India and Pakistan both have nuclear weapons. If a conflict between them escalates, then even a limited nuclear exchange would lead to tens of millions of casualties, mass starvation, widespread electronic outages, and releasing millions of tons of black smoke into the atmosphere; crop yields worldwide would be severely reduced.
There have been a few studies on a nuclear war between India and Pakistan, although as you'd expect, they're mostly from antinuclear advocacy groups. There are many unknown factors and a wide range of estimates, so I'd take all numbers with a grain of salt.
https://pubmed.ncbi.nlm.nih.gov/31616796/
https://psr.org/wp-content/uploads/2018/04/two-billion-at-ri...
There's also an active National Academies study on the environmental effects of nuclear war (https://www.nationalacademies.org/our-work/independent-study...), although they haven't released anything yet.