(FORTUNE Magazine) – If you've read the news lately, you know the scale of the problem. Catastrophes that once seemed far away are creeping uncomfortably close to our lifetimes. The permanent polar ice cap will disappear by 2040. The seas could be practically devoid of fish by 2048. Manhattan and Miami will be flooded by 2050. Add in widespread disease and famine, and you have a script for the apocalypse. But before you get too depressed, consider that business—until now part of the problem—is scrambling for answers. Clean-technology investments soared by more than 50% in the first three quarters of 2006. And venture capital giant Kleiner Perkins Caufield & Byers has announced a doubling of its renewable-energy fund, to $200 million. Kleiner partner Ray Lane told the Wall Street Journal that clean tech will be "bigger than the Internet, by an order of magnitude."

For the following stories, we identified the most intractable problems facing the human race. Beyond climate change, there are the pollution troubles: mountains of trash, haze-choked skies, and dirty water. Disease includes not just viral epidemics but also new strains of ultra-resistant bacteria. And our global food problem isn't just about Third World famine; it's also about conditions that could wipe out the $158 billion fishing industry.

It made for a disquieting list—until we found companies developing workable, scalable solutions. For each, we teased out not just the size of the potential windfall but also entrepreneurial insights from the pioneers. And we found lots of technologies on the horizon, still too new to be commercialized, that could emerge in just a few years . Our most disastrous century yet? Maybe. It could also be our finest hour.

THE BIGGEST PROBLEMS

1. GLOBAL WARMING

2. HUNGER AND MALNUTRITION

3. WASTE DISPOSAL

4. DIRTY WATER

5. DIRTY AIR

6. EPIDEMICS

7. OVERFISHING