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Yes, the increases in atmospheric carbon dioxide (CO2) and
other greenhouse gases during the industrial era are caused by human activities. In fact,
the observed increase in atmospheric CO2 concentrations does
not reveal the full extent of human emissions in that it accounts for only 55% of the
CO2 released by human activity since 1959. The rest has been
taken up by plants on land and by the oceans. In all cases, atmospheric concentrations of
greenhouse gases, and their increases, are determined by the balance between sources (emissions
of the gas from human activities and natural systems) and sinks (the removal of the gas from
the atmosphere by conversion to a different chemical compound). Fossil fuel combustion (plus
a smaller contribution from cement manufacture) is responsible for more than 75% of human-caused
CO2 emissions. Land use change (primarily deforestation) is
responsible for the remainder. For methane, another important greenhouse gas, emissions
generated by human activities exceeded natural emissions over the last 25 years. For nitrous
oxide, emissions generated by human activities are equal to natural emissions to the atmosphere.
Most of the long-lived halogen-containing gases (such as chloro-fluorcarbons) are manufactured
by humans, and were not present in the atmosphere before the industrial era. On average,
present-day tropospheric ozone has increased 38% since pre-industrial times, and the increase
results from atmospheric reactions of short-lived pollutants emitted by human activity. The
concentration of CO2 is now 379 parts per million (ppm) and
methane is greater than 1,774 parts per billion (ppb), both very likely much higher than any
time in at least 650 kyr (during which CO2 remained between
180 and 300 ppm and methane between 320 and 790 ppb). The recent rate of change is dramatic
and unprecedented; increases in CO2 never exceeded 30 ppm in
1 kyr – yet now CO2 has risen by 30 ppm in just the last 17 years.
Carbon Dioxide
Emissions of CO2 (Figure 1a) from fossil fuel combustion,
with contributions from cement manufacture, are responsible for more than 75% of the
increase in atmospheric CO2 concentration since pre-industrial
times. The remainder of the increase comes from land use changes dominated by deforestation
(and associated biomass burning) with contributions from changing agricultural practices.
All these increases are caused by human activity. The natural carbon cycle cannot explain
the observed atmospheric increase of 3.2 to 4.1 GtC yr−1
in the form of CO2 over the last 25 years. (One GtC equals
1015 grams of carbon, i.e., one billion tonnes.)
Natural processes such as photosynthesis, respiration, decay and sea surface gas exchange
lead to massive exchanges, sources and sinks of CO2 between
the land and atmosphere (estimated at ~120 GtC yr−1)
and the ocean and atmosphere (estimated at ~90 GtC yr−1;
see figure 7.3). The natural sinks of carbon produce a small net uptake of
CO2 of approximately 3.3 GtC yr−1
over the last 15 years, partially offsetting the human-caused emissions. Were it not for the
natural sinks taking up nearly half the human-produced CO2 over
the past 15 years, atmospheric concentrations would have grown even more dramatically.
The increase in atmospheric CO2 concentration is known to be
caused by human activities because the character of CO2 in
the atmosphere, in particular the ratio of its heavy to light carbon
atoms, has changed in a way that can be attributed to
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for thousands of years. Concentrations of several important halogen-containing gases,
including CFCs, are now stabilising or decreasing at the Earth’s surface as a result of
the Montreal Protocol on Substances that Deplete the Ozone Layer and its Amendments.
Concentrations of HCFCs, production of which is to be phased out by 2030, and of the
Kyoto Protocol gases HFCs and PFCs, are currently increasing.
Methane
Methane (CH4) sources to the atmosphere generated by human
activities exceed CH4 sources from natural systems (Figure 1c).
Between 1960 and 1999, CH4 concentrations grew an average of
at least six times faster than over any 40-year period of the two millennia before 1800,
despite a near-zero growth rate since 1980. The main natural source of CH4
to the atmosphere is wetlands. Additional natural sources include termites, oceans, vegetation
and CH4 hydrates. The human activities that produce
CH4 include energy production from coal and natural gas,
waste disposal in landfills, raising ruminant animals (e.g., cattle and sheep), rice
agriculture and biomass burning. Once emitted, CH4 remains
in the atmosphere for approximately 8.4 years before removal, mainly by chemical oxidation
in the troposphere. Minor sinks for CH4 include uptake by
soils and eventual destruction in the stratosphere.
Nitrous Oxide
Nitrous oxide (N2O) sources to the atmosphere from human
activities are approximately equal to N2O sources from
natural systems (Figure 1d). Between 1960 and 1999, N2O
concentrations grew an average of at least two times faster than over any 40-year period
of the two millennia before 1800. Natural sources of N2O
include oceans, chemical oxidation of ammonia in the atmosphere, and soils. Tropical
soils are a particularly important source of N2O to the
atmosphere. Human activities that emit N2O include transformation
of fertilizer nitrogen into N2O and its subsequent emission from
agricultural soils, biomass burning, raising cattle and some industrial activities, including
nylon manufacture. Once emitted, N2O remains in the atmosphere
for approximately 114 years before removal, mainly by destruction in the stratosphere.
Tropospheric Ozone
Tropospheric ozone is produced by photochemical reactions in the atmosphere involving forerunner
chemicals such as carbon monoxide, CH4, volatile organic compounds
and nitrogen oxides. These chemicals are emitted by natural biological processes and by human
activities including land use change and fuel combustion. Because tropospheric ozone is relatively
short-lived, lasting for a few days to weeks in the atmosphere, its distributions are highly
variable and tied to the abundance of its forerunner compounds, water vapour and sunlight.
Tropospheric ozone concentrations are significantly higher in urban air, downwind of urban
areas and in regions of biomass burning. The increase of 38% (20–50%) in tropospheric ozone
since the pre-industrial era (Figure 1e) is human-caused.
It is very likely that the increase in the combined radiative forcing from
CO2, CH4 and
N2O was at least six times faster between 1960 and 1999
than over any 40-year period during the two millennia prior to the year 1800.
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