6.4.1.1 How Do Glacial-Interglacial Variations in the Greenhouse Gases Carbon Dioxide, Methane and Nitrous Oxide Compare with the Industrial Era Greenhouse Gas Increase?

The present atmospheric concentrations of CO₂, CH₄ and nitrous oxide (N₂O) are higher than ever measured in the ice core record of the past 650 kyr (Figures 6.3 and 6.4). The measured concentrations of the three greenhouse gases fluctuated only slightly (within 4% for CO₂ and N₂O and within 7% for CH₄) over the past millennium prior to the industrial era, and also varied within a restricted range over the late Quaternary. Within the last 200 years, the late Quaternary natural range has been exceeded by at least 25% for CO₂, 120% for CH₄ and 9% for N₂O. All three records show effects of the large and increasing growth in anthropogenic emissions during the industrial era.

Variations in atmospheric CO₂ dominate the radiative forcing by all three gases (Figure 6.4). The industrial era increase in CO₂, and in the radiative forcing (Section 2.3) by all three gases, is similar in magnitude to the increase over the transitions from glacial to interglacial periods, but started from an interglacial level and occurred one to two orders of magnitude faster (Stocker and Monnin, 2003). There is no indication in the ice core record that an increase comparable in magnitude and rate to the industrial era has occurred in the past 650 kyr. The data resolution is sufficient to exclude with very high confidence a peak similar to the anthropogenic rise for the past 50 kyr for CO₂, for the past 80 kyr for CH₄ and for the past 16 kyr for N₂O. The ice core records show that during the industrial era, the average rate of increase in the radiative forcing from CO₂, CH₄ and N₂O is greater than at any time during the past 16 kyr (Figure 6.4). The smoothing of the atmospheric signal (Schwander et al., 1993; Spahni et al., 2003) is small at Law Dome, a high-accumulation site in Antarctica, and decadal-scale rates of change can be computed from the Law Dome record spanning the past two millennia (Etheridge et al., 1996; Ferretti et al., 2005; MacFarling Meure et al., 2006). The average rate of increase in atmospheric CO₂ was at least five times larger over the period from 1960 to 1999 than over any other 40-year period during the two millennia before the industrial era. The average rate of increase in atmospheric CH₄ was at least six times larger, and that for N₂O at least two times larger over the past four decades, than at any time during the two millennia before the industrial era. Correspondingly, the recent average rate of increase in the combined radiative forcing by all three greenhouse gases was at least six times larger than at any time during the period AD 1 to AD 1800 (Figure 6.4d).

Figure 6.4. The concentrations and radiative forcing by (a) CO₂, (b) CH₄ and (c) nitrous oxide (N₂O), and (d) the rate of change in their combined radiative forcing over the last 20 kyr reconstructed from antarctic and Greenland ice and firn data (symbols) and direct atmospheric measurements (red and magenta lines). The grey bars show the reconstructed ranges of natural variability for the past 650 kyr (Siegenthaler et al., 2005a; Spahni et al., 2005). Radiative forcing was computed with the simplified expressions of Chapter 2 (Myhre et al., 1998). The rate of change in radiative forcing (black line) was computed from spline fits (Enting, 1987) of the concentration data (black lines in panels a to c). The width of the age distribution of the bubbles in ice 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 Law Dome ice and firn data, covering the past two millennia, and recent instrumental data have been splined with a cut-off period of 40 years, with the resulting rate of change in radiative forcing shown by the inset in (d). The arrow shows the peak in the rate of change in radiative forcing after the anthropogenic signals of CO₂, CH₄ and N₂O have been smoothed with a model describing the enclosure process of air in ice (Spahni et al., 2003) applied for conditions at the low accumulation Dome C site for the last glacial transition. The CO₂ data are from Etheridge et al. (1996); Monnin et al. (2001); Monnin et al. (2004); Siegenthaler et al. (2005b; South Pole); Siegenthaler et al. (2005a; Kohnen Station); and MacFarling Meure et al. (2006). The CH₄ data are from Stauffer et al. (1985); Steele et al. (1992); Blunier et al. (1993); Dlugokencky et al. (1994); Blunier et al. (1995); Chappellaz et al. (1997); Monnin et al. (2001); Flückiger et al. (2002); and Ferretti et al. (2005). The N₂O data are from Machida et al. (1995); Battle et al. (1996); Flückiger et al. (1999, 2002); and MacFarling Meure et al. (2006). Atmospheric data are from the National Oceanic and Atmospheric Administration’s global air sampling network, representing global average concentrations (dry air mole fraction; Steele et al., 1992; Dlugokencky et al., 1994; Tans and Conway, 2005), and from Mauna Loa, Hawaii (Keeling and Whorf, 2005). The globally averaged data are available from http://www.cmdl.noaa.gov/.