Geoscience Reference
In-Depth Information
because the Arctic current has warmed
over the past three decades, causing the re-
lease of methane by breaking down meth-
ane hydrate in the sediment beneath the
seabed. This Siberian discovery is an ex-
ample of the evolving nature of our knowl-
edge of greenhouse gas emissions. We still
have much to learn.
The sudden release of massive amounts
of methane from marine methane ice is the
suspected cause of two of the Earth's major
extinction events. The Paleocene-Eocene
Thermal Maximum of 55 million years ago
led to the extinction of numerous marine
and land-based organisms. In this instance
the collapse of methane ice deposits seems
highly probable as the cause of the occur-
rence of spectacular atmospheric warm-
ing, which took perhaps 100,000 years to
recover from. The much larger Permian-
Triassic Extinction Event 250 million years
ago resulted in the extinction of 70% of all
land vertebrate species. Although the cause
is much less certain because the event hap-
pened so long ago, conceivably methane ice
melting and a runaway greenhouse effect
were behind it as well.
You can find dramatic videos on YouTube
of scientists from the University of Alaska,
Fairbanks, punching holes in frozen lakes
and lighting up the methane released by
melting permafrost, gas which is tempo-
rarily trapped by the ice.
Nitrous oxide is currently responsible for
about 6% of the heating caused by green-
house gases. It is a pollutant from industry
and particularly from agricultural fertiliz-
ers. It is also released naturally from soils
and from the oceans. Besides its role as a
greenhouse gas it reacts with and destroys
ozone in the atmosphere.
Freons are a number of compounds that
have no natural source but are now in the
atmosphere. These include chlorofluoro-
carbons (cfcs) used in refrigerators; their
replacements, hydrofluorocarbons (hfcs);
nitrogen trifluoride from flat-screen televi-
sions; and halons from fire extinguishers.
All are very powerful greenhouse gases
(high global warming potential) but they
occur in such small quantities in the atmo-
sphere that to date their impact on warm-
ing has not been important.
The following list is a generalized sum-
mary of the global sources of greenhouse
emissions. Almost all the listed sources pro-
duce emissions primarily through the burn-
ing of fossil fuels such as coal, oil, and natu-
ral gas. Only one, land use change, produces
emissions when the carbon-sequestering
abilities of the land are altered.
Agriculture
13%
Industry
19%
Transport
13%
Residential and commercial buildings
8%
Energy production
26%
Land use change
18%
Waste management
3%
There are other ways to analyze sources
of global emissions. For example, accord-
ing to the United Nations Food and Ag-
riculture Organization, the meat in our
diet is a major source of greenhouse gases.
