Biology Now, 2e

Amazon on Fire ■ 329

Absorbed energy is reemitted

to the atmosphere as infrared

radiation.

(^1) About a third of the
incoming solar radiation
is reflected back into
space by the atmosphere
and Earth’s surface.
(^2) Some of the incoming
solar radiation is absorbed
by Earth’s surface, thereby
warming the planet.
(^3) A portion of the infrared radiation
is absorbed by greenhouse gases
in the atmosphere, and some is
reemitted. The reemitted infrared
radiation further warms the
atmosphere and Earth’s surface.

4

Some of the infrared radiation is lost

to outer space. When the

concentration of greenhouse gases

increases, more infrared radiation is

absorbed by these gases and less is

lost at this step.

5

Earth

Sun

Atmo

sphere

Greenhouse^ gases

Figure 18.7

How greenhouse gases warm the surface of Earth

Carbon dioxide (CO 2 ), water vapor (H 2 O), methane (CH 4 ), and nitrous oxide (N 2 O) are known as greenhouse gases because they

absorb and trap heat that would otherwise radiate away from Earth. M

Q1: How much of the incoming solar energy is reflected back to outer space?

Q2: What kind of energy is reemitted to the atmosphere after being absorbed by Earth’s surface?

Q3: How are greenhouse gases like a blanket on your bed at night?

climate, and increasing wildfires are devastat-

ing that tree cover.

“The big question is, What’s the limit for

these tropical forests? How much fire can they

withstand before they convert to something

else?” says Balch. That something else would

be grasslands, also called savanna. If major

parts of the Amazon rainforest were to convert

to savanna, the climate would be significantly

affected, and there would be no easy way—

perhaps no way at all—to revert that land back

to rainforest.