If you're referring to investment hedging, then you should diversify into things that would profit if expected event hit. For example alternative energy sources would benefit greatly from increased evidence of global warming, or the onset of peak oil.
Preparing for calamities that would render the stock market inaccessible, the answer is quite different. Simply own more of things that people would want than you need. A list of possibilities would include:
- Non perishable food (canned, dried, or otherwise)
- Weapons and ammunition
- Medical supplies
- Hand tools
Precious metals are also a way to secure value outside the financial markets, but would not be readily sellable until the immediate calamity had passed.
All this should be balanced on an honest evaluation of the risks, including the risk of nothing happening. I've heard of people not saving for retirement because they don't expect the financial markets to be available then, but that's not a risk I'm willing to take.