By way of introduction to CESM, here's
a cross post from the "The Distributed Arctic Sea Ice Model" thread:
Why trust climate models? According to Ars Technica "
It's a matter of Simple Science"!
It's possible that shrinking sea ice in the Arctic could increase snowfall over Siberia, pushing the jet stream southward, creating summer high pressures in Europe that allow India’s monsoon rains to linger, and on it goes… It's hard to examine those connections in the real world, but it's much easier to see how things play out in a climate model.
The article explains the inner workings of climate models, with the emphasis on the Community Earth System Model. Here's one of the graphics:

Courtesy of
Kate Alexander and
Steve Easterbrook.
Those links are well worth exploring too.