Biology 12

526 MHR • Unit 5 Population Dynamics

(of human origin) sources of carbon dioxide to

natural ones. The burning of fossil fuels and

deforestation(the cutting down of forests) have

been major contributors of carbon dioxide all over

the world. Since living trees take up carbon

dioxide for photosynthesis, they act as sinks that

reduce the level of this gas in the atmosphere. As

a result, forest fires and the burning of felled trees

are doubly damaging, because they destroy the

sinks and release large amounts of carbon dioxide.

Most scientists believe that excessive production

of greenhouse gases could enhance the greenhouse

effect and thus raise the average temperature on

Earth, producing what is known as global warming.

The effects of this are still being debated, but they

may include major climate alterations, rising sea

levels (enough to destroy coastal ecosystems and

flood coastal cities), increased drought in some areas,

and changed conditions for farming (possibly

accompanied by a drop in production) in many

regions. The Thinking Lab on page 527 will help

you understand some of the potential problems

related to global warming.

Natural sources of carbon dioxide include cellular

respiration (which produces about 60 billion tonnes per

year) and the ocean (in which roughly 90 billion tonnes

diffuse annually). Anthropogenic sources make up about

seven billion tonnes per year. Of this latter amount, about

three billion tonnes remain in the atmosphere and roughly

two billion tonnes are absorbed into the oceans (which act

as a sink for much more than this). Scientists think that

much of the remaining two billion tonnes is absorbed by

land vegetation. Recent studies conducted in Canada have

indicated that the trees and soil of the boreal forest, for

example, act as a sink for about 0.6 billion tonnes of carbon

dioxide per year.

BIO FACT

Your Electronic Learning Partner has animation clips that

will enhance your understanding of the greenhouse effect

on Earth.

ELECTRONIC LEARNING PARTNER

Figure 15.18How do greenhouse gases affect the temperature of Earth?

sunlight

heat

radiated heat

heat

CO 2 andothe

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