As discussed in #269, the processing of CMIP3 datasets works only if `max_parallel_tasks = 1` is set in `config-user.yml`. This could be a serious limitation, e.g. for the ipcc recipe developed [here](https://github.com/ESMValGroup/ESMValTool/pull/1384).
