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particularly difficult to estimate because of the limited understanding of how aerosols affect cloud properties, which affect the transfer of radiation through the atmosphere. In addition to changes in atmospheric composition, land use changes can be a significant factor in causing climate change at the earth's surface.

Despite the many unresolved issues touched on in this chapter and discussed in more detail in chapters 5–9, the progress that has been achieved over the past few years provides a basis for drawing some tentative conclusions concerning the nature of the observed differences between surface and upper air temperature trends, and their implications for the detection and attribution of global climate change.break



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