Greenhouse Gas - Natural and Anthropogenic Sources

Natural and Anthropogenic Sources

Aside from purely human-produced synthetic halocarbons, most greenhouse gases have both natural and human-caused sources. During the pre-industrial Holocene, concentrations of existing gases were roughly constant. In the industrial era, human activities have added greenhouse gases to the atmosphere, mainly through the burning of fossil fuels and clearing of forests.

The 2007 Fourth Assessment Report compiled by the IPCC (AR4) noted that "changes in atmospheric concentrations of greenhouse gases and aerosols, land cover and solar radiation alter the energy balance of the climate system", and concluded that "increases in anthropogenic greenhouse gas concentrations is very likely to have caused most of the increases in global average temperatures since the mid-20th century". In AR4, "most of" is defined as more than 50%.

Abbreviations used in the two tables below: ppm = parts-per-million; ppb = parts-per-billion; ppt = parts-per-trillion; W/m2 = watts per square metre

Current greenhouse gas concentrations
Gas Pre-1750
tropospheric
concentration
Recent
tropospheric
concentration
Absolute increase
since 1750
Percentage
increase
since 1750
Increased
radiative forcing
(W/m2)
Carbon dioxide (CO2) 280 ppm 392.6 ppm 112.6 ppm 40.2% 1.85
Methane (CH
4)
700 ppb 1874 ppb /
1758 ppb
1174 ppb /
1058 ppb
167.7% /
151.1%
0.51
Nitrous oxide (N
2O)
270 ppb 324 ppb /
323 ppb
54 ppb /
53 ppb
20.0% /
19.6%
0.18
Tropospheric
ozone (O
3)
25 ppb 34 ppb 9 ppb 36% 0.35
Relevant to radiative forcing and/or ozone depletion; all of the following have no natural sources and hence zero amounts pre-industrial
Gas Recent
tropospheric
concentration
Increased
radiative forcing
(W/m2)
CFC-11
(trichlorofluoromethane)
(CCl
3F)
238 ppt /
236 ppt
0.060
CFC-12 (CCl
2F
2)
531 ppt /
529 ppt
0.17
CFC-113 (Cl
2FC-CClF
2)
75 ppt /
75 ppt
0.024
HCFC-22 (CHClF
2)
226 ppt /
203 ppt
0.041
HCFC-141b (CH
3CCl
2F)
23 ppt /
20 ppt
0.0025
HCFC-142b (CH
3CClF
2)
23 ppt /
21 ppt
0.0031
Halon 1211 (CBrClF
2)
4.2 ppt /
4.0 ppt
0.001
Halon 1301 (CBrClF
3)
3.3 ppt /
3.2 ppt
0.001
HFC-134a (CH
2FCF
3)
68 ppt /
58 ppt
0.0055
Carbon tetrachloride (CCl
4)
86 ppt /
84 ppt
0.012
Sulfur hexafluoride (SF
6)
7.47 ppt /
7.09 ppt
0.0029
Other halocarbons Varies by
substance
collectively
0.021

Ice cores provide evidence for greenhouse gas concentration variations over the past 800,000 years (see the following section). Both CO2 and CH
4 vary between glacial and interglacial phases, and concentrations of these gases correlate strongly with temperature. Direct data does not exist for periods earlier than those represented in the ice core record, a record that indicates CO2 mole fractions stayed within a range of 180 ppm to 280 ppm throughout the last 800,000 years, until the increase of the last 250 years. However, various proxies and modeling suggests larger variations in past epochs; 500 million years ago CO2 levels were likely 10 times higher than now. Indeed higher CO2 concentrations are thought to have prevailed throughout most of the Phanerozoic eon, with concentrations four to six times current concentrations during the Mesozoic era, and ten to fifteen times current concentrations during the early Palaeozoic era until the middle of the Devonian period, about 400 Ma. The spread of land plants is thought to have reduced CO2 concentrations during the late Devonian, and plant activities as both sources and sinks of CO2 have since been important in providing stabilising feedbacks. Earlier still, a 200-million year period of intermittent, widespread glaciation extending close to the equator (Snowball Earth) appears to have been ended suddenly, about 550 Ma, by a colossal volcanic outgassing that raised the CO2 concentration of the atmosphere abruptly to 12%, about 350 times modern levels, causing extreme greenhouse conditions and carbonate deposition as limestone at the rate of about 1 mm per day. This episode marked the close of the Precambrian eon, and was succeeded by the generally warmer conditions of the Phanerozoic, during which multicellular animal and plant life evolved. No volcanic carbon dioxide emission of comparable scale has occurred since. In the modern era, emissions to the atmosphere from volcanoes are only about 1% of emissions from human sources.

Read more about this topic:  Greenhouse Gas

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