Page 141 - Environment: The Science Behind the Stories

Atmosphere

Respiration
Respiration RespirationRespiration
GPP
GPP

Consumers Producers
Atmosphere
Decomposers
750 + 5.0/yr
90
Oceans 90
Ocean-
Reduced
Ocean- Reduced
atmosphere uptake by
uptake by
atmosphere
exchange Fossil fuel plants Respiration RespirationRespiration
plants
exchange
Fossil fuel
Respiration
0.9
60
combustion
60
92
Volcanic 92 combustion 0.9 60 GPP 60
Volcanic
GPP
9.1
and hydrothermal 9.1 120
and hydrothermal
120
emissions
emissions
< 0.1
< 0.1 Rivers Net
deforestation
Runoff
Runoff
0.8
0.8 Land
Weathering
Oceans Weathering plants Consumers
< 0.1
38,000 < 0.1 615
Anthropogenic
sources
Decomposition
Decomposition
Fossil fuel
Burial 0.1
Burial 0.1 Fossil fuel
extraction
extraction
Soil and
Oceans Fossil fuels soil biota
(coal, oil, natural gas) 2344
Sedimentary rock 4000
80,600,000
Figure 5.17 The carbon cycle summarizes the many routes that carbon atoms take as they move
through the environment. Gray arrows represent fluxes among reservoirs, or pools, for carbon. In the carbon
cycle, plants use carbon dioxide from the atmosphere for photosynthesis (gross primary production, or “GPP” in
the figure). Carbon dioxide is returned to the atmosphere through cellular respiration by plants, their consumers,
and decomposers. The oceans sequester carbon in their water and in deep sediments. The vast majority of the
planet’s carbon is stored in sedimentary rock. In the figure, pool names are printed in black type, and numbers
in black type represent pool sizes expressed in petagrams (units of 10 g) of carbon. Processes, printed in italic
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red type, give rise to fluxes, printed in italic red type and expressed in petagrams of carbon per year. Data from
Schlesinger, W.H., 2013. Biogeochemistry: An analysis of global change, 3rd ed. Academic Press, London.
many factors, including temperature and the numbers of marine In addition, cutting down forests removes carbon from
organisms converting CO into carbohydrates and carbonates. the pool of vegetation and releases it to the air. And if less
2
vegetation is left on the surface, there are fewer plants to draw
We are shifting carbon from the lithosphere CO back out of the atmosphere.
2
to the atmosphere As a result, scientists estimate that today’s atmospheric
carbon dioxide reservoir is the largest that Earth has expe-
By mining fossil fuel deposits, we are essentially removing rienced in the past 800,000 years, and likely in the past 20
carbon from an underground reservoir with a residence time million years (p. 504). The ongoing flux of carbon into the
of millions of years. By combusting fossil fuels in our auto- atmosphere is the driving force behind today’s anthropogenic
mobiles, homes, and industries, we release carbon dioxide and global climate change (Chapter 18).
greatly increase the flux of carbon from the ground to the air. Some of the excess CO in the atmosphere is now being
2
Since the mid-18th century, our fossil fuel combustion has absorbed by ocean water. This is causing ocean water to
added over 250 billion metric tons (276 billion tons) of carbon become more acidic, leading to problems that threaten many
to the atmosphere. The movement of CO from the atmos- marine organisms (pp. 446–447).
2
phere back to the hydrosphere, lithosphere, and biosphere has Our understanding of the carbon cycle is not yet complete.
140 not kept pace. Scientists remain baffled by the so-called missing carbon sink.
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