HUMAN BIOLOGY

478 Chapter 25

The troughs and peaks in Figure 25.10 (facing page) are

annual lows and highs of global carbon dioxide concentra­

tions. For the first time, scientists have been able to see the

big picture regarding effects of the carbon balances for an

entire hemisphere. Notice the midline of the troughs and

peaks in the cycle. It shows that the concentration of carbon

dioxide is steadily increasing, as are the concentrations of

other greenhouse gases.

Greenhouse gas levels are far higher than they were in

most of the past. Carbon dioxide may be at its highest level

since 420,000 years ago, perhaps even longer.

n    Human activities have increased the concentrations of

greenhouse gases in the lower atmosphere so that the

average temperature is warming. This warming is driving

major changes in Earth’s climate.

A variety of gases in Earth’s atmosphere play a key

role in shaping the average temperature near its surface.

Temperature, in turn, has huge effects on global and

regional climates.

Atmospheric molecules of carbon dioxide, water, ozone,

methane, nitrous oxide, and chlorofluoro carbons are the

major players in interactions that affect global temperature.

Collectively, the gases act like the panes of glass in a

greenhouse—hence their name, “greenhouse gases.” Wave­

lengths of visible light pass through these gases to Earth’s

surface, which absorbs them and then emits them as heat.

Greenhouse gases slow the escape of this heat back into

space. Instead, much of it radiates back toward Earth’s sur­

face (Figure 25.9).

With time, heat builds up in the lower atmosphere and

the air temperature near the surface rises. This warming

action is known as the greenhouse effect. Without it,

Earth’s surface would be too cold to support life.

In the 1950s, researchers in a laboratory on Hawaii’s

highest volcano began a long­term program of measuring

the concentrations of greenhouse gases in the atmosphere.

They found that carbon dioxide concentrations follow the

annual cycle of plant growth (that is, primary production)

in the Northern Hemisphere. This includes the growth

of phytoplankton at the ocean surface. CO 2 levels drop in

summer, when photosynthesis rates are highest (and plants

use lots of CO 2 ). They rise in winter, when photosynthesis

by plants slows.

F i g u r e 25.10 Human activities produce large amounts of

greenhouse gases. Recorded changes in global temperature

between 1875 and 2008 (facing page). At this writing, 2005

and 2010 were tied as the hottest years on record in the

Northern Hemisphere. Above, Mexico City on a smoggy

morning. In 2013, roughly 20 million people were living in

Mexico City and its environs.

Figure 25.9 Animated! The greenhouse effect warms Earth’s surface. (© Cengage Learning)

A Rays of sunlight

penetrate the lower

atmosphere and

warm Earth’s surface.

B The surface radiates heat (infrared wavelengths) to the

lower atmosphere. Some heat escapes into space. But

greenhouse gases and water vapor absorb some infrared

energy and radiate a portion of it back toward Earth.

C Increased concentrations of greenhouse gases

trap more heat near Earth’s surface. Sea surface

temperature rises, more water evaporates into the

atmosphere, and Earth’s surface temperature rises.

Yann Arthus-Bertrand/Latitude/Corbis

25.5 Global Warming and Climate Change

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