Additional info from the Union of Concerned Scientists
The most important goal of the Intergovernmental Panel on Climate Change (IPCC) is to synthesize the state of climate change science in a way that adheres to the highest standards of scientific credibility. The IPCC aims for consensus among its members about its key findings as well as those areas that require further research. There are two instances in which consensus is being sought.
Scientific ConsensusThe first type of consensus is sought among the scientists writing the report. In this case, consensus does not mean that everyone agrees with every single aspect of the report — a clearly unrealistic aim. It is well understood that science is not something to be decided by voting, but by logical reasoning. The core, then, of scientific consensus among IPCC scientists is that they agree that the report is of the highest scientific integrity and reflects the state of knowledge fairly and adequately. Points of dispute in the science of climate change are usually resolved either by developing appropriate intervals of uncertainty around certain projections or by crafting language that reflects the different viewpoints of experts within the scientific community and the reasons that the differences exist.
Governmental ConsensusThe second type of consensus is sought among the governmental representatives who need to sign off on the report's Summaries for Policymakers (SPM). In the final stages of preparing the assessment report, these governmental representatives meet in plenary and seek unanimity on the exact wording of the SPM. The scientific experts serving as lead authors of the underlying technical report are at hand to ensure scientific integrity and to resolve questions. It is particularly impressive that full consensus has been achieved in the final "sign-off" of all IPCC documents. Such consensus is not required — countries are allowed to formally register their dissents; in all cases to date, however, unanimous agreement has been reached.
Adapted from © Union of Concerned Scientists