I'll add another argument for not releasing scientific code, specially of a matter so polarizing as climate change: programmers aren't scientists. We aren't familiar with neither the field's literature nor the science being used. For example, the famous comment from the CRU code said:

; Plots 24 yearly maps of calibrated (PCR-infilled or not) MXD reconstructions ; of growing season temperatures. Uses “corrected” MXD – but shouldn’t usually ; plot past 1960 because these will be artificially adjusted to look closer to ; the real temperatures.

Do you expect the average programmer to take a 2 year course in climate science to truly understand what MXD means instead of instantly running around screaming "wolf!"? If you did so, be disappointed (a careful reading of Google results for "CRU code" might be enlightening).