Figuring out what influences climate, how it has changed in the past, whether it's changing now, and how it might affect our future has required the expertise of thousands of scientists and other experts from dozens of fields who were brought together in one body: the IPCC.

Since it was formed in 1988, the IPCC's purpose has been to bring together the experts who can provide decision makers and others with an objective source of information. It does not, however, conduct any research itself. The IPCC's Assessment Reports (issued in 1990, 1995, 2001, and 2007) also don't tell us what to do. They discuss options, but leave prescribing policies for dealing with climate change up to individual nations.

But who are the experts involved in the IPCC and what do they do? These specialists (more than 2,000 of them) represent about 140 countries, a variety of fields, and a range of views. Their function is to assess the latest peer-reviewed literature and to achieve consensus about where the weight of the evidence points and where uncertainties lie.

Remember that this means they are looking at papers that have already been reviewed in depth by other experts before the articles were published in a scientific journal. For the 2007 Report, they examined 29,000 data sets. The assessment also included computer modeling to determine the cause of climate change and to predict future impacts.

Levels of Confidence
Terminology	Degree of confidence in being correct
Very high confidence	At least 9 of 10 chances of being correct
High confidence	About 8 of 10 chances
Medium confidence	About 5 of 10 chances
Low confidence	About 2 out of 10 chances
Very low confidence	Less than 1 out of 10 chance

Likelihood Scale
Terminology	Likelihood of occurrence/outcome
Virtually certain	>99% probability of occurrence
Very likely	>90% probability
Likely	>66% probability
About as likely as not	33-66% probability
Unlikely	<33% probability
Very unlikely	<10% probability
Exceptionally unlikely	<1% probability

The IPCC was very careful in its use of language. Phrases such as "likely" and "low confidence" have very specific meanings. For example,

Similarly, expressions of the likelihood of something occurring are also well-defined:

So when the IPCC states, "Changes in snow, ice and frozen ground have with high confidence increased the number and size of glacial lakes," the phrases mean that the scientists believe there is an 8 out of 10 chance the statement is correct.