This forum seems to be sorely lacking in nuclear war-related threads, so I'm going to start one.
How much of humanity could survive a nuclear conflict? I've been doing some research, and scientists and military strategists don't seem to have come to an agreement. Depending on the size of the conflict, blast effects, gamma rays, radioactive isotopes, and radioactive dust could kill billions of people and cause first-world society to collapse.
However, "nuclear winter" may be the most dangerous effect of nuclear war. Recent research from scientists Alan Robock and Owen Toon suggests that even a regional nuclear war, such as one between India and Pakistan, could wreak havoc worldwide.
According to this research, the explosions would stoke massive firestorms, pumping 5 million tons of smoke into the atmosphere. The soot would absorb much sunlight over the entire world. Average global temperatures would drop by about 1.25*C, crops would fail, and famine and starvation would ensue. The climate might not recover for a decade or more.
All in all, according to Toon and Robock's January 2010 Scientific American article,
"1 billion people worldwide with marginal food supplies today could die of starvation because of ensuing agricultural collapse." (emphasis in original)
To support their theory, they cite analogies from volcanic eruptions such as 1815's Tambora eruption, which resulted in the "Year without a Summer." Freak frosts caused crop failures all over the world. Imagine what would happen if summer did not occur for 10 years.
This research, published in Science magazine (link) and other reputable scientific journals, does not seem to have been challenged. On the other hand, interest in nuclear war is not what it was during the Cold War, when the most research was done on this topic. Cynics will sense a political motive behind Robock & Toon's research, since they are using it to promote total nuclear disarmament.
EDIT: I originally wrote that the nuclear exchange would purportedly cause "....a shroud of 5 million tons of dust and smoke." However, the research by Robock et al focuses almost exclusively on smoke effects. Nuclear war would purportedly create massive firestorms in megacities such as the population centers of India and Pakistan. It is the smoke from these fires that would cause nuclear winter effects.