2. Human activities are increasing the amount of greenhouse gases in the atmosphere

• Certain human activities release greenhouse gases into the atmosphere. For example, burning fossil fuels (coal, oil, gas), cement production and removing forests release carbon dioxide, and decay of organic matter in anaerobic conditions in paddy fields releases methane (see Box 1, right, for examples of the chemical reactions that underpin these processes).
• Careful accounting of these processes allows the amount of greenhouse gases being released into the atmosphere to be calculated. Table 2 (below) shows the annual global emissions of the main greenhouses gases, along with their main sources, and the change in their concentration since pre-industrial times.

Table 2 (above): Emissions, main sources and change in the atmospheric concentration of the main anthropogenic greenhouse gases.

* Data for 2005 (from EDGARv4) excluding land use, land use change and forestry (LULUCF) and “international bunkers” (relating to aviation and marine emissions), expressed as carbon dioxide equivalents, which is a measure of their effect on climate relative to that of CO₂. Figures are taken from the AVOID Global Greenhouse Gases Emissions Review.

** Calculated using data from NOAA/ESRL for 2009 and IPCC AR4 for pre-industrial. 2009 values are: CO₂ = 386 ppm, CH₄ = 1790 ppb, N₂O = 322 ppb. Pre-industrial values are: CO₂ = 278 ppm, CH₄ = 715 ppb, N₂O = 270 ppb. Uncertainties will attach to these estimates, which are likely to be particularly significant for N₂O. Several databases exist presenting similar data, each with its strengths and weaknesses.

• The atmospheric concentration of CO₂-  the most important anthropogenic greenhouse gas - being released into the atmosphere by human activities each year is increasing at a rate of about 2% per year (average rate of increase over 2000-2008 for emissions from fossil fuel combustion, cement production and land use change) [9] (Figure 3, below).

Figure 3 (above): Global annual CO₂ emissions from fossil fuel combustion and cement production (black line) and land use change (brown line). Note that 1 petagram of carbon (PgC) = 3.7 billion tonnes of CO₂. (Source: adapted from Le Quéré et al. (2009). Data: CDIAC, FAO, Woods Hole Research Center, available at Global Carbon Project)

• The rate at which gases accumulate in the atmosphere and the time that they remain there are affected by the rate at which they are removed. In the case of CO₂, about half of a given addition (or ‘pulse’) to the atmosphere is removed within a few decades, primarily through uptake by the oceans and terrestrial biosphere [10]. A further 30% is removed within a few centuries, and the remaining 20% may stay in the atmosphere for many thousands of years. Methane and nitrous oxide are removed from the atmosphere more quickly (the lifetime of methane is about 12 years and that of nitrous oxide is about 114 years) [11]. See further explanation on atmospheric lifetimes.
• Studies indicate that as the Earth warms, the net uptake of CO₂ by the oceans may decrease, which would mean that, for each tonne of anthropogenic CO₂ released, the concentration of CO₂ in the atmosphere would increase more rapidly than at present. There are indications that the efficiency of CO₂ ‘sinks’ in the Southern Ocean and the North Atlantic have begun to decrease [12], but longer, more comprehensive records of CO₂ uptake are needed to confirm this.
• Direct observations show that the concentration of greenhouse gases in the atmosphere has been increasing over the past 50 years [13] (Figure 4, below). Regular and continuous measurements of CO₂ concentration began at Mauna Loa in Hawaii in 1958 and more extensive global measurements now exist, confirming the trend in all regions. Over the first full year of measurements, 1959, the average concentration was found to be 316 parts per million (ppm). In 2009 it was 387 ppm; an increase of 23% over 50 years [13].

Figure 4 (above): Atmospheric concentration of CO₂ measured at Mauna Loa Observatory, Hawaii (red line = monthly average values, black line = monthly average values after correction for the average seasonal cycle). Fluctuations in the red line reflect a marked seasonal variation: a decrease in Northern Hemisphere spring, when plants take up carbon dioxide, and an increase in Northern Hemisphere autumn and winter due to increased respiration, which releases CO₂. The global signal is dominated by this Northern Hemisphere seasonal cycle because there is less land-based vegetation in the Southern Hemisphere. Source: NOAA/ESRL (www.esrl.noaa.gov/gmd/ccgg/trends/) and Scripps Institution of Oceanography. (Larger version of Figure 4 (PDF, 129 Kb) )

• The record of atmospheric CO₂ concentration has been extended back in time by analysing the composition of air bubbles trapped in ice sheets. This record shows that the concentration of CO₂ has increased by 39% since pre-industrial times (when it was approximately 280 ppm) [14] (Figure 5, below). Two thirds of that increase has occurred in the past 50 years [13]. Changes in the atmospheric concentrations of methane and nitrous oxide have been assessed in the same way, showing that their concentrations are now 150% and 19% higher, respectively, than they were during pre-industrial times. [14].

Figure 5 (above): Record of the concentration of carbon dioxide in the atmosphere over the past 1,000 years based on direct observations (line) and ice core data (squares). Ice core data are from Law Dome, Antarctica (from Etheridgeet al., 1996 and MacFarling Meureet al., 2006) and the direct observations are atmospheric annual average CO₂ measurements made at Mauna Loa, Hawaii (from NOAA/ESRL). (Larger version of Figure 5 (PDF, 11 Kb) )

• A number of lines of evidence confirm that the additional CO₂ in the atmosphere has come from human activities rather than natural sources such as the ocean:
  
  
  • The emissions of CO₂ due to human activity [13] and the increase of atmospheric CO₂ are well known. The atmospheric increase accounts for only about half of the emissions due to human activity. The remainder has been taken up by the ocean and biosphere in amounts broadly consistent with our understanding.
  • The ratio of oxygen to nitrogen in the atmosphere has declined as CO₂ has increased (Figure 6a). This is because oxygen is used up during combustion of organic material (Box 1) – albeit that the amount of reduction relative to its atmospheric concentration is in itself very small. The change in O₂/N₂ ratio observed is consistent with the additional CO₂ being emitted from burning fossil fuels and/or vegetation.
  • A heavier form (isotope) of carbon known as “carbon-13” is less abundant (relative to the lighter form “carbon-12”) in vegetation and fossil fuels than it is in volcanic emissions and the oceans. The relative amount of carbon-13 in the atmosphere has been declining since CO₂ levels began to rise, strongly supporting the case that the additional CO₂ is primarily derived from fossil fuels and/or vegetation, rather than volcanic emissions or the oceans (Figure 6b).
  • The radioactive isotope of carbon, carbon-14, is present in CO₂ emitted from most natural sources, but not in fossil fuels (because fossil fuels are ancient and the carbon-14 within them has therefore decayed). Until the atmospheric testing of nuclear weapons in the 1950s disrupted the signal, the relative amount of carbon-14 in the atmosphere was decreasing, consistent with the addition of non-radioactive CO₂ from fossil fuels.
  • Observations show that there is slightly more CO₂ in the Northern Hemisphere than in the Southern Hemisphere. This difference is consistent with most of the human activities that produce CO₂ being in the north. It takes about a year for these emissions to circulate through the atmosphere and reach southern latitudes.

Figure 6(a) (above, top): CO₂ concentrations measured by at Mauna Loa (black) and Baring Head, New Zealand (blue) and atmospheric oxygen (O₂) measurements from Canada (pink) and Australia (cyan). O₂ concentration is measured as ‘per meg’ deviations in the O₂/N₂ ratio from an arbitrary reference; analogous to the ‘per mil’ unit typically used in stable isotope work, but where the ratio is multiplied by 10⁶ instead of 10³ because much smaller changes are measured.

Figure 6(b)(above, bottom): Annual global CO₂ emissions from fossil fuel burning and cement manufacture in GtC yr^–1 (black) and annual averages of the ratio of carbon-13 to carbon-12 in atmospheric CO₂ measured at Mauna Loa, expressed as the per mil deviation from a calibration standard. The scale is inverted to improve clarity. (Source: IPCC AR4)

• The acidity of the surface ocean is expected to increase as the concentration of CO₂ in the atmosphere increases (if the CO₂ is not derived from the ocean), because CO₂ reacts with water to form carbonic acid (Box 2, right). Models predict that the change in atmospheric CO₂ concentration since the pre-industrial period would have caused the global average pH of the surface ocean to decrease by 0.1 pH units [15], which is equivalent to a 30% increase in the concentration of hydrogen ions. This is consistent with the changes that have been observed at a number of locations. The impact that acidification has on ecosystems remains uncertain, but it probably makes it more difficult for organisms like corals and shellfish to form their calcium carbonate shells and skeletons [16].
• Water vapour is a strong greenhouse gas (see section 1) and is also a product of the burning of fossil fuels. Unlike CO₂, CH₄ and the other greenhouse gases discussed above, however, man-made emissions of water vapour are not a major direct cause of global warming. This is because the concentration of water vapour in the atmosphere is primarily determined by temperature, air motions and the large supply of water to the lower atmosphere via evaporation from the oceans. Also, water vapour released into the atmosphere by human activities ‘rains out’ within a few days. The only exceptions to this are the small amounts added to the stratosphere (upper atmosphere) [17][18], and evaporation of water vapour during irrigation in very dry regions, which can affect temperatures locally [19].
• Water vapour does, however, act as an important ‘feedback’ on global warming.  If surface temperatures rise, both the amount of evaporation from the oceans and the capacity of the atmosphere to hold water vapour increase. Observations suggest around a 5% average global increase in water vapour in the atmosphere corresponds to a global temperature rise of 1°C, with higher increases over oceans than land [20]. This, in turn, increases the strength of the greenhouse effect, which increases surface temperatures, leading to a positive feedback.

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Key references 

Footnotes

1. 9. Source: Data from CDIAC, FAO, Woods Hole Research Center, available at Global Carbon Project
2. 10. Natural land and ocean CO₂sinks removed 57% of all CO₂emitted from human activities during the period 1958-2008 (Source: Global Carbon Project).
3. 11. IPCC AR4, Working Group I (2007) and see also Archeret al. (2009).
4. 12. See Le Quéréet al. (2009) for a summary of the available evidence.
5. 13. Data from NOAA/ESRL (www.esrl.noaa.gov/gmd/ccgg/trends/)
6. 14. See Table 3 for data sources.
7. 15. IPCC AR4, Working Group I Report (2007)
8. 16. Royal Society (2005)
9. 17. Water vapour is added to the stratosphere through oxidation of anthropogenic methane and aircraft emissions, and the stratosphere is dry enough and has a long enough residence time (a few years) for them to have an effect.
10. 18. A paper by Solomonet al. (2010) concluded that a decrease in stratospheric water vapour concentrations between 2000 and 2009 acted to slow the rate of increase in global surface temperature by about 25% compared to that which would have occurred due only to greenhouse gases, and that a possible increase in stratospheric water vapour between 1980 and 2000 would have enhanced the decadal rate of surface warming during the 1990s by about 30% as compared to estimates neglecting this change.
11. 19. This causes both radiative warming and evaporative cooling (Boucheret al., 2004).
12. 20. IPCC Fourth Assessment Report 2007; WG1 3.4.2.1
    
    

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