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1 For example, Tim Lenton et al., 'Tipping elements in the Earth's climate system', Proceedings of the National Academy of the Sciences, vol. 105, no. 6, 12 February 2008.

2 Jorgen Peder Steffensen et al., 'High-Resolution Greenland Ice Core Data Show Abrupt Climate Change Happens in Few Years', Science, vol. 321, no. 5889, 1 August 2008. Graeme Pearman notes that these abrupt changes may have been due to rapid deglaciation coming out of the last ice age, 'a process that would not be applicable to Earth which is already substantially more deglaciated' (pers. comm.).

3 James Hansen, 'Scientific reticence and sea level rise', Environmental Research Letters, April-June 2007.

4 Hansen, 'Scientific reticence and sea level rise', p. 5. Slightly modified.

5 Others, such as the UK Met Office's Vicki Pope, believe the recent sharp decline could be due to natural variation but that over the longer term human-induced warming will lead to the disappearance of Arctic sea-ice in the summer. Vicki Pope, 'Scientists must rein in misleading climate change claims', Guardian, 11 February 2009.

6 Quoted in Deborah Zabarenko, 'Arctic ice second-lowest ever; polar bears affected', Reuters, 27 August 2008.

7 Quoted in the New York Sun, 22 December 2007.

8 Andrew Weaver of the University of Victoria in British Columbia quoted by Richard Monastersky, 'A Burden Beyond Bearing', Nature, vol. 458, 30 April 2009, p. 1094.

9 Ross Garnaut, The Garnaut Climate Change Review, Cambridge University Press, Melbourne, 2008, Table 3.1, p. 56.

10 Garnaut, The Garnaut Climate Change Review, p. 66.

11 Garnaut, The Garnaut Climate Change Review, Table 3.2, p. 65. In which year, the United States is expected to account for 11 per cent (down from 18) and India 8 per cent (up from 4.6 per cent). Everything is going the wrong way in China: it has the biggest population, the fastest rate of economic growth and its energy is overwhelmingly based on fossil fuels.

12 Garnaut, The Garnaut Climate Change Review, p. 58.

13 National Oceanic and Atmospheric Administration, 'Trends in Atmospheric Carbon Dioxide—Mauna Loa', http://www.esrl.noaa. gov/gmd/ccgg/trends/.

14 Susan Solomon, Gian-Kasper Plattner, Reto Knutti and Pierre Friedlingstein, 'Irreversible climate change due to carbon dioxide emissions', Proceedings of the National Academy of Sciences, vol. 106, no. 6, 10 February 2009.

15 Richard Monastersky, 'A Burden Beyond Bearing', Nature, vol. 458, 30 April 2009, p. 1094.

16 Garnaut, The Garnaut Climate Change Review, p. 64.

17 Steve Connor and Chris Green, 'Climate scientists: it's time for plan B', Independent, 2 January 2009, http://www.independent. co.uk/environment/climate-change/climate-scientists-its-time-for-plan-b-1221092.html.

18 The extent to which higher concentrations of greenhouse gases in the atmosphere lead to warming is known as climate sensitivity. The best estimate is that a doubling of CO2 concentrations from 280 to 560 will bring about warming of 3°C, with a likely range of 1.5°C to 4.5°C, although this has been challenged as much too conservative. See David Spratt and Philip Sutton, Climate Code Red (Scribe, Melbourne, 2008), pp. 45-8.

19 Bjorn Lomborg, The Skeptical Environmentalist, Cambridge University Press, Cambridge, 2001, pp. 284, 286.

20 Lomborg, The Skeptical Environmentalist, p. 286.

21 See also Katherine Richardson et al., Synthesis Report, from the Climate Change: Global Risks, Challenges & Decisions conference, University of Copenhagen, Copenhagen, 2009, Box 2.

22 Recent observed climate trends are compared to the model projections contained in the 2001 IPCC report in S. Rahmstorf, A. Cazenave, J.A. Church, J.E. Hansen, R.F. Keeling, D.E. Parker and R.J.C. Somerville, 'Recent climate observations compared to projections', Science, vol. 316, no. 5825, 2007.

23 IPCC, 'Summary for Policymakers', Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, Table SPM1, p. 8. See also V. Ramanathan and Y. Feng, 'On avoiding dangerous anthropogenic interference with the climate system: Formidable challenges ahead', Proceedings of the National Academy of Sciences, vol. 105, no. 38, 23 September 2008.

24 Although it in turn has a range of 2.4-6.4°C above 1990 levels, or 3.0-7.0°C above pre-industrial levels.

25 David Archer, The Long Thaw, Princeton University Press, Princeton, 2009, p. 1; Solomon et al., 'Irreversible climate change due to carbon dioxide emissions'.

26 Archer, The Long Thaw, p. 6. Anthropogenic forcing is currently at around 1.6 Watt/m2 relative to 1750 (accounting for all gases), to which should be added the short-term masking effect of aerosols, 1.2 Watt/m2, and the effects of Arctic and other albedo loss, and open-water infrared radiative gain, totalling 3 Watt/m2 or more. The natural orbital forcing associated with the end of glacial periods was around 6 Watt/m2. So to date we have had about half the impact that led to the end of the last ice age (Andrew Glikson, pers. comm., based on James Hansen et al., 'Target Atmospheric CO2: Where Should Humanity Aim?', The Open Atmospheric Science Journal, vol. 2, pp. 217-31, 2008).

27 David Adam, 'Amazon could shrink by 85% due to climate change, scientists say', Guardian, 11 March 2009.

28 Global Carbon Project, 'Carbon budget and trends 2007', 26 September 2008, www.globalcarbonproject.org.

29 IPCC, Climate Change 2007: Synthesis Report, Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Core writing team, R.K. Pachauri and A. Reisinger (eds), IPCC, Geneva, 2007, p. 67.

30 See, for example, Ramanathan and Feng, 'On avoiding dangerous anthropogenic interference with the climate system'. Graeme Pearman points out that setting a target like this is not a rigorous process because it involves estimating risks associated with a wide range of impacts across the globe. Some of the risks embodied in it are very large for some impacts in some parts of the world (pers. comm.).

31 The IPCC report shows that there is almost a 50 per cent chance of exceeding 2°C warming if the concentration stabilises at 450 ppm.

32 Richardson et al., Synthesis Report, p. 18.

33 Richardson et al., Synthesis Report, p. 18.

34 Lenton et al., 'Tipping elements in the Earth's climate system', Table 1.

35 James Hansen, 'Global Warming Twenty Years Later: Tipping Points Near', Speech to the National Press Club, Washington, 23 June 2008.

36 Richardson et al., Synthesis Report, p. 18.

37 J.C. Zachos, G.R. Dickens and R.E. Zeebe, 'An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics', Nature, vol. 451, 2008, pp. 279-83.

38 Hansen et al., 'Target Atmospheric CO2'.

39 See, for example, Bill Hare, Michiel Schaeffer and Malte Meins-hausen, 'Emission reductions by the USA in 2020 and the risk of exceeding 2°C warming', Climate Analytics, March 2009.

40 Hansen et al., 'Target Atmospheric CO2'.

41 Richardson et al., Synthesis Report, p. 18.

42 Kevin Anderson and Alice Bows, 'Reframing the climate change challenge in light of post-2000 emission trends', Philosophical Transactions of the Royal Society, Royal Society, 2008.

43 All figures below are taken from the analysis by Anderson and Bows.

44 Anderson and Bows, 'Reframing the climate change challenge', p. 5.

45 Also called a gigatonne. It is organic carbon rather than CO2 that is locked up in forests, but the analysis here expresses it in terms of the CO2 that results from oxidising the carbon through burning or decay.

46 Anderson and Bows, 'Reframing the climate change challenge', p. 7. After rising for many years, in 1999 global methane emissions plateaued. They began to rise again in 2007, possibly due to the melting of the Siberian permafrost. See M. Rigby et al., 'Renewed growth of atmospheric methane', Geophysical Research Letters, vol. 35, L22805, 2008.

47 Anderson and Bows, 'Reframing the climate change challenge', pp. 8-9.

48 In 2007 the world's population was 6.6 billion and the UN's middle estimate for 2050 is 9.2 billion (United Nations, World Population Prospects: The 2006 Revision, UN Department of Economic and Social Affairs, New York, 2007).

49 It is, for example, the most optimistic outcome modelled by Hare et al., 'Emission reductions by the USA in 2020 and the risk of exceeding 2°C warming'.

50 United Nations, World Population Prospects.

51 Robert Socolow and Steve Pacala, 'Stabilisation Wedges: Solving the Climate Problem for the Next Fifty Years with Current Technology', Science, vol. 305, 13 August 2004.

52 Nicholas Stern, The Economics of Climate Change: The Stern Review, Cambridge University Press, Cambridge, 2007, Box 8.3, p. 231. Anderson and Bows also draw from it.

53 See the World Bank figures reported by the BBC (http://news.bbc. co.uk/2/shared/spl/hi/guides/457000/457038/html/default.stm).

54 Stern, The Economics of Climate Change, p. 232.

55 Anderson and Bows, 'Reframing the climate change challenge', Table 7, p. 14.

56 One study concluded that, to keep warming from exceeding 2°C, total anthropogenic emissions must be limited to 3.67 trillion tonnes of CO2 (ignoring non-CO2 forcing agents). Half of this amount has already been emitted since the start of the Industrial Revolution in the eighteenth century, leaving a budget of 1.85 trillion tonnes for the twenty-first century, or 1.46 trillion tonnes if non-CO2 forcings are taken into account (Myles Allen et al., 'Warming caused by cumulative carbon emissions towards the trillionth tonne', Nature, vol. 458, 30 April 2009). Anderson and Bows' analysis shows that, if global emissions peak in 2020 and fall by 3 per cent per annum thereafter, an additional 3 trillion tonnes of CO2-e will be added to the atmosphere—60 per cent more than allowed by the 2°C target.

57 Anderson and Bows, 'Reframing the climate change challenge', p. 13.

58 See especially, Joel B. Smith et al., 'Assessing dangerous climate change through an update of the Intergovernmental Panel on Climate Change (IPCC) "reasons for concern"', Proceedings of the National Academy of Sciences, Early edition, 26 February 2009.

59 Alice Bows, Kevin Anderson and Sarah Mander, 'Aviation in turbulent times', Technology Analysis & Strategic Management, vol. 21, no. 1, 2009, pp. 17-37.

60 J.G. Canadell et al., 'Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks', Proceedings of the National Academy of Sciences, vol. 104, 2007, pp. 18866-70; M.R. Raupach, J.G. Canadell and C. Le Quere, 'Anthropogenic and biophysical contributions to increasing atmospheric CO2 growth rate and airborne fraction', Biogeosciences, vol. 5, 2008, pp. 1601-13.

61 Stern, The Economics of Climate Change, p. 194.

62 I am grateful to Andrew Glikson and Graeme Pearman for discussions on these themes.

63 Other factors driving abrupt climate changes include vulcanism, asteroid and comet impacts, methane releases from sediments and the effects of radiation from supernovae. G. Keller, 'Impacts, Volcanism and Mass Extinction: Random coincidence or cause and effect?', Australian Journal of Earth Sciences, vol. 52, issues 4 & 5, 2005, pp. 725-57; Andrew Glikson, 'Asteroid/comet impact clusters, flood basalts and mass extinctions: Significance of isotopic age overlaps', Earth and Planetary Science Letters, vol. 236, 2005, pp. 933-7; Archer, The Long Thaw; Steffensen et al., 'HighResolution Greenland Ice Core Data Show Abrupt Climate Change Happens in Few Years'; Hansen et al., 'Target Atmospheric CO2'.

64 Lenton et al., 'Tipping elements in the Earth's climate system'; Ramanathan and Feng, 'On avoiding dangerous anthropogenic interference with the climate system'.

65 Solomon et al., 'Irreversible climate change due to carbon dioxide emissions', pp. 1704-9.

66 Solomon et al., 'Irreversible climate change due to carbon dioxide emissions', pp. 1708-9.

67 IPCC, Climate Change 2007: Mitigation of climate change, Working Group III Contribution to the Fourth Assessment Report of the IPCC,

Cambridge University Press, Cambridge, 2007, p. 173.

68 Lenton et al., 'Tipping elements in the Earth's climate system', p. 1792.

69 David King, Speech to the Decarbonising the UK conference, Church House, Westminster, 21 September 2005. King, of course, is one of the more enlightened policy advisers. He accepts that aiming even for 450 ppm is a big risk. In 2007 he was urging the world to aim at somewhere between 450 and 550 ppm (Science, vol. 317, no. 5842, 31 August 2007, pp. 1184-7).

70 Stern, The Economics of Climate Change, p. 194.

71 Todd Stern, 'Keynote Remarks at U.S. Climate Action Symposium', Senate Hart Office Building, Washington, DC, 3 March 2009.

72 J.A. Lowe, C. Huntingford, S.C.B. Raper, C.D. Jones, S.K. Liddicoat and L.K. Gohar, 'How difficult is it to recover from dangerous levels of global warming?', Environmental Research Letters, vol. 4, 2009. In the simulation where emissions are set to zero in 2050 as concentrations reach 550 ppm CO2, the temperature rises by an additional 0.2°C through to 2150.

73 Ramanathan and Feng, 'On avoiding dangerous anthropogenic interference with the climate system'.

74 Hans Joachim Schellnhuber, 'Global warming: Stop worrying, start panicking?', Proceedings of the National Academy of the Sciences, vol. 105, no. 38, 23 September 2008, pp. 14239-40.

75 See, for example, A. Dupont and G.I. Pearman, Heating up the Planet: Climate Change and Security, Lowy Institute Paper 12, Lowy Institute, Sydney, 2007; and R. Schubert et al., Climate Change as a Security Risk, Earthscan, London, 2008.

76 See, for example, J. Overpeck, D.C. Whitlock and B. Huntley, 'Terrestrial biosphere dynamics in the climate system: Past and future', in K.D. Bradley and R.S. and T.F. Pedersen (eds), Paleo-climate, Global Change and the Future, Springer-Verlag, Berlin, 2003.

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