From Figure 3.11 it will be seen that:

• The dominant forcing for climate change over the last two centuries has been that from the increase of long-lived greenhouse gases, especially carbon dioxide.

• Since the mid twentieth century, significant offset to the positive forcing from greenhouse gases has arisen from negative forcing due to aerosols, especially from sulphates.

• Other smaller forcings are due to changes in ozone (stratospheric and tropo-spheric), stratospheric water vapour and land surface albedo (for definition see Glossary) (Figure 3.12) and persistent contrails from aircraft.

• Estimates of changes in solar irradiance are smaller than estimated in the 2001 IPCC report (see Chapter 6, page 166).

• Significant progress has been made in the understanding and estimating of indirect aerosol forcing since the 2001 IPCC report - although substantial uncertainties remain.

For the future, different assumptions about future emissions of greenhouse gases and aerosols are used to generate emission scenarios. From these scenarios estimates are made (for carbon dioxide, for instance, using a computer model of the carbon cycle) of likely increases in greenhouse gas concentrations in the future. Details of radiative forcings projected for the twenty-first century are presented in Chapter 6 (page 142). Chapters 5 and 6 will explain how estimates of radiative forcing can be incorporated into computer climate models so as to predict the climate change that is likely to occur because of human activities. However, before considering predictions of future climate change, it is helpful to gain perspective by looking at some of the climate changes that have occurred in the past.

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