When interfacing with C languages, that can be a big penalty (depending on the complexity of the algorithm).
It will probably take two generations of developers to make this happen, but well worth it. When I was studying atmospheric science I tried to understand OpenWRF (weather research forecasting) and the barrier was too high... And I had programming experience, imagine those with none!
An open source project is only as useful as it is accessible to it's average developer base.
The reason is that for every hyper optimized climate model there are a thousand small or medium models. For these Julia offers sufficient performance and excellent developer experience.