Subtly and Creatively Increasing Existential Risks

The World Series of World Wars could be broadcast like an international event and have exciting opening ceremonies. A few hundred weapons on cities which are targeted would be enough to cause smoke that would reduce the sunlight reaching the earth's surface by about 70%, killing all the harvests worldwide. I'm talking now about a war between the US and Russia where thousands of weapons would be involved. Nearly everyone would starve to death except for a small fraction, perhaps 1%.