Geoscience Reference
In-Depth Information
10.1 THE ATMOSPHERE'S CHANGING
CHEMICAL COMPOSITION
Our concern about anthropogenic climate change stems from the observed
changes in the concentrations of greenhouse gases in the atmosphere. Of partic-
ular importance are increasing levels of carbon dioxide (CO
2
), methane (CH
4
),
nitrous oxide (N
2
O), chlorofluorocarbons (CFCs), and tropospheric ozone (O
3
).
CARBON DIOXIDE (CO
2
)
Regular measurements of the atmospheric CO
2
concentration were established
at the Mauna Loa Observatory (elev. 3400 m) in Hawaii in 1958. Measure-
ments are obtained by determining the degree of absorption of an infrared
beam as it passes through an air sample. This record, which is known as the
mean CO
2
levels, while the gray curve indicates monthly mean values. The unit
is parts per million (
ppm
) of CO
2
in the air by volume. A CO
2
concentration of
1 ppm means that in 10
6
(1 million) liters of air the CO
2
occupies a volume of
1 liter. Between 1958 and 2011, annual mean CO
2
levels increased about 25%,
from 315 ppm to 392 ppm.
Seasonal oscillations of about 5 ppm are apparent in the monthly mean
data. Because there is much more land and, therefore, vegetation in the North-
ern Hemisphere than in the Southern Hemisphere, CO
2
levels drop during
Northern Hemisphere summer due to photosynthetic activity.
The Mauna Loa CO
2
measurements have proved to be invaluable for es-
tablishing incontrovertibly that atmospheric CO
2
levels are rising because the
record is consistent and continuous. The Mauna Loa data reflect global CO
2
levels because CO
2
is well mixed in the atmosphere. Molecules that do not react
chemically in the atmosphere, or do not rain out, remain in the atmosphere for
long periods of time and become uniformly distributed across the globe. Char-
acteristic mixing times for the atmosphere range from a few days (e.g., for gases
mixing zonally in middle latitudes), to a few months (mixing between hemi-
spheres), to a few years (mixing between the troposphere and stratosphere).
Residence time
measures the average length of time a given molecule released
into the atmosphere will remain there. For CO
2
, residence times range from 5
to 300 years. The range is wide because the removal processes, which include
uptake by the oceans and photosynthesis, operate on disparate time scales, but
even the shortest residence time is longer than the characteristic global mixing
1975-2011, is similar to the Mauna Loa record except for the greatly reduced
amplitude of the seasonal signal.
1
Charles D. Keeling (1928-2005), with support from Roger Revelle and Harry Wexler, origi-
nated and sustained precise measurements of atmospheric CO
2
concentrations at the Mauna Loa
Observatory, Hawaii, beginning in 1958, to address the hypothesis that human activity is changing
the chemical composition of the atmosphere.
