The next wave of successful start-ups will be those that help make the transition to a carbon-free energy affordable, said BlackRock CEO Larry Fink in his most recent highly anticipated annual letter.

"The next 1,000 unicorns won't be search engines or social media companies, they'll be sustainable, scalable innovators – startups that help the world decarbonize and make the energy transition affordable for all consumers," Fink wrote, using the tech-industry term "unicorns" to refer to start-ups valued at $1 billion or more.

Currently, clean energy solutions tend to be more expensive than the greenhouse-gas emitting incumbents, and that's a tough problem, Fink said.

While Fink is clear that the global economy must and will shift to a decarbonized future, he doesn't believe it's going to be a fast shift.