What’s The Worst That Could Happen?

The Economist's More Intelligent Life hosts a forum on the question. Edward Carr nominates war, specifically between China and the US:

The shadow of nuclear devastation is one reason to be fearful. But even if we avoided that last, hideous step, the cost would be immense. That is partly because today’s conventional weapons are so potent, but also because China and America depend on each other in ways that Russia and America never did. The flow of goods to our shops would dry up, as globalisation failed. The financial system might collapse, because America could not borrow from China, and China would have nowhere to put its savings. Cyber-warriors might wreck communications and infrastructure. Collaboration on trade, science and action on climate change would be swept aside. Global economic depression would drag billions back into poverty.