How long would the southern hemisphere survive after a nuclear war that, while intense, was limited to the northern hemisphere?

Question

I remember hearing/reading something somewhere a long time ago about how the weather patterns on earth mostly cycle away between the equator and the poles, and that this means if there was a northern hemisphere nuclear war (US/Europe/Russia/EU/China/India/Pakistan)the radioactive isotopes would mostly remain in the northern hemisphere, for quite a long time. This could mean the southern hemisphere is either spared entirely, or has a grace period, before it too becomes uninhabitable.

I'm working on something set in the second scenario, based around the idea that the survivors would be building space station arcs, and vying for inclusion. Assuming the violence was enough to effectively eliminate the entire northern hemisphere, how long could the southern hemisphere remain viable?

UPDATE: I figured some more specificity would help so lets say that atleast 50% of the worlds nuclear arsenal was fired, but this was exclusively in the northern hemisphere, perhaps not even near the equator in case that makes a difference, the lowest latitude hit could could be Mumbai.

Answer 1

OK, let's start with what we expect to happen:

• A nuclear war in the north causes the expected nuclear winter, spreading soot into the upper troposphere, blocking the sun, and seriously reducing surface temperature.

• The upper troposphere has powerful winds, but they're strongly temperature based. This means that the hot air of the equator is falling at the poles (not to ground level, but in the troposphere).

On the surface you would think this would preserve the southern hemisphere, and it likely would, but not for long.

As the north cooled (and it would cool very quickly thanks to the nuclear winter), the temperature gradient would begin to move south. Let's simplify today's behavior by saying the equatorial temperature is 100° and the poles are -40°. That's a gradient of 140° over a 90° arc of the earth.

But that winter starts to close the arc. As it does, the equator begins to cool (even if the winter is, in the beginning, all in the north). Fairly quickly you'd have something closer to a 15° arc with the "centerline" of the equator being pushed to cooler southern latitudes. Eventually the "equatorial" temperature begins to revolve around the Tropic of Capricorn — and the collapse is very quick after that.

The issue is that the change in temperature gradient will change the wind patterns. The cooling will also change the ocean current patterns. Between the two, southern hemisphere patterns will begin to change.

How long would this take? A meteorologist could give you a more accurate answer. You should review articles about volcanic ash affecting global weather to see if that study can tell you how much the patterns change, and a study of the projections of what happens when the temperature of the North Atlantic Current changes, but my suspicion is that it would take no more than a few years.

Finally, in a moment of prognostication, I can imagine the tropic of capricorn getting really hot before it began to cool down because energy must go somewhere — and it's not heading north. My prediction is that in the event you describe, Australia becomes a dustbowl before becoming a sheet of ice.

Answer 2

Nevil Shute explored this question in his novel On the Beach. The way it played out in the book, the Southern Hemisphere didn't even last a year, because of the the seasonal movement of the Inter Tropical Convergence Zone (ICTZ), which is the basically the barrier between the airmasses of the Northern and Southern Hemisphere. In the Southern Hemisphere summer, with the ICTZ to the south, air could come down to the tropics from the Northern Hemisphere and deposit radioactive particles on the ground. Then in the Northern Hemisphere summer, the ICTZ moves far to the north (Himalayas and Taiwan), so air from the Southern Hemisphere can move north over the now irradiated tropics.