Why has there been controversy about what the climate scientists know and project?

The earth's climate system is exceedingly complex. Determining global temperature and sea level is not a simple matter. Determining how one factor affects others is more difficult and subject to differing standards of evidence and interpretation.

Predicting what will happen in the future is even more complex and basically not possible. This is partly because predictable changes will have unpredictable effects. For example, higher air temperatures will lead to more clouds, but it is not possible to know in advance if more clouds, which both reflect light and trap heat, will hasten or slow the warming trend. The biggest uncertainty is not knowing about consequences of future human activity.

The International Panel on Climate Change (IPCC) makes its projections in numerical ranges because areas of known uncertainties make more specific predictions unwarranted. A number of large corporations have opposed the conclusions of the IPCC as “unscientific,” and have helped fund and publicize the work of a very few scientists who challenge its findings. News reports that present "both sides" make it seem as though there is much more disagreement among climate scientists than really exists.