Page 505 - Environment: The Science Behind the Stories

400 2000 CO concentration is at its highest level by far in over 800,000
2
Carbon dioxide (CO ) 1800 years, and likely the highest in the last 20 million years.
2
Why have atmospheric carbon dioxide levels risen so
CO 2 (ppm), N 2 O (ppb) 350 Nitrous oxide (N 2 O) 1600 CH 4 (ppb) layers of the lithosphere (p. 140). The deposition, partial
Methane (CH )
4
rapidly? Most carbon is stored for long periods in the upper
1400
decay, and compression of organic matter (mostly plants and
phytoplankton) that grew in wetland or marine areas hun-
1200
dreds of millions of years ago led to the formation of coal,
300
1000
human activity, these carbon reservoirs would remain buried
800 oil, and natural gas in buried sediments. In the absence of
for many millions more years. However, over the past two
250 600 centuries we have extracted these fossil fuels from the ground
0 500 1000 1500 2000
Year and burned them in our homes, factories, and automobiles,
transferring large amounts of carbon from one reservoir (the
Figure 18.2 Since the start of the industrial revolution, underground deposits that stored the carbon for millions of
global atmospheric concentrations of carbon dioxide, years) to another (the atmosphere). This sudden flux of car-
methane, and nitrous oxide have increased markedly. Data bon from lithospheric reservoirs into the atmosphere is the
from Intergovernmental Panel on Climate Change (IPCC), 2007. Fourth assess- main reason atmospheric carbon dioxide concentrations have
ment report. FAQ 2.1, Fig 1, in The physical science basis: Contribution of Work- increased so dramatically.
ing Group I.
At the same time, people have cleared and burned forests
By about what percentage has atmospheric carbon to make room for crops, pastures, villages, and cities. Forests
dioxide concentration increased since 1750? serve as a reservoir for carbon as plants conduct photosyn-
thesis (p. 50) and store carbon in their tissues. Thus, when
we clear forests it reduces the biosphere’s ability to remove
Greenhouse gas concentrations are rising fast carbon dioxide from the atmosphere. In this way, deforesta-
tion (pp. 329–332) contributes to rising atmospheric CO
2
The greenhouse effect is a natural phenomenon, and green- concentrations. Figure 18.3 summarizes scientists’ current
house gases have been present in our atmosphere for all of understanding of the fluxes (both natural and anthropogenic)
Earth’s history. It’s a good thing, too. Without the natural of carbon dioxide between the atmosphere and reservoirs on
greenhouse effect, our planet would be too cold to support Earth’s surface.
life as we know it. Thus, it is not the natural greenhouse effect Methane concentrations are also rising—2.5-fold since
that concerns scientists today, but rather the anthropogenic 1750 (see Figure 18.2)—and today’s atmospheric concentra-
(human-generated) intensification of the greenhouse effect. tion is the highest by far in over 800,000 years. We release
By adding novel greenhouse gases (certain halocarbons) methane by tapping into fossil fuel deposits, raising livestock
to the atmosphere, and by increasing the concentrations of that emit methane as a metabolic waste product, disposing of
several natural greenhouse gases over the past 250 years organic matter in landfills, and growing crops such as rice.
(Figure 18.2), we are intensifying our planet’s greenhouse Human activities have also enhanced atmospheric concen-
effect beyond what our species has ever experienced. trations of nitrous oxide. This greenhouse gas, a by-product of
We have boosted Earth’s atmospheric concentration of car- feedlots, chemical manufacturing plants, auto emissions, and
bon dioxide from 280 parts per million (ppm) in the late 1700s synthetic nitrogen fertilizers, has risen by nearly 20% since
to 396 ppm in 2013 (see Figure 18.2). Today’s atmospheric 1750 (see Figure 18.2).

Natural fluxes Anthropogenic fluxes Figure 18.3 Human activities are
Units are in billions of metric tons of CO per year sending more carbon dioxide from
2
Earth’s surface to its atmosphere
Atmosphere than is moving from the atmos-
phere to the surface. Shown are
26 6 0.3 ~15 all current fluxes of CO , with arrows
Photosynthesis 440 Increased uptake by plants Absorption 260 Absorption 70 = arrows indicate natural fluxes, and red
2
sized according to mass. Green
arrows indicate anthropogenic fluxes.
Adapted from IPCC, 2007. Fourth assessment
Respiration Industry Volcanoes 260 Release 80 Release For every metric ton of carbon
440 Changing land use Weathering Net accumulation report.
dioxide we emit due to
0.7
10
changing land use (e.g. deforestation),
504 Land Ocean how much do we emit from industry?

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