First Assessment Report - IPCC

EXECUTIVE SUMMARY
The Earth's climate is dependent upon the radiative balance of the
atmosphere, which in turn depends upon the input of solar
radiation and the atmospheric abundances ot ladiatively active
trace gases (1 e , greenhouse gases), clouds and aerosols
Since the industrial revolution the atmospheric concentrations
of several greenhouse gases, i e , carbon dioxide (CO2) methane
(CH4), chlorofluorocarbons (CFCs), nitrous oxide (N2O), and
tropospheric ozone (O3), have been increasing primarily due to
human activities Several ol these gieenhouse gases have long
atmospheric lifetimes, decades to centuries, which means that
their atmospheric concentrations respond slowly to changes in
emission rates In addition theie is evidence that the
concentrations of troposphenc aerosols have increased at least
regionally
Carbon Dioxide
The atmospheric C02 concentration at'W ppm\ in 1990 is now
about 25% greater than the pre industiial (1750 1800) value of
about 280 ppmv, and higher than at any tune in at least the last
160,000 years Carbon dioxide is currently rising at about 1 8
ppmv (0 5%) per year due to anthropogenic emissions
Anthropogenic emissions ol CCn are estimated to be 5 7±0 5 Gt
C (in 1987) due to fossil fuel burning, plus 0 6 2 5 Gt C (in
1980) due to deforestation The atmospheric inuease during the
past decade corresponds to (48±8)% ol the total emissions during
the same period with the remamdei being taken up by the oceans
and land Indirect evidence suggests that the land and oceans
sequester CO2 in roughly equal pioportions though the
mechanisms are not all well undeistood The time taken lor
atmospheric CO2 to adjust to changes in sources 01 sinks is ot
order 50 200 years, determined mainly by the slow exchange of
carbon between surface watcis and deepei layeis ot the ocean
Consequently CO2 emitted into the atmosphere today will
influence the atmospheric concentiation ot CO2 tor centuries into
the future Three models have been used to estimate that even if
anthropogenic emissions of CO2 could be kept constant at present
day rates, atmospheric CO2 would increase to 415 - 480 ppmv by
the year 2050, and to 460 - 560 ppmv by the year 2100 In order
to stabilize concentrations at present day levels an immediate
reduction in global anthropogenic emissions by 60 80 perceni
would be necessary
Methane
Current atmospheric CH4 concentration, at 1 72 ppim is now
more than double the pre-industnal (1750 1800) value ot about
0 8 ppmv, and is increasing at a rate of about 0 015 ppmv (0 9%)
per year The major sink for CH4, reaction with hydroxyl (OH)
radicals in the troposphere, results in a relatively short
atmospheric lifetime of about 10 years Human activities such as
rice cultivation, domestic ruminant rearing, biomass burning, coal
mining, and natural gas venting have increased the input of CH4
into the atmosphere, which combined with a possible decrease in
the concentration ot tropospheric OH yields the observed rise in
global CH4 However the quantitative importance of each ot the
factors contributing to the observed increase is not well known at
present In order to stabilize concentrations at present day levels
an immediate reduction in global anthropogenic emissions by 15
20 percent would be necessary
Chlorofluorocarbons
The current atmospheric concentrations of the anthropogenically
produced halocarbons CCI3F (CFC 11) CCI2F2 (CFC 12)
C2CI1F3 (CFC 1H) and CCI4 (carbon tetrachloride) are about
280 pptv 484 pptv 60 pptv and 146 pptv respectively Over the
past few decades their concentrations, except tor CCI4 have
increased more rapidly (on a percentage basis) than the other
gieenhouse gases, currently at rates ot at least 4% per year The
fully halogenated CFCs and CCI4 are primarily removed by
photolysis in the stratosphere, and have atmospheric lifetimes in
excess of 50 years Future emissions will, most likely, be
eliminated or significantly lower than todays because of current
international negotiations to strengthen regulations on
chlorofluorocarbons However, the atmospheric concentrations ot
CFCs 11 12 and 1 H will still be significant O0 - 40% ot current)
lor at least the next centuiy because ol their long atmosphenc
lifetimes
Nitrous Oxide
The current atmospheric N2O concentration, at ^ 10 ppbv, is now
about 8% greater than in the pre-industnal era, and is increasing al
a rate of about 0 8 ppbv (0 25%) per year The major sink foi
N2O, photolysis in the stratosphere, results in a relatively long
atmospheric lifetime ot about 150 years It is difficult to
quantitatively account tor the source of the current increase in the
atmospheric concentiation of N2O but it is thought to be due to
human activities Recent data suggest thai the total annual flu\ ot
N2O from combustion and biomass burning is much less than
pieviously believed Agricultural practices may stimulate
emissions ot N2O from soils and play a major role In order to
stabi'ize concentrations at present day levels an immediate