TS.2.1.1 Changes in Atmospheric Carbon Dioxide, Methane and Nitrous Oxide

Current concentrations of atmospheric CO₂ and CH₄ far exceed pre-industrial values found in polar ice core records of atmospheric composition dating back 650,000 years. Multiple lines of evidence confirm that the post-industrial rise in these gases does not stem from natural mechanisms (see Figure TS.1 and Figure TS.2). {2.3, 6.3–6.5, FAQ 7.1}

Glacial-Interglacial Ice Core Data

Figure TS.1. Variations of deuterium (δD) in antarctic ice, which is a proxy for local temperature, and the atmospheric concentrations of the greenhouse gases carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) in air trapped within the ice cores and from recent atmospheric measurements. Data cover 650,000 years and the shaded bands indicate current and previous interglacial warm periods. {Adapted from Figure 6.3}

The total radiative forcing of the Earth’s climate due to increases in the concentrations of the LLGHGs CO₂, CH₄ and N₂O, and very likely the rate of increase in the total forcing due to these gases over the period since 1750, are unprecedented in more than 10,000 years (Figure TS.2). It is very likely that the sustained rate of increase in the combined radiative forcing from these greenhouse gases of about +1 W m^–2 over the past four decades is at least six times faster than at any time during the two millennia before the Industrial Era, the period for which ice core data have the required temporal resolution. The radiative forcing due to these LLGHGs has the highest level of confidence of any forcing agent. {2.3, 6.4}

Changes in Greenhouse Gases from Ice Core and Modern Data

Figure TS.2. The concentrations and radiative forcing by (a) carbon dioxide (CO₂), (b) methane (CH₄), (c) nitrous oxide (N₂O) and (d) the rate of change in their combined radiative forcing over the last 20,000 years reconstructed from antarctic and Greenland ice and firn data (symbols) and direct atmospheric measurements (panels a,b,c, red lines). The grey bars show the reconstructed ranges of natural variability for the past 650,000 years. The rate of change in radiative forcing (panel d, black line) has been computed from spline fits to the concentration data. The width of the age spread in the ice data varies from about 20 years for sites with a high accumulation of snow such as Law Dome, Antarctica, to about 200 years for low-accumulation sites such as Dome C, Antarctica. The arrow shows the peak in the rate of change in radiative forcing that would result if the anthropogenic signals of CO₂, CH₄, and N₂O had been smoothed corresponding to conditions at the low-accumulation Dome C site. The negative rate of change in forcing around 1600 shown in the higher-resolution inset in panel d results from a CO₂ decrease of about 10 ppm in the Law Dome record. {Figure 6.4}

The concentration of atmospheric CO₂ has increased from a pre-industrial value of about 280 ppm to 379 ppm in 2005. Atmospheric CO₂ concentration increased by only 20 ppm over the 8000 years prior to industrialisation; multi-decadal to centennial-scale variations were less than 10 ppm and likely due mostly to natural processes. However, since 1750, the CO₂ concentration has risen by nearly 100 ppm. The annual CO₂ growth rate was larger during the last 10 years (1995–2005 average: 1.9 ppm yr^–1) than it has been since continuous direct atmospheric measurements began (1960–2005 average: 1.4 ppm yr^–1). {2.3, 6.4, 6.5}

Increases in atmospheric CO₂ since pre-industrial times are responsible for a radiative forcing of +1.66 ± 0.17 W m^–2; a contribution which dominates all other radiative forcing agents considered in this report. For the decade from 1995 to 2005, the growth rate of CO₂ in the atmosphere led to a 20% increase in its radiative forcing. {2.3, 6.4, 6.5}

Emissions of CO₂ from fossil fuel use and from the effects of land use change on plant and soil carbon are the primary sources of increased atmospheric CO₂. Since 1750, it is estimated that about 2/3rds of anthropogenic CO₂ emissions have come from fossil fuel burning and about 1/3rd from land use change. About 45% of this CO₂ has remained in the atmosphere, while about 30% has been taken up by the oceans and the remainder has been taken up by the terrestrial biosphere. About half of a CO₂ pulse to the atmosphere is removed over a time scale of 30 years; a further 30% is removed within a few centuries; and the remaining 20% will typically stay in the atmosphere for many thousands of years. {7.3}

In recent decades, emissions of CO₂ have continued to increase (see Figure TS.3). Global annual fossil CO₂ emissions^[3] increased from an average of 6.4 ± 0.4 GtC yr^–1 in the 1990s to 7.2 ± 0.3 GtC yr^–1 in the period 2000 to 2005. Estimated CO₂ emissions associated with land use change, averaged over the 1990s, were^[4] 0.5 to 2.7 GtC yr^–1, with a central estimate of 1.6 Gt yr^-1. Table TS.1 shows the estimated budgets of CO₂ in recent decades. {2.3, 6.4, 7.3, FAQ 7.1}

Since the 1980s, natural processes of CO₂ uptake by the terrestrial biosphere (i.e., the residual land sink in Table TS.1) and by the oceans have removed about 50% of anthropogenic emissions (i.e., fossil CO₂ emissions and land use change flux in Table TS.1). These removal processes are influenced by the atmospheric CO₂ concentration and by changes in climate. Uptake by the oceans and the terrestrial biosphere have been similar in magnitude but the terrestrial biosphere uptake is more variable and was higher in the 1990s than in the 1980s by about 1 GtC yr^–1. Observations demonstrate that dissolved CO₂ concentrations in the surface ocean (pCO₂) have been increasing nearly everywhere, roughly following the atmospheric CO₂ increase but with large regional and temporal variability. {5.4, 7.3}

Carbon uptake and storage in the terrestrial biosphere arise from the net difference between uptake due to vegetation growth, changes in reforestation and sequestration, and emissions due to heterotrophic respiration, harvest, deforestation, fire, damage by pollution and other disturbance factors affecting biomass and soils. Increases and decreases in fire frequency in different regions have affected net carbon uptake, and in boreal regions, emissions due to fires appear to have increased over recent decades. Estimates of net CO₂ surface fluxes from inverse studies using networks of atmospheric data demonstrate significant land uptake in the mid-latitudes of the Northern Hemisphere (NH) and near-zero land-atmosphere fluxes in the tropics, implying that tropical deforestation is approximately balanced by regrowth. {7.3}

Short-term (interannual) variations observed in the atmospheric CO₂ growth rate are primarily controlled by changes in the flux of CO₂ between the atmosphere and the terrestrial biosphere, with a smaller but significant fraction due to variability in ocean fluxes (see Figure TS.3). Variability in the terrestrial biosphere flux is driven by climatic fluctuations, which affect the uptake of CO₂ by plant growth and the return of CO₂ to the atmosphere by the decay of organic material through heterotrophic respiration and fires. El Niño-Southern Oscillation (ENSO) events are a major source of interannual variability in atmospheric CO₂ growth rate, due to their effects on fluxes through land and sea surface temperatures, precipitation and the incidence of fires. {7.3}

CO2 Emissions and Increases

Figure TS.3. Annual changes in global mean CO₂ concentration (grey bars) and their fiveyear means from two different measurement networks (red and lower black stepped lines). The five-year means smooth out short-term perturbations associated with strong ENSO events in 1972, 1982, 1987 and 1997. Uncertainties in the five-year means are indicated by the difference between the red and lower black lines and are of order 0.15 ppm. The upper stepped line shows the annual increases that would occur if all fossil fuel emissions stayed in the atmosphere and there were no other emissions. {Figure 7.4}

The direct effects of increasing atmospheric CO₂ on large-scale terrestrial carbon uptake cannot be quantified reliably at present. Plant growth can be stimulated by increased atmospheric CO₂ concentrations and by nutrient deposition (fertilization effects). However, most experiments and studies show that such responses appear to be relatively short lived and strongly coupled to other effects such as availability of water and nutrients. Likewise, experiments and studies of the effects of climate (temperature and moisture) on heterotrophic respiration of litter and soils are equivocal. Note that the effect of climate change on carbon uptake is addressed separately in section TS.5.4. {7.3}

The CH₄ abundance in 2005 of about 1774 ppb is more than double its pre-industrial value. Atmospheric CH₄ concentrations varied slowly between 580 and 730 ppb over the last 10,000 years, but increased by about 1000 ppb in the last two centuries, representing the fastest changes in this gas over at least the last 80,000 years. In the late 1970s and early 1980s, CH₄ growth rates displayed maxima above 1% yr^–1, but since the early 1990s have decreased significantly and were close to zero for the six-year period from 1999 to 2005. Increases in CH₄ abundance occur when emissions exceed removals. The recent decline in growth rates implies that emissions now approximately match removals, which are due primarily to oxidation by the hydroxyl radical (OH). Since the TAR, new studies using two independent tracers (methyl chloroform and ¹⁴CO) suggest no significant long-term change in the global abundance of OH. Thus, the slowdown in the atmospheric CH₄ growth rate since about 1993 is likely due to the atmosphere approaching an equilibrium during a period of near-constant total emissions. {2.3, 7.4, FAQ 7.1}

Increases in atmospheric CH₄ concentrations since pre-industrial times have contributed a radiative forcing of +0.48 ± 0.05 W m^–2. Among greenhouse gases, this forcing remains second only to that of CO₂ in magnitude. {2.3}

Current atmospheric CH₄ levels are due to continuing anthropogenic emissions of CH₄, which are greater than natural emissions. Total CH₄ emissions can be well determined from observed concentrations and independent estimates of removal rates. Emissions from individual sources of CH₄ are not as well quantified as the total emissions but are mostly biogenic and include emissions from wetlands, ruminant animals, rice agriculture and biomass burning, with smaller contributions from industrial sources including fossil fuel-related emissions. This knowledge of CH₄ sources, combined with the small natural range of CH₄ concentrations over the past 650,000 years (Figure TS.1) and their dramatic increase since 1750 (Figure TS.2), make it very likely that the observed long-term changes in CH₄ are due to anthropogenic activity. {2.3, 6.4, 7.4}

In addition to its slowdown over the last 15 years, the growth rate of atmospheric CH₄ has shown high interannual variability, which is not yet fully explained. The largest contributions to interannual variability during the 1996 to 2001 period appear to be variations in emissions from wetlands and biomass burning. Several studies indicate that wetland CH₄ emissions are highly sensitive to temperature and are also affected by hydrological changes. Available model estimates all indicate increases in wetland emissions due to future climate change but vary widely in the magnitude of such a positive feedback effect. {7.4}

The N₂O concentration in 2005 was 319 ppb, about 18% higher than its pre-industrial value. Nitrous oxide increased approximately linearly by about 0.8 ppb yr^–1 over the past few decades. Ice core data show that the atmospheric concentration of N₂O varied by less than about 10 ppb for 11,500 years before the onset of the industrial period. {2.3, 6.4, 6.5}

The increase in N₂O since the pre-industrial era now contributes a radiative forcing of +0.16 ± 0.02 W m^–2 and is due primarily to human activities, particularly agriculture and associated land use change. Current estimates are that about 40% of total N₂O emissions are anthropogenic but individual source estimates remain subject to significant uncertainties. {2.3, 7.4}