the ease with which humankind can bring about a nuclear winter?
Well an india/pakistan war would be catastrophic, but lets think beyond the estimated 10-15 million dead from each country. India has about 90 warheads, pakistan about 120 warheads, so you're a little behind there. India has only LR bomber capabilities, just 1 of the 3 components of a nuclear triad deterence of SLBM, ICBM, and LR Bomber... this just wont work for indias NoFirstUse policy, because you simply dont have second strike capability!
If a limited regional nuclear volley were to happen between india/pakistan with say a combined 50 of your Hiroshima-sized bombs (15kT) as airbursts on dense urban areas, it would set off an small nuclear winter comparable to the little ice age, this exchange would bring approximately 5 Tg (1 Tg = 1012 grams) of black carbon (BC) particulate aka soot into the atmosphere. For comparison without a banana handy, the eruption of Eyjafjallajökull in iceland in 2010 produced about 0.5Tg of BC.
About 20% of the BC would fall with precipitation rather quickly as 'fallout' as the wind blows, but the remaining bit would linger for most of a decade in the mid to high stratosphere. If these were ground blasts, the air particulates would be orders of magnitude more severe.
regardless, a small exchange like this you could expect 30 million dead, a large ozone hole, unusable rice plantations, 30% +UV in summers, cooling of 3-5°C over large areas of North America and Eurasia. It would halve the worlds wheat/corn crops from shortening the growing season by a month... and these areas export to well everybody. india's rice lands wouldnt be usable for ten's of years.
we know most of todays icbm/slbm rockets have far greater than the 15 kT payload from the previous example, for example americas UGM-133 Trident II SLBM can carry up to 12 W88 warheads @ 475kT each using MIRV tech so that it splits midair like a shotgun bird shot made up of thermonuke pellets, these 12 reentry vehicles/RVs are quasi powered and considered a 4th stage of the rocket. The explosions are set to happen at the optimal height to maximize the ~10psi radius of over pressure. While a 5psi wave is sufficent to blow the walls off an apartment building, the 10psi is usually the gauge for reinforced structures like military and hospital destruction. A single hypothetical 475kt blast optimized for 10psi overpressure would occur at 5,600 ft-msl with a ground diameter of about 5 miles. with the MIRV, you could set the Trident II's 12 RV's to give a triangular grid airburst spaced at 5 miles, sort of like a bowling pin arrangement covering a 30 mile diameter of nothingness. Each of the 14 Ohio CLass submarine can carry 24 Trident II SLMB. Heres a map to show what a single W88 will do, http://nuclearsecrecy.com/nukemap/, remember to imagine a 12x pattern of this optimized at a 10psi spread.
There is also americas LGM30 Minuteman III in service since 1970, it can carry up to 3 separate W78 or W87 Warheads @ 350kt with MIRV tech, but these are 'old' and largely for patriotic show. However, Americas strategic command just has launched one yesterday 9-23-14 from North Dakota to the marshall islands without a payload. With a range of 13,000 km they can hit almost anywhere except small portions of the pacific (earth circumfrence = 40,000km), but the silos are spread out past their range limitations throughout americas heartland.
in total, america is estimated to have 1.2 gT at the ready., with 1,250 submarine warheads, 500 missile silo, 500 LR bomber ready to pull trigger in 5 minutes. theres also another 2500 in storage and 2700 retired but intact.
in total, America is estimated to have 1,200 mT (1.2 gT) at the ready. most are slbm in submarines spread around the globe, but 500 icbm, several thousand in various stages of disarmerment that can be reversed, about 2,500 in storage, 200 hidden in europe,
The newest models at NASA Goddard Institute for Space Studies ModelE, is able to represent the atmosphere up to 80 km, and simulates plume rise to the middle and upper stratosphere, producing a long aerosol lifetime. They use these to simulate large volcanic super eruptions like the eruption of Eyjafjallajökull in iceland in 2010, this produced about 0.5Tg of BC.
http://onlinelibrary.wiley.com/doi/10.1029/2006JD008235/abstract
http://www.pecad.fas.usda.gov/cropexplorer/global_reservoir/
