The authors of IPCC WG2 AR5 have now been announced.
The selection process is something as follows.
As a first step, the IPCC member states nominate people, and indicate their role (convening lead author, lead author, review editor). Every member state does this differently. I know of one case in which somebody was nominated by his government, without him knowing. I know of one case in which someone was nominated by a junior civil servant, against the will of more senior civil servants. I know of one case in which someone agreed to be be nominated, but declined the invitation by the IPCC.
As a second step, the IPCC Technical Support Unit (TSU) and WG Bureau select the Convening Lead Authors (CLA). For some chapters, there is a choice, but for other chapters there is one candidate only -- or none.
As a third step, the CLAs select a team of Lead Authors from the nominations (having access to summary and full CVs), and submit the list to the TSU.
As a fourth step, the TSU and WG Bureau reconcile the conflicts between the lists of authors. Strange things happen in this phase. Apparently, a number of people were nominated for multiple roles -- and the good ones were selected for multiple roles. This makes a mockery of the notions that the IPCC has a clear outline and that IPCC authors are experts in their fields. Other candidates were excluded because there were too many people from country X -- even if country X has a long tradition in climate research and therefore a large pool of experts, and even if there is no expert from another country as a substitute. Finally, some people were selected as a lead author without being nominated for that chapter.
As a fifth step, TSU and CLA agree on the list, adding some people and removing others.
Because the nomination and selection process is about people, it cannot be fully open and transparent. I think that the current process can be improved, however. Particularly, it strikes me that there should be a strict limit on the number of chapters for which someone can be nominated. The structure of the reports should be much more crisp, so that people would clearly recognize where they can best contribute. While there should be a soft limit on the number of participants from a particular country, it cannot be that there are gaps of expertise because of such constraints.