2.3 Chemically and Radiatively Important Gases

2.3.1 Atmospheric Carbon Dioxide

This section discusses the instrumental measurements of CO₂, documenting recent changes in atmospheric mixing ratios needed for the RF calculations presented later in the section. In addition, it provides data for the pre-industrial levels of CO₂ required as the accepted reference level for the RF calculations. For dates before about 1950 indirect measurements are relied upon. For these periods, levels of atmospheric CO₂ are usually determined from analyses of air bubbles trapped in polar ice cores. These time periods are primarily considered in Chapter 6.

A wide range of direct and indirect measurements confirm that the atmospheric mixing ratio of CO₂ has increased globally by about 100 ppm (36%) over the last 250 years, from a range of 275 to 285 ppm in the pre-industrial era (AD 1000–1750) to 379 ppm in 2005 (see FAQ 2.1, Figure 1). During this period, the absolute growth rate of CO₂ in the atmosphere increased substantially: the first 50 ppm increase above the pre-industrial value was reached in the 1970s after more than 200 years, whereas the second 50 ppm was achieved in about 30 years. In the 10 years from 1995 to 2005 atmospheric CO₂ increased by about 19 ppm; the highest average growth rate recorded for any decade since direct atmospheric CO₂ measurements began in the 1950s. The average rate of increase in CO₂ determined by these direct instrumental measurements over the period 1960 to 2005 is 1.4 ppm yr^-1.

High-precision measurements of atmospheric CO₂ are essential to the understanding of the carbon cycle budgets discussed in Section 7.3. The first in situ continuous measurements of atmospheric CO₂ made by a high-precision non-dispersive infrared gas analyser were implemented by C.D. Keeling from the Scripps Institution of Oceanography (SIO) (see Section 1.3). These began in 1958 at Mauna Loa, Hawaii, located at 19°N (Keeling et al., 1995). The data documented for the first time that not only was CO₂ increasing in the atmosphere, but also that it was modulated by cycles caused by seasonal changes in photosynthesis in the terrestrial biosphere. These measurements were followed by continuous in situ analysis programmes at other sites in both hemispheres (Conway et al., 1994; Nakazawa et al., 1997; Langenfelds et al., 2002). In Figure 2.3, atmospheric CO₂ mixing ratio data at Mauna Loa in the Northern Hemisphere (NH) are shown with contemporaneous measurements at Baring Head, New Zealand in the Southern Hemisphere (SH; Manning et al., 1997; Keeling and Whorf, 2005). These two stations provide the longest continuous records of atmospheric CO₂ in the NH and SH, respectively. Remote sites such as Mauna Loa, Baring Head, Cape Grim (Tasmania) and the South Pole were chosen because air sampled at such locations shows little short-term variation caused by local sources and sinks of CO₂ and provided the first data from which the global increase of atmospheric CO₂ was documented. Because CO₂ is a LLGHG and well mixed in the atmosphere, measurements made at such sites provide an integrated picture of large parts of the Earth including continents and city point sources. Note that this also applies to the other LLGHGs reported in Section 2.3.

Figure 2.3. Recent CO_2 concentrations and emissions. (a) CO_2 concentrations (monthly averages) measured by continuous analysers over the period 1970 to 2005 from Mauna Loa, Hawaii (19°N, black; Keeling and Whorf, 2005) and Baring Head, New Zealand (41°S, blue; following techniques by Manning et al., 1997). Due to the larger amount of terrestrial biosphere in the NH, seasonal cycles in CO_2 are larger there than in the SH. In the lower right of the panel, atmospheric oxygen (O_2) measurements from flask samples are shown from Alert, Canada (82°N, pink) and Cape Grim, Australia (41°S, cyan) (Manning and Keeling, 2006). The O_2 concentration is measured as ‘per meg’ deviations in the O_2/N_2 ratio from an arbitrary reference, analogous to the ‘per mil’ unit typically used in stable isotope work, but where the ratio is multiplied by 10^6 instead of 10^3 because much smaller changes are measured. (b) Annual global CO_2 emissions from fossil fuel burning and cement manufacture in GtC yr^–1 (black) through 2005, using data from the CDIAC website (Marland et al, 2006) to 2003. Emissions data for 2004 and 2005 are extrapolated from CDIAC using data from the BP Statistical Review of World Energy (BP, 2006). Land use emissions are not shown; these are estimated to be between 0.5 and 2.7 GtC yr^{–1 }for the 1990s (Table 7.2). Annual averages of the ^13C/^12C ratio measured in atmospheric CO_2 at Mauna Loa from 1981 to 2002 (red) are also shown (Keeling et al, 2005). The isotope data are expressed as δ^13C(CO_2) ‰ (per mil) deviation from a calibration standard. Note that this scale is inverted to improve clarity.

In the 1980s and 1990s, it was recognised that greater coverage of CO₂ measurements over continental areas was required to provide the basis for estimating sources and sinks of atmospheric CO₂ over land as well as ocean regions. Because continuous CO₂ analysers are relatively expensive to maintain and require meticulous on-site calibration, these records are now widely supplemented by air sample flask programmes, where air is collected in glass and metal containers at a large number of continental and marine sites. After collection, the filled flasks are sent to central well-calibrated laboratories for analysis. The most extensive network of international air sampling sites is operated by the National Oceanic and Atmospheric Administration’s Global Monitoring Division (NOAA/GMD; formerly NOAA/Climate Monitoring and Diagnostics Laboratory (CMDL)) in the USA. This organisation collates measurements of atmospheric CO₂ from six continuous analyser locations as well as weekly flask air samples from a global network of almost 50 surface sites. Many international laboratories make atmospheric CO₂ observations and worldwide databases of their measurements are maintained by the Carbon Dioxide Information Analysis Center (CDIAC) and by the World Data Centre for Greenhouse Gases (WDCGG) in the WMO Global Atmosphere Watch (GAW) programme.^[6]

The increases in global atmospheric CO₂ since the industrial revolution are mainly due to CO₂ emissions from the combustion of fossil fuels, gas flaring and cement production. Other sources include emissions due to land use changes such as deforestation (Houghton, 2003) and biomass burning (Andreae and Merlet, 2001; van der Werf, 2004). After entering the atmosphere, CO₂ exchanges rapidly with the short-lived components of the terrestrial biosphere and surface ocean, and is then redistributed on time scales of hundreds of years among all active carbon reservoirs including the long-lived terrestrial biosphere and deep ocean. The processes governing the movement of carbon between the active carbon reservoirs, climate carbon cycle feedbacks and their importance in determining the levels of CO₂ remaining in the atmosphere, are presented in Section 7.3, where carbon cycle budgets are discussed.

The increase in CO₂ mixing ratios continues to yield the largest sustained RF of any forcing agent. The RF of CO₂ is a function of the change in CO₂ in the atmosphere over the time period under consideration. Hence, a key question is ‘How is the CO₂ released from fossil fuel combustion, cement production and land cover change distributed amongst the atmosphere, oceans and terrestrial biosphere?’. This partitioning has been investigated using a variety of techniques. Among the most powerful of these are measurements of the carbon isotopes in CO₂ as well as high-precision measurements of atmospheric oxygen (O₂) content. The carbon contained in CO₂ has two naturally occurring stable isotopes denoted ¹²C and ¹³C. The first of these, ¹²C, is the most abundant isotope at about 99%, followed by ¹³C at about 1%. Emissions of CO₂ from coal, gas and oil combustion and land clearing have ¹³C/¹²C isotopic ratios that are less than those in atmospheric CO₂, and each carries a signature related to its source. Thus, as shown in Prentice et al. (2001), when CO₂ from fossil fuel combustion enters the atmosphere, the ¹³C/¹²C isotopic ratio in atmospheric CO₂ decreases at a predictable rate consistent with emissions of CO₂ from fossil origin. Note that changes in the ¹³C/¹²C ratio of atmospheric CO₂ are also caused by other sources and sinks, but the changing isotopic signal due to CO₂ from fossil fuel combustion can be resolved from the other components (Francey et al., 1995). These changes can easily be measured using modern isotope ratio mass spectrometry, which has the capability of measuring ¹³C/¹²C in atmospheric CO₂ to better than 1 part in 10⁵ (Ferretti et al., 2000). Data presented in Figure 2.3 for the ¹³C/¹²C ratio of atmospheric CO₂ at Mauna Loa show a decreasing ratio, consistent with trends in both fossil fuel CO₂ emissions and atmospheric CO₂ mixing ratios (Andres et al., 2000; Keeling et al., 2005).

Atmospheric O₂ measurements provide a powerful and independent method of determining the partitioning of CO₂ between the oceans and land (Keeling et al., 1996). Atmospheric O₂ and CO₂ changes are inversely coupled during plant respiration and photosynthesis. In addition, during the process of combustion O₂ is removed from the atmosphere, producing a signal that decreases as atmospheric CO₂ increases on a molar basis (Figure 2.3). Measuring changes in atmospheric O₂ is technically challenging because of the difficulty of resolving changes at the part-per-million level in a background mixing ratio of roughly 209,000 ppm. These difficulties were first overcome by Keeling and Shertz (1992), who used an interferometric technique to show that it is possible to track both seasonal cycles and the decline of O₂ in the atmosphere at the part-per-million level (Figure 2.3). Recent work by Manning and Keeling (2006) indicates that atmospheric O₂ is decreasing at a faster rate than CO₂ is increasing, which demonstrates the importance of the oceanic carbon sink. Measurements of both the ¹³C/¹²C ratio in atmospheric CO₂ and atmospheric O₂ levels are valuable tools used to determine the distribution of fossil-fuel derived CO₂ among the active carbon reservoirs, as discussed in Section 7.3. In Figure 2.3, recent measurements in both hemispheres are shown to emphasize the strong linkages between atmospheric CO₂ increases, O₂ decreases, fossil fuel consumption and the ¹³C/¹²C ratio of atmospheric CO₂.

From 1990 to 1999, a period reported in Prentice et al. (2001), the emission rate due to fossil fuel burning and cement production increased irregularly from 6.1 to 6.5 GtC yr^–1 or about 0.7% yr^–1. From 1999 to 2005 however, the emission rate rose systematically from 6.5 to 7.8 GtC yr^–1 (BP, 2006; Marland et al., 2006) or about 3.0% yr^–1, representing a period of higher emissions and growth in emissions than those considered in the TAR (see Figure 2.3). Carbon dioxide emissions due to global annual fossil fuel combustion and cement manufacture combined have increased by 70% over the last 30 years (Marland et al., 2006). The relationship between increases in atmospheric CO₂ mixing ratios and emissions has been tracked using a scaling factor known as the apparent ‘airborne fraction’, defined as the ratio of the annual increase in atmospheric CO₂ to the CO₂ emissions from annual fossil fuel and cement manufacture combined (Keeling et al., 1995). On decadal scales, this fraction has averaged about 60% since the 1950s. Assuming emissions of 7 GtC yr^–1 and an airborne fraction remaining at about 60%, Hansen and Sato (2004) predicted that the underlying long-term global atmospheric CO₂ growth rate will be about 1.9 ppm yr^–1, a value consistent with observations over the 1995 to 2005 decade.

Carbon dioxide emissions due to land use changes during the 1990s are estimated as 0.5 to 2.7 GtC yr^–1 (Section 7.3, Table 7.2), contributing 6% to 39% of the CO₂ growth rate (Brovkin et al., 2004). Prentice et al. (2001) cited an inventory-based estimate that land use change resulted in net emissions of 121 GtC between 1850 and 1990, after Houghton (1999, 2000). The estimate for this period was revised upwards to 134 GtC by Houghton (2003), mostly due to an increase in estimated emissions prior to 1960. Houghton (2003) also extended the inventory emissions estimate to 2000, giving cumulative emissions of 156 GtC since 1850. In carbon cycle simulations by Brovkin et al. (2004) and Matthews et al. (2004), land use change emissions contributed 12 to 35 ppm of the total CO₂ rise from 1850 to 2000 (Section 2.5.3, Table 2.8). Historical changes in land cover are discussed in Section 2.5.2, and the CO₂ budget over the 1980s and 1990s is discussed further in Section 7.3.

In 2005, the global mean average CO₂ mixing ratio for the SIO network of 9 sites was 378.75 ± 0.13 ppm and for the NOAA/GMD network of 40 sites was 378.76 ± 0.05 ppm, yielding a global average of almost 379 ppm. For both networks, only sites in the remote marine boundary layer are used and high-altitude locations are not included. For example, the Mauna Loa site is excluded due to an ‘altitude effect’ of about 0.5 ppm. In addition, the 2005 values are still pending final reference gas calibrations used to measure the samples.

New measurements of CO₂ from Antarctic ice and firn (MacFarling Meure et al., 2006) update and extend those from Etheridge et al. (1996) to AD 0. The CO₂ mixing ratio in 1750 was 277 ± 1.2 ppm.^[7] This record shows variations between 272 and 284 ppm before 1800 and also that CO₂ mixing ratios dropped by 5 to 10 ppm between 1600 and 1800 (see Section 6.3). The RF calculations usually take 1750 as the pre-industrial index (e.g., the TAR and this report). Therefore, using 1750 may slightly overestimate the RF, as the changes in the mixing ratios of CO₂, CH₄ and N₂O after the end of this naturally cooler period may not be solely attributable to anthropogenic emissions. Using 1860 as an alternative start date for the RF calculations would reduce the LLGHG RF by roughly 10%. For the RF calculation, the data from Law Dome ice cap in the Antarctic are used because they show the highest age resolution (approximately 10 years) of any ice core record in existence. In addition, the high-precision data from the cores are connected to direct observational records of atmospheric CO₂ from Cape Grim, Tasmania.

The simple formulae for RF of the LLGHG quoted in Ramaswamy et al. (2001) are still valid. These formulae are based on global RF calculations where clouds, stratospheric adjustment and solar absorption are included, and give an RF of +3.7 W m^–2 for a doubling in the CO₂ mixing ratio. (The formula used for the CO₂ RF calculation in this chapter is the IPCC (1990) expression as revised in the TAR. Note that for CO₂, RF increases logarithmically with mixing ratio.) Collins et al. (2006) performed a comparison of five detailed line-by-line models and 20 GCM radiation schemes. The spread of line-by-line model results were consistent with the ±10% uncertainty estimate for the LLGHG RFs adopted in Ramaswamy et al. (2001) and a similar ±10% for the 90% confidence interval is adopted here. However, it is also important to note that these relatively small uncertainties are not always achievable when incorporating the LLGHG forcings into GCMs. For example, both Collins et al. (2006) and Forster and Taylor (2006) found that GCM radiation schemes could have inaccuracies of around 20% in their total LLGHG RF (see also Sections 2.3.2 and 10.2).

Using the global average value of 379 ppm for atmospheric CO₂ in 2005 gives an RF of 1.66 ± 0.17 W m^–2; a contribution that dominates that of all other forcing agents considered in this chapter. This is an increase of 13 to 14% over the value reported for 1998 in Ramaswamy et al. (2001). This change is solely due to increases in atmospheric CO₂ and is also much larger than the RF changes due to other agents. In the decade 1995 to 2005, the RF due to CO₂ increased by about 0.28 W m^–2 (20%), an increase greater than that calculated for any decade since at least 1800 (see Section 6.6 and FAQ 2.1, Figure 1).

Table 2.1 summarises the present-day mixing ratios and RF for the LLGHGs, and indicates changes since 1998. The RF from CO₂ and that from the other LLGHGs have a high level of scientific understanding (Section 2.9, Table 2.11). Note that the uncertainty in RF is almost entirely due to radiative transfer assumptions and not mixing ratio estimates, therefore trends in RF can be more accurately determined than the absolute RF. From Section 2.5.3, Table 2.8, the contribution from land use change to the present CO₂ RF is likely to be about 0.4 W m^–2 (since 1850). This implies that fossil fuel and cement production have likely contributed about three-quarters of the current RF.

Table 2.1. Present-day concentrations and RF for the measured LLGHGs. The changes since 1998 (the time of the TAR estimates) are also shown.

Notes:

^a See Table 2.14 for common names of gases and the radiative efficiencies used to calculate RF.

^b Mixing ratio errors are 90% confidence ranges of combined 2005 data, including intra-annual standard deviation, measurement and global averaging uncertainty. Standard deviations were multiplied by 1.645 to obtain estimates of the 90% confidence range; this assumes normal distributions. Data for CO₂ are combined measurements from the NOAA Earth System Research Laboratory (ESRL) and SIO networks (see Section 2.3.1); CH₄ measurements are combined data from the ESRL and Advanced Global Atmospheric Gases Experiment (AGAGE) networks (see Section 2.3.2); halocarbon measurements are the average of ESRL and AGAGE networks. University of East Anglia (UEA) and Pennsylvania State University (PSU) measurements were also used (see Section 2.3.3).

^c Pre-industrial values are zero except for CO₂ (278 ppm), CH₄ (715 ppb; 700 ppb was used in the TAR), N₂O (270 ppb) and CF₄ (40 ppt).

^d 90% confidence ranges for RF are not shown but are approximately 10%. This confidence range is almost entirely due to radiative transfer assumptions, therefore trends remain valid when quoted to higher accuracies. Higher precision data are used for totals and affect rounding of the values. Percent changes are calculated relative to 1998.

^e Data available from AGAGE network only.

^f Data for 1998 not available; values from 1999 are used.

^g Data from UEA only.

^h Data from 2003 are used due to lack of available data for 2004 and 2005.

ⁱ Data from ESRL only.

^j 1997 data from PSU (Khalil et al., 2003, not updated) are used.

^k CFC total includes a small 0.009 W m^–2 RF from CFC-13, CFC-114, CFC-115 and the halons, as measurements of these were not updated.