Biology Now, 2e

328 ■ CHAPTER 18 General Principles of Ecology

ECOLOGY

distant past, up to hundreds of thousands of

years ago, by measuring CO 2 concentrations in

air bubbles trapped in ice. This evidence shows a

near-perfect historical correlation between CO 2

levels and the surface temperature on Earth.

Ye t du r i ng t he la s t 20 0 ye a r s , le vel s of at mo-

spheric CO 2 have risen greatly—from roughly

280 to 400 parts per million (Figure 18.8).

Measurements from ice bubbles show that this

rate of increase is greater than even the most

sudden increase that occurred naturally during

the past 420,000 years. Carbon dioxide levels

are now higher than those estimated for any

time during that period.

The cause of the increasing levels of atmo-

spheric CO 2 has been linked to the burning

of fossil fuels like coal and oil, which releases

CO 2 into the air. About 75 percent of the

current yearly increase in atmospheric CO 2 is

due to the burning of fossil fuels. Logging and

burning of forests are responsible for most of

the remaining 25 percent of the increase, but

industrial processes also make a significant

contribution. “Humans have a giant impact on

the environment,” says Coe. “With more than

seven billion people on the planet, individual

decisions add up to big global changes.” (For

more on the environmental impact of personal

choices, see “How Big Is Your Ecological Foot-

print?” on page 332.)

All this is to say that human burning of fossil

fuels has led to an increase in carbon dioxide in

our atmosphere. That carbon dioxide acts as a

greenhouse gas and traps additional heat as it

leaves the atmosphere, causing temperatures on

Earth to rise. Since the early twentieth century,

Earth’s mean surface temperature has increased

by about 1.4°F (0.8°C), and it is estimated to rise

another 2°F–11.5°F (1.1°C–6.4°C) in the future

(Figure 18.9). As of the date this book went

to publication, 2016 was the hottest year ever

recorded; 2015 was previously the hottest year

ever; 2014 was the winner before that. We keep

beating our own record.

Our globe is warming. And the effects

of that warming on the biosphere are now

evident, especially in the Amazon, where trees

dominate the landscape. Trees act as a layer

connecting the atmosphere above them and the

ground beneath them, absorbing water from

the ground and releasing oxygen into the air.

Any change in tree cover can change the local

function much as the walls of a greenhouse or

the windows of a car do: they let in sunlight and

trap heat in a process known as the greenhouse

effect (Figure 18.7).

Greenhouse gases are not inherently bad;

in fact, they have existed in Earth’s atmosphere

for more than 4 billion years, and they play an

important role in maintaining temperatures that

are warm enough for life to thrive on Earth. Yet

human activities, primarily the burning of fossil

fuels, have released an excess of greenhouse gases

into the atmosphere, especially the infamous king

of greenhouse gases: carbon dioxide (CO 2 ).

Scientists have estimated atmospheric CO 2

levels for both the recent and the relatively

The sun’s rays hit Earth at an

angle near the poles, resulting

in cooler temperatures.

The sun’s rays

strike Earth directly

near the equator,

resulting in warmer

temperatures.

Figure 18.6

Sunlight strikes Earth most directly at

the equator

The angle at which the rays of the sun strike Earth

determines how much energy or heat reaches

Earth’s surface. The more direct the rays are when

they strike Earth, the more heat they deliver.

Q1: Why is it colder at the poles than at the

equator?

Q2: Why is it warmer at the equator than at

the poles?

Q3: During part of the year the Northern

Hemisphere is tilted at a more direct angle

to the sun than the Southern Hemisphere

is, and for the other part of the year the

opposite is true. How does this tilt explain

temperature differences in summer and

winter?