This chapter has been considering how uncertainty concerning future climate change is weighed against the costs of the likely damage due to climate change, of adaptation and of mitigation action. Its conclusions can be summarised as follows:

• The four comprehensive IPCC assessments since 1990 have provided increasingly accurate and detailed information about the climate change occurring now and likely future change - although substantial uncertainties still remain.

• Four principles for international action have been identified: the Precautionary Principle, the Principle of Sustainable Development, the Polluter-Pays Principle and the Principle of Equity.

• In assessing costs the following items in the climate change balance sheet have been identified as follows:

(1) Due to damage from the impacts of climate change by 2050, loss in GDP in developed countries has been estimated as typically in the range 1% to 4% of GDP and of 5% to 10% or more in many developing countries.

(2) Climate change impacts that cannot be valued in money terms; for instance, those with social consequences or that affect human amenity or 'natural' capital or those with implications for national security. For instance, it is estimated that there could be over 150 million environmental refugees by 2050.

(3) The cost of adaptation to anthropogenic climate change. As climate change is beginning to be realised, planning for adaptation is urgently required that in some areas and sectors will lead to substantial cost.

(4) The costs of mitigation of anthropogenic climate change. Providing mitigation action is pursued urgently and properly planned, for reductions in emissions leading to stabilisation of atmospheric carbon dioxide concentration mitigation costs are typically less than one or two year's economic growth by 2050 - considerably less than estimates of climate change damage in (1) above.

(5) Refinements of all the above estimates and the assumptions on which they are based in the above list are urgently required.

The next chapter will consider some of the actions in more detail in the light of the principles we have enunciated in this chapter and in the context of the international FCCC.

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