The best investment is almost always in reducing uncertainty.

Lets say you think there's a 95% chance of a sea level rise, which will cost $100 billion to mitigate. It's a good investment to spend $1 billion (i.e. 1%) in research if you can increase that certainty to 99% (or decrease it 90%).

Don't forget the time-value of money either. Say you can buy land in Antarctica for $1/acre today, and it will be worth $10,000/acre in 200 years when that 1 km of ice melts off. Is that a good investment?

$1 invested at 5% per year = $17,293 after 200 years. Of course, none of us will be around to enjoy it, but this is all about our children, and our children's children etc. right?

It is heartening to see that, while so many of us can't be bothered to save for our own retirement, we still want to spend so much money on problems that will only affect the next generations. I guess we're not as selfish as we think we are.