There is a tremendous amount of uncertainty concerning the future of human-induced climate change. No serious scientist doubts the reality of anthropogenic climate change, but the magnitude of the climate response is the subject of serious study and predictions vary widely.
The purpose of this page is give some idea of the magnitude of the variation in predictions. For example, the above plot (courtesy of IPCC web-based publications, see Note) shows the variation in the predicted amount of CO2 that is sequestered (that is, stored) for a number of the climate models used by the IPCC for EXACTLY the same assumptions with regard to population increase, land-use changes, emissions, and so forth. The predictions vary from a positive uptake (CO2 stored) to atmospheric CO2 actually increasing due to some positive feedback effect.
The figure below is an even more dramatic example of future uncertainties. The IPCC has published the Special Report on Emissions Scenarios (usually referred to as the SRES). This report develops a large number of possible scenarios of future emissions of green house gases (GHG). By far the most important human-produced GHG is carbon dioxide (CO2). The variation in possible CO2 emissions published by the IPCC is tremendous. The IPCC has not selected any particular subset of these scenarios as being representative.
[This figure is taken from the SRES, Excutive Summary.]
Because of the large varation in possible emissions, attempts to build highly complex models of biotic systems and their response to global climate change is a largely futile exercise. This should NOT be taken as concluding that coral reefs, for example, will not be heavily impacted by climate change. Almost any of these future scenarios will result in significant declines in coral reefs already heavily impacted by human effects.
NOTE: The Intergovernmental Panel on Climate Change (IPCC) is a scientific intergovernmental body set up by the World Meteorological Organization (WMO) and by the United Nations Environment Programme (UNEP).