Notes For Chapter

1 It is convenient to define radiative forcing as the radiative imbalance at the top of the troposphere rather than at the top of the whole atmosphere. The formal definition of radiative forcing as in the IPCC reports is 'the change in net (down minus up) irradiance (solar plus longwave in W m-2) at the tropopause after allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values'.

2 See Denman, K. L., Brasseur, G. et al. 2007. Chapter 7, Section 7.3.12, in Solomon et al. (eds.) Climate Change 2007: The Physical Science Basis.

3 See Jansen, E., Overpeck, J. et al. 2007. Chapter 6 (especially Box 6.2), in Solomon et al. (eds.) Climate Change 2007: The Physical Science Basis.

4 The process has been called the 'plankton multiplier': Woods, J., Barkmann, W. 1993. The plankton multiplier: positive feedback in the greenhouse. Journal of Plankton Research, 15, 1053-74.

5 For more details see House, J. I. et al. 2003. Reconciling apparent inconsistencies in estimates of terrestrial CO2 sources and sinks. Tellus, 55B, 345-63.

6 See Jones, C. D., Cox, P. M. 2001. Atmospheric science letters. doi.1006/asle. 2001.0041; the respiration rate varies approximately with a factor 2T-10/10.

7 Cox, P. M., Betts, R. A., Collins, M., Harris, P., Huntingford, C., Jones, C. D. 2004. Amazon dieback under climate-carbon cycle projections for the 21st century. Theoretical and Applied Climatology, 78, 137-56.

8 Some recent papers have pointed out the need for urgent research into aspects of the climate/carbon-cycle feedback, for instance the importance of linking together all components of the ecosystem chemistry and dynamics including, for instance, the influences of water and nitrogen. See Heimann, M., Reichstein, M. 2008. Nature, 451, 289-92; Gruber, N., Galloway, J. N. 2008. Nature, 451, 293-6; and Friedlingstein, P. 2008. Nature, 451, 297-8.

9 In the absence of climate feedbacks the Hadley model is similar to the other models with slightly above average land carbon uptake and slightly below average ocean uptake.

10 The extensive and protracted forest fires in Indonesia and neighbouring areas in 1997-8 have been estimated to have resulted in the emission to the atmosphere of 0.8 to 2.6 GtC; this may be one of the reasons for particularly high growth in atmospheric carbon dioxide in 1998.

11 In the 1990s the rate of increase substantially slowed. The reason for this is not known but one suggestion is that, because of the collapse of the Russian economy, the leakage from Siberian natural gas pipelines was much reduced.

12 The ratio of the enhanced greenhouse effect of a molecule of methane compared to a molecule of carbon dioxide is known as its global warming potential (GWP); a definition of GWP is given later in the chapter. The figure of about 8 given here for the GWP of methane is for a time horizon of 100 years - see Lelieveld, J., Crutzen, P.J. 1992. Nature, 355, 339-41; see also Prather et al., Chapter 4, and Ramaswamy et al., Chapter 6, in, Houghton, J. T., Ding, Y., Griggs, D. J., Noguer, M., van der Linden, P., Dai, X., Maskell, K., Johnson, C. A. (eds.) 2001. Climate Change 2001: The Scientific Basis. Contribution of Working Group 1 to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press. The GWP is also often expressed as the ratio of the effect for unit mass of each gas in which case the GWP for methane (whose molecular mass is 0.36 of that of carbon dioxide) becomes about 23 for the 100-year time horizon. About 75% of the contribution of methane to the greenhouse effect is because of its direct effect on the outgoing thermal radiation. The other

25% arises because of its influence on the overall chemistry of the atmosphere. Increased methane eventually results in small increases in water vapour in the upper atmosphere, in tropospheric ozone and in carbon dioxide, all of which in turn add to the greenhouse effect.

13 Taking into account the loss processes due to reaction with OH in the troposphere, chemical reactions and soil loss lead to a lifetime of about ten years. However, the effective lifetime of methane against a perturbation in concentration in the atmosphere (the number quoted here) is complex because it depends on the methane concentration. This is because the concentration of the radical OH (interaction with which is the main cause of methane destruction), due to chemical feedbacks, is itself dependent on the methane concentration (more details in Prather et al. 2001. Chapter 4, in Houghton et al. (eds.), Climate Change 2001: The Scientific Basis).

14 For more detail, see Scientific Assessment of Ozone Depletion: 2003. Geneva: World Meteorological Organization.

15 Prather et al., Chapter 4, in Houghton et al. (eds.), Climate Change 2001: The Scientific Basis.

16 More detail on this and the radiative effects of minor gases and particles can be found in Ramaswamy et al. 2001, Chapter 6, in Houghton et al. (eds.), Climate Change 2001: The Scientific Basis.

17 More detail in Penner, J. E. et al. (eds.), 1999. Aviation and the Global Atmosphere. An IPCC Special Report. Cambridge: Cambridge University Press.

18 See for instance Ramanathan, V. et al. 2007, Warming trends in Asia amplified by brown cloud solar absorption. Nature, 448, 575-8.

19 For variations of radioactive forcing from different forcing agents over the period 1880-2004, see James Hansen et al., Science 308, 1431.

20 More detail in Penner et al. (eds.), Aviation and the Global Atmosphere.

21 More detail on GWPs can be found in Ramaswamy et al., Chapter 6, in Houghton et al. (eds.), Climate Change 2001: The Scientific Basis.

Satellite image of the termini of retreating glaciers in the Himalayan Mountains of Bhutan.

TO OBTAIN some perspective against which to view future climate change, it is helpful to look at some of the climate changes that have occurred in the past. This chapter will briefly consider climatic records and climate changes in three periods: the last hundred years, then the last thousand years and finally the last million years. At the end of the chapter some interesting evidence for the existence of relatively rapid climate change at various times during the past one or two hundred thousand years will be presented.

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