66°F). Th e amount of warming to expect in

the future is diffi cult to predict because Earth’s climate is criti-

cally sensitive to a number of diff erent factors, not just the

abundance of greenhouse gases. For example, a planet’s albedo

is the fraction of the sunlight hitting it that gets refl ected away.

A planet with an albedo of 1 would be perfectly white, and a

planet with an albedo of 0 would be perfectly black. Earth’s

■ Figure 20-10

The greenhouse effect: (a) Visual-wavelength sunlight can enter a green-
house and heat its contents, but the longer-wavelength infrared radiation
cannot get out. (b) The same process can heat a planet’s surface if its
atmosphere contains greenhouse gases such as CO 2. (c) The concentration
of CO 2 in Earth’s atmosphere as measured in Antarctic ice cores remained
roughly constant for thousands of years until the beginning of the Industrial
Revolution around the year 1800. Since then it has increased by more than
30 percent. Evidence from proportions of carbon isotopes and oxygen in
the atmosphere proves that most of the added CO 2 is the result of burning
fossil fuels. (Graph adapted from a fi gure by Etheridge, Steele, Langenfelds, Francey,
Barnola, and Morgan)

1000 1200

340

1400 1600

earY

1800 2000

a

b

c

CO

(ppm) 2

Industrial

Revolution

Visual-wavelength

sunlight

Greenhouse

gas molecules

Visual-

wavelength

sunlight

Greenhouse

Atmosphere

of planet

Infrared

radiation

Infrared

radiation

330

390

320

310

380

370

360

350

300

290

280

270