Geoscience Reference
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is about one-tenth that of carbon dioxide. In the lower atmosphere, the increase in
ozone concentration has a warming effect equivalent to or slightly greater than that
of N
2
O. Destruction of ozone in the upper atmosphere (stratosphere) as a result of
ozone-depleting chlorofluorocarbons (CFCs) has a cooling effect at high elevations.
Some gases and aerosols act to offset the warming effect resulting from the increase
in CO
2
,CH
4
and N
2
O, most notably sulphur dioxide (SO
2
), which has a regional
cooling effect.
Le Quer´eetal.(
2008
) warned that there had been a 29 per cent increase in fossil fuel
emissions between 2000 and 2008. Total carbon (C) emissions in 2010 amounted to
about 9.28 billion metric tonnes, or gigatonnes (9.28 Gt
10
9
t C). (To convert
tonnes of carbon to equivalent tonnes of carbon dioxide, simply multiply by 3.67).
Burning fossil fuels and cement manufacture are the major sources of atmospheric
carbon dioxide emissions (about 90 per cent) and amounted to roughly 8.35 Gt C,
of which slightly more than half remains in the atmosphere (see Global Carbon
Emissions,
2010
, which updates the data in Canadell et al.,
2007
,andLeQuer´eetal.,
2008
). Biomass burning and land use change, especially deforestation, are estimated to
have contributed an additional 0.9 Gt C or so in 2010. During the last 600 years, there
have been considerable variations in Southern Hemisphere biomass burning, with
peaks centred around 1300 and 1900 AD and minima between 1300 and 1600 AD
and after 1900 AD (Prentice, 2010; Wang et al.,
2010
). At the present time, there is
a very real danger that increased carbon emissions from fires in areas of secondary
forest regrowth in Amazonian forests and elsewhere may offset the carbon savings
from curbing deforestation (Aragao and Shimabukuro,
2010
). However, the work of
Wang et al. (
2010
) has demonstrated that Southern Hemisphere biomass burning is at
present lower than at any time in the previous 650 years, although, as Prentice (
2010
)
has cautioned, this could change for the worse if global mean temperatures increase
by several degrees. Apart from the carbon stored in the atmosphere, the sinks for
carbon were estimated at about 2.23 Gt C for the oceans in 2010 and about 2.41 Gt
C/year for the biosphere.
Major sources for methane are more varied than they are for carbon, and they
include fossil fuel mining and distribution, livestock, landfills, rice paddies, wetlands
and biomass burning, in broadly similar proportions. There is considerable concern
over current permafrost thawing in high northern latitudes, which could release large
additional supplies of methane hitherto locked up in frozen swamps (Schuur et al.,
2011
). Ruddiman and Thomson (
2001
) have suggested that the increase in methane
(CH
4
) emissions evident in Northern Hemisphere ice cores from about 5,000 years
ago onwards was a result of inefficient rice cultivation in flooded areas with abundant
CH
4
-producing weeds. They based this conclusion on the fact that the 100 ppbv
increase in atmospheric CH
4
after 5 ka followed a pattern not seen previously in the
orbitally driven ice core record. This attractive hypothesis is still in need of rigorous
testing.
=
9.28
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