Sagan was political. His nuclear winter theory was almost entirely bullshit cobbled together for a political movement. While his peers did rip his theory to shreds from a scientific standpoint, they clearly didn't grasp that all of them are clueless from a policy standpoint. They simply don't understand how nuclear war works at even an elementary level, and so Sagan got a pass on inventing a strategy to make his model work.
The theory lives on to this day via the flawed works of several other scientists, who are creeping back into some of the scientific flaws, and still have no understanding of the strategy, and so they simply make up their own...which happens to benefit their alarmist models.
Sorry, but this was the topic that destroyed any image I had of scientists being competent and unbiased. Seriously, go read a few books about nuclear weapons and strategy, then read the scientific papers, and then the rebuttals from colleagues and independent sources. The ignorance and hubris will blow your mind.