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cycling, see Chapter 21.) One way that carbon

is transferred between the biotic and abiotic

worlds is through combustion—the burning of

carbon-rich materials, living or not.

Some of the organic matter from ancient

organisms has been transformed by geologic

processes into deposits of fossil fuels such as

petroleum, coal, and natural gas. When we

extract these fossil fuels and burn them to meet

our energy needs, the carbon that was locked

in these deposits for hundreds of millions of

years is released into the atmosphere as carbon

dioxide.

Plants also release carbon back into the

atmosphere when they are burned. To assess the

amount of carbon released by a burning rainfor-

est, Balch and her team measured the amount

of leaf litter and branches—the biomass—on

the forest floor both before and after a burn,

as well as the number of trees affected. Half of

that biomass is carbon, so half of the difference

between the biomass before and after the burn is

the amount of carbon that was released into the

atmosphere. It was a lot.

The very first burn, for example, released

“about 20 tons of carbon per hectare,” says Balch.

That’s approximately equal to the carbon emit-

ted by a passenger car driven 864,000 miles

(burning about 40,000 gallons of gasoline). And

burns have another, indirect, detrimental effect

that Balch wasn’t able to directly measure: they

destroy trees that normally absorb carbon diox-

ide from the air. In a normal year, trees in the

Amazon absorb approximately 1.5 billion tons of

carbon dioxide, helping to slow climate change.

In this way, they act as a carbon sink, a natural

or artificial reservoir that absorbs more carbon

than it releases.

But during the 2005 drought, as trees died

and rotted, the rainforest stopped absorbing

more carbon dioxide than it gave off, and it

actually released carbon into the atmosphere.

In 2005, 5 billion tons of carbon were released.

Instead of acting as a carbon sink, the Amazon

became a carbon source, a reservoir that releases

more carbon than it absorbs (Figure 18.15).

And with increasing temperatures, decreasing

rainfall, and increasing wildfires, ecologists fear

that the Amazon could transition more and more

into a carbon source instead of a carbon sink.

If that happens, our planet will lose one of its

greatest buffers against future climate change.

Figure 18.15

Carbon sinks and sources

Q1: How does a carbon source contribute to global warming?

Q2: How does a carbon sink protect against global warming?

Q3: How can trees act as both a source and a sink?

A carbon sink

absorbs more CO 2

than it gives off.

A carbon source

gives off more CO 2

than it absorbs.

Waiting and Watching

Balch and her crew have stopped burning the

plots and have begun to record what happens

to them as they recover. In addition to making

direct observations of which plants grow back

and how their growth affects the water and

carbon cycles, the team is conducting new exper-

iments on the burned plots to see whether it is

possible to help prevent the invasion of grasses

and encourage a return to rainforest.

In one area, the scientists are planning to

plant different types of trees to see whether

particular species can successfully reestab-

lish themselves in a mat of grasses and retake

that land. Another experiment, also still in the

planning phase, will test different amounts of

watering to see whether the lack of precipitation