Geoscience Reference
In-Depth Information
the terrestrial biosphere (~1.0 Pg/yr). The global carbon budget is discussed in
more detail in
chapter 12.
Ice cores from Vostok, Antarctica, provide a record of atmospheric CO
2
levels back more than 400,000 years. These values are based on the analysis
of air bubbles trapped when ice forms. The data are highly reliable since the
ice formation process essentially samples the air at the time of its formation
and protects the sample. Similar records are available from the Arctic, but the
Vostok ice cores are the longest (reaching a depth of nearly 4 km), having been
taken from the very stable and deep East Antarctic ice.
Figure 10.1c
shows a
record of atmospheric CO
2
concentration from Vostok. The most recent value
recorded in the ice core is about 280 ppm, which is the pre-industrial CO
2
concentration. Recent increases (
Figs. 10.1a
and
b)
are so rapid they are not
resolved in the ice cores. Proceeding backward in time, from left to right i
n Fig-
about 20,000 years ago. CO
2
values remained below 250 ppm during the entire
glacial period and rose to about 280 ppm during the last interglacial period,
about 130,000 years ago. CO
2
levels have oscillated with the coming and going
of glacial periods, increasing to about 280 ppm during interglacials and falling
to about 200 ppm during glacial periods. This glacial-interglacial cycling is
ultimately forced by well-known changes in the earth's orbital parameters (see
section 3.5). The CO
2
oscillation amplifies the externally forced temperature
changes and occurs primarily because a warmer ocean releases dissolved CO
2
in the same way that soda in an open glass goes flat when it is allowed to warm
to room temperature.
Following the most recent glacial period, which ended about 20,000 years
ago, atmospheric CO
2
concentrations remained at the background interglacial
value of about 280 ppm until the late 1700s.
Figure 10.1d
combines measure-
ments from Mauna Loa (after 1958) with information gathered from ice cores
at various Antarctic stations. These data show dramatically that the current
increases in atmospheric CO
2
are unprecedented and exponential.
METHANE (CH
4
)
Methane is a powerful greenhouse gas with a more complicated story than
CO
2
. It has numerous sources and sinks, is not well mixed globally, and under-
goes chemical reactions in the atmosphere. The largest sources of atmospheric
CH
4
are emissions from wetlands and rice paddies. Other important sources
are animals, termites, biomass burning, landfills, and fossil fuel (coal and gas)
production. The primary sink for atmospheric CH
4
is its photochemical oxida-
tion by the hydroxyl radical (OH) in the troposphere (see
chapter 12)
. As a
result, the atmospheric residence time for CH
4
is about 12 years, much shorter
than for CO
2
.
10.2a
for stations at Mauna Loa and Barrow, Alaska. The fact that these values
are different indicates that CH
4
is not well mixed in the atmosphere, but at
both locations CH
4
increased steadily from 1987 until about 2000, stabilized
from about 2000 to 2007, and then resumed rising.
