The range of projections of the future change in global-mean surface air temperature and sea level. The central line indicates the most likely estimate and the shaded area shows the range of possibilities. Based on the 1992 IPCC assessment. Turning to the future, the projections of climate change for the 21st century are uncertain because of limited understanding of the manner in which the climate of the Earth will respond to the change in the composition of the atmosphere. Furthermore, determining the ultimate impact of global warming on humanity is not an easy matter.
Forecasts are necessarily speculative.
They rest on uncertain projections of changes in regional climate, highly simplified computer models of basic physical, biological and social processes, experience of past impacts and conjecture. Where there are threats of serious or irreversible damage, lack of full scientific certainty should not be used as a reason for postponing cost-effective measures to prevent such degradation. The international community has advised that, at this time, precautionary action should be taken in response to global warming. It would not be wise to delay action. On the other hand, there are many scientific uncertainties and it would be unwise to devote considerable resources to combating a threat which may not be as severe as currently envisaged.