While there is an extremely high degree of certainty(95%)that the atmo sphere has been warming primarily due to human activities, there is also a high degree of uncertainty in climate model results about projected future changes in the average atmospheric temperature. This is indicated by the wide range of projected temperature changes in Figure 15. C and the range of the proected rise in sea level(see Figure 16,p. S48, in Supplement 5). Climate experts are working to reduce such uncerainty by learning more about how the earth's climate system works and by improving climate data and models. Despite their limitations, these models are the best and only tools that we have for projecting likely average atmospheric temperatures in coming decades.