Indo-Pak war would cause irreversible impacts --- death, soot and smoke, freezing, rain, famine, and ozone damage.

One study published two years ago looked at the likely impacts of a nuclear exchange of about 100 Hiroshima-sized detonations (15 kt yield each) on the most-populated urban areas of India and Pakistan. Each detonation was estimated to incinerate an area of 13 square km, with this scenario generating about 5 Tg (teragrams) of soot as smoke from wildfires and burning buildings entered the atmosphere.

Direct human deaths in this “limited” nuclear war scenario are not quantified in the study, but would presumably number in the tens to hundreds of millions. The planetary impacts are also severe: as the soot reaches the stratosphere it circulates globally, blocking incoming solar radiation and dropping the Earth’s surface temperature by 1.8C in the first five years.

This would be a greater cooling than caused by any recent volcanic eruption, and more than any climate perturbation for at least the last 1,000 years. Rainfall patterns are drastically altered, and total precipitation declines by about 8 percent. (These results come from widely-used climate models of the same types used to project long-term impacts of greenhouse gas emissions.)

Food exports collapse as stocks are depleted within a single year, and by year four a total of 1.3 billion people face a loss of about a fifth of their current food supply. The researchers conclude that “a regional conflict using <1 percent of the worldwide nuclear arsenal could have adverse consequences for global food security unmatched in modern history.”

A 2014 study of the same scenario (of a 100-weapon nuclear exchange between India and Pakistan) found that the soot penetrating the stratosphere would cause severe damage to the Earth’s ozone layer, increasing UV penetration by 30-80 percent over the mid-latitudes. This would cause “widespread damage to human health, agriculture, and terrestrial and aquatic ecosystems,” the researchers wrote. “The combined cooling and enhanced UV would put significant pressures on global food supplies and could trigger a global nuclear famine.”