Other factors that might influence climate change

So far climate change due to human activities has been considered. Are there other factors, external to the climate system, which might induce change? Chapter 4 showed that it was variations in the incoming solar energy as a result of changes in the Earth's orbit that triggered the ice ages and the major climate changes of the past. These variations are, of course, still going on: what influence are they having now?

Over the past 10 000 years, because of these orbital changes, the solar radiation incident at 60° N in July has decreased by about 35 W m-2, which is quite a large amount. But over 100 years the change is only at most a few tenths of a watt per square metre, which is much less than the changes due to the increases in greenhouse gases (remember that doubling carbon dioxide alters the thermal radiation, globally averaged, by about 4 W m-2 - see Chapter 2). Looking to the future and the effect of the Earth's orbital variations, over at least the next 50 000 years the solar radiation incident in summer on the polar regions will be unusually constant so that the present interglacial is expected

Does the Sun's output change?

Some scientists have suggested that all climate variations, even short-term ones, might be the result of changes in the Sun's energy output. Such suggestions are bound to be somewhat speculative because the only direct measurements of solar output that are available are those since 1978, from satellites outside the disturbing effects of the Earth's atmosphere. These measurements indicate a very constant solar output, changing by about 0.1% between a maximum and a minimum in the cycle of solar magnetic activity indicated by the number of sunspots.

It is known from astronomical records and from measurements of radioactive carbon in the atmosphere that solar sunspot activity has, from time to time over the past few thousand years, shown large variations. Of particular interest is the period known as the Maunder Minimum in the seventeenth century when very few sunspots were recorded.33 At the time of the IPCC TAR in 2001, studies of recent measurements of solar output correlated with other indicators of solar activity, when extrapolated to this earlier period, suggested that the Sun was a little less bright in the seventeenth century, perhaps by about 0.4% in the average solar energy incident on the Earth's surface and that this reduction in solar energy may have been a cause of the cooler period at that time known as the 'Little Ice Age'.34 More recently some of the assumptions in this work have been questioned and estimates made that over the past two centuries variations in the solar energy incident on the Earth's surface are unlikely to be greater than about 0.1% (Figure 6.15).35 This is about the same as the change in the energy regime at the Earth's surface due to two or three years' increase in greenhouse gases at the current rate.



Flux transport simulations (Wang et al 2005)

Range of cycle + background (Lean 2000)

Flux transport simulations (Wang et al 2005)

Range of cycle + background (Lean 2000)


1800 Year


Figure 6.15 Reconstructions of the total solar irradiance from 1600 to the present showing estimates (blue) of the range of the irradiance variations arising from the 11-year solar activity cycle and the period in the seventeenth century when no sunspots were recorded. The lower envelope is the reconstruction by J. Lean, in which the long-term trend was inferred from brightness changes in sun-like stars. The recent reconstruction by Y. Wang et al. (purple) is based on solar considerations alone.




to last for an exceptionally long period.32 Suggestions therefore that the current increase of greenhouse gases might delay the onset of the next ice age are unfounded.

These orbital changes only alter the distribution of incoming solar energy over the Earth's surface; the total amount of energy reaching the Earth is hardly affected by them. Of more immediate interest are suggestions that the actual energy output of the Sun might change with time. As I mentioned in Chapter 3 (see Figure 3.11) and as is described in the box, such changes, if they occur, are estimated to be much smaller than changes in the energy regime at the Earth's surface due to the increase in greenhouse gases.

There have also been suggestions of indirect mechanisms whereby effects on the Sun might influence climate on Earth. Changes in solar ultraviolet radiation will influence atmospheric ozone and hence could have some influence on climate. It has been suggested that the galactic cosmic ray flux, modified by the varying Sun's magnetic field, could influence cloudiness and hence climate. Although studies have been pursued on such connections, their influence remains speculative. So far as the last few decades are concerned, there is firm evidence from both observational and theoretical studies that none of these mechanisms could have contributed significantly to the rapid global temperature rise that has been observed.36

Another influence on climate comes from volcanic eruptions. Their effects, lasting typically a few years, are relatively short term compared with the much longer-term effects of the increase of greenhouse gases. The large volcanic eruption of Mount Pinatubo in the Philippines which occurred in June 1991 has already been mentioned (Figure 5.21). Estimates of the change in the net amount of radiation (solar and thermal) at the top of the atmosphere resulting from this eruption are of about 0.5 W m-2. This perturbation lasts for about two or three years while the major part of the dust settles out of the atmosphere; the longer-term change in radiation forcing, due to the minute particles of dust that last for longer in the stratosphere, is much smaller.


• Increase in greenhouse gases is by far the largest of the factors that can lead to climate change during the twenty-first century.

• Likely climate changes for a range of scenarios of greenhouse gas emissions have been described in terms of global average temperature and in terms of regional change of temperature and precipitation and the occurrence of extremes.

• The rate of change is likely to be larger than any the Earth has seen at any time during the past 10 000 years.

• The changes that are likely to have the greatest impact will be changes in the frequencies, intensities and locations of climate extremes, especially droughts and floods.

• Sufficient fossil fuel reserves are available to provide for continuing growth in fossil fuel emissions of carbon dioxide well into the twenty-second century. If this occurred the climate change could be very large indeed and lead to unpredictable features or 'surprises'.

The next chapter will look at the impact of such changes on sea level, on water, on food supplies and on human health. Later chapters of the book will then suggest what action might be taken to slow down and eventually to terminate the rate of change.


1 Suggest, for Figure 6.8, an appropriate temperature scale for a place you know. Define what might be meant by a very hot day and estimate the percentage increase in the probability of such days if the average temperature increases by 1, 2 and 4 °C.

2 It is stated in the text describing extremes that in convective regions, with global warming, as the updraughts become more moist the downdraughts tend to be drier. Why is this?

3 Look at the assumptions underlying the full range of SRES emission scenarios in the IPCC 2007 Report. Would you want to argue that some of the scenarios are more likely to occur than the others? Which (if any) would you designate as the most likely scenario?

4 It is sometimes suggested that northwest Europe could become colder in the future while most of the rest of the world becomes warmer. What could cause this and how likely do you think it is to occur?

5 How important do you consider it is to emphasise the possibility of 'surprises' when presenting projections of likely future climate change?

6 Estimate the effect, on the projected carbon dioxide concentrations for 2100 shown in Figure 6.2, the projected radiative forcing for 2100 shown in Table 6.1 and the projected temperatures for 2100 shown in Figure 6.4a, of assuming the climate feedback on the carbon cycle illustrated in Figure 3.5 (note: first turn the accumulated atmospheric carbon in Figure 3.5 into an atmospheric concentration).

7 From newspapers or websites look up articles purporting to say that there is no human-induced climate change or that what there is does not matter. Assess them in the light of this and other chapters in this book. Do you think any of their arguments are credible?


Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K. B., Tignor, M., Miller, H.L. (eds.) 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press.

Technical Summary (summarises basic information about future climate projections) Chapter 10 Global Climate Projections Chapter 11 Regional Climate Projections Nakicenovic, N. et al. (eds.) 2000. IPCC Special Report on Emissions Scenarios

Cambridge: Cambrige University Press. WMO/UNEP, 2007. Climate Change 2007, IPCC Synthesis Report, www.ipcc.ch McGuffie, K., Henderson-Sellers, A. 2005. A Climate Modeling Primer, third edition. New York: Wiley.

Palmer, T. Hagedorn, R. (eds.) 2006. Predictability of Weather and Climate. Cambridge:

Cambridge University Press. Schnellnhuber, H.J. et al. (eds.) 2006. Avoiding Dangerous Climate Change. Cambridge: Cambridge University Press.


1 Nakicenovic, N. et al. (eds.) 2000. Special Report on Emission Scenarios (SRES): A Special Report of the IPCC. Cambridge: Cambridge University Press.

2 For details of IS 92a see Leggett, J., Pepper, W. J., Swart, R. J. 1992. Emission scenarios for the IPCC: an update. In Houghton, J. T., Callender, B. A., Varney,

S. K. (eds.) Climate Change 1992: The Supplementary Report to the IPCC Assessments. Cambridge: Cambridge University Press, pp. 69-95. Small modifications have been made to the IS 92a scenario to take into account developments in the Montreal Protocol.

3 The +30% amounting to an addition of between 200 and 300 ppm to the carbon dioxide concentration in 2100 (see box on carbon feedbacks on page 48).

4 This summary is based on the Summary of SRES in the Summary for policymakers. In Houghton et al. (eds.), Climate Change 2001: The Scientific Basis, p. 18. Also Summary for policymakers, in Solomon et al. (eds.) Climate Change 2007: The Physical Science Basis.

5 The World Energy Council Report. 1995 Global Energy Perspectives to 2050 and Beyond. London: World Energy Council projects at 2050 global sulphur emissions that are little more than half the 1990 levels. Also see United Nations Environmental Programme. 2007. Global Environmental Outlook GEO4, Nairobi, Kenya: UNEP, Chapter 9, pp. 435-445.

6 See Figure 3.11.

7 Metz, B., David, O., Bosch, P., Dave, R., Meyer, L. (eds.) 2007. Climate Change 2007: Mitigation of Climate Change. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press, Figure 3.12, Chapter 3.

8 Detailed values listed in Ramaswamy, V. et al. 2001. Chapter 6, in Houghton et al. (eds.), Climate Change 2001: The Science Basis; also see Meehl, G.A., Stocker, T.F. et al. 2007. Chapter 10, in Solomon et al. (eds.) Climate Change 2007: The Physical Science Basis.

9 Note that because the response of global average temperature to the increase of carbon dioxide is logarithmic in the carbon dioxide concentration, the increase of global average temperature for doubling of carbon dioxide concentration is the same whatever the concentration that forms the base for the doubling, e.g. doubling from 280 ppm or from 360 ppm produces the same rise in global temperature. For a discussion of 'climate sensitivity' see Cubasch, U., Meehl, G. A. et al. 2001. Chapter 9, in Houghton et al. (eds.) Climate Change 2001: The Scientific Basis, also in Meehl, et al., Chapter 10, in Solomon et al. (eds.) Climate Change 2007: The Physical Science Basis.

10 Summary for Policy Makers, ibid., p. 9.

11 Ibid.

12 James Hansen, Bjerknes Lecture at American Geophysical Union, 17 December 2008 at www. columbia.edu/~jehl/2008/AGUBjerknes_20081217.pdf.

13 See Harvey, D. D. 1997. An introduction to simple climate models used in the IPCC Second Assessment Report. In IPCC Technical Paper 2. Geneva: IPCC.

14 The assumption that greenhouse gases may be treated as equivalent to each other is a good one for many purposes. However, because of the differences in their radiative properties, accurate modelling of their effect should treat them separately. More details of this problem are given in Gates, W. L. et al.

1992. Climate modelling, climate prediction and model validation. In Houghton et al. (eds.) Climate Change 1992: The Supplementary Report, pp. 171-5. Also see Forster, P., Ramaswamy, V. et al., Chapter 2, Section 2.9, in Solomon et al. (eds.) Climate Change 2007: The Physical Science Basis.

15 Alternatively gases other than carbon dioxide can be converted to equivalent amounts of carbon dioxide by using their Global Warming Potentials (see Chapter 3 and Table 10.2).

16 The relationship between radiative forcing R and CO2 concentration C is R = 5.3 ln(C/C0) where C0 = the pre-industrial concentration of 280 ppm.

17 See for instance, Metz et al. (eds.) Climate Change 2007: Mitigation, Chapter 3, Fig 3.12. For more information about aerosol assumptions for the twenty-first century and how aerosol forcing is treated in models see Johns, T.C. et al. 2003. Climate Dynamics, 20, 583-612.

18 Related through the Clausius-Clapeyron equation, e_1 de/dT = L/RT2, where e is the saturation vapour pressure at temperature T, L the latent heat of evaporation and R the gas constant.

19 Allen, M. R., Ingram, W. J. 2002. Nature, 419, 224-32.

20 Christensen, J.H., Hewitson, B. et al. 2007. Regional climate projections. Chapter 11, Executive Summary, in Solomon et al. (eds.) Climate Change 2007: The Physical Science Basis.

21 For more on this see Palmer, T. N. 1993. Weather, 48, 314-25; and Palmer, T. N. 1999. Journal of Climate, 12, 575-91.

22 See Figure 10.16 in Meehl, et al., Chapter 10, in Solomon et al. (eds.) Climate Change 2007: The Physical Science Basis.

23 For a review of the science of extreme events see Mitchell, J.F.B., et al, 2006, Philosophical Transactions of the Royal Society A, 364, 2117-33; also see Meehl, et al., Chapter 10 in Solomon et al. (eds.) Climate Change 2007: The Physical Science Basis.

24 For a more detailed discussion of the effect of global warming on the hydrological cycle, see Allen, M. R., Ingram, W. J. 2002.

25 Tebaldi, C. et al. 2002. Climatic Change, 79, 185-211.

26 Milly, P. C. D. et al. 2002. Increasing risk of great floods in a changing climate. Nature, 415, 514-17;

see also Meehl et al., in Solomon et al. (eds.) Climate Change 2007: The Physical Science Basis.

27 Burke, E. J., Brown, S. J. Christidis, N. 2006. Modeling the recent evolution of global drought and projections for the 21st century with the Hadley Centre climate model. Journal of Hydrometrology, 7, 1113-25.

28 Defined with relation to the Palmer Drought Severity Index.

29 Knutson, T.R., Tuleya, R.E. 2004. Journal of Climate, 17, 3477-95. See also Meehl, et al., in Solomon et al. (eds.) Climate Change 2007: The Physical Science Basis.

30 For definition of continental and regional scales see Note 28 in Chapter 5.

31 See also Table 7.4.

32 Berger, A., Loutre, M. F. 2002. Science, 297, 1287-8.

33 Studies of other stars are providing further information; see Nesme-Ribes, E. et al. 1996. Scientific American, August, 31-6.

34 Lean, J. 2000 Evolution of the Sun's spectral irradiance since the Maunder Minimum. Geophysics Research Letters, 27, 2425-8.

35 Wang Y. et al 2005. Modeling the Sun's magnetic field and irradiance since 1713, Astrophysical Journal, 625, 522-38.

36 Lockwood M., Frohlich, C. 2007. Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature. Proceedings of the Royal Society A doi:10.1098/rspa.2007.1880.

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  • Linda
    What are 4 factors that are expected to influence global warming in the future?
    3 years ago