In the latest edition of their annual letter published today, Bill and Melinda Gates argue that the world needs “an energy miracle,” and are willing to bet that such a breakthrough will arrive within 15 years.

Bill Gates cites scientists’ estimates that to avoid the worst effects of climate change the biggest carbon-emitting countries must reduce greenhouse gas emissions by 80% by 2050, and the world must more or less stop such emissions entirely by 2100. And that’s not going to happen if we continue on our current trajectory.

Gates says he was stunned to discover how little research and development money is going toward breakthroughs in cheaper, scaleable clean-energy sources. Gates announced last year that he was committing $1 billion of his own money over five years to invest in clean-energy technology, and has been pushing governments to increase their funding.

To explain the need for a breakthrough in energy technology, he uses an equation (similar to the Kaya identity equation) that represents the factors determining how much carbon dioxide the world emits every year.

You can see Gates explain the equation in the Quartz video above.

Gates believes that cleaner options such as electric cars and LED lighting won’t bring down energy consumption enough to hit those climate-change goals. In fact, he doesn’t see any current clean-energy technology that will enable the world to eliminate carbon dioxide emissions by 2100, partly because it’s not consistent or inexpensive enough.

Gates has personally invested in next-generation nuclear power technology, which he describes as “a very promising path.” He is also backing efforts to improve battery technology, so that energy from intermittent clean sources such as solar and wind can be affordably stored at large scale for use over time. “I think we need to pursue many different paths,” says Gates in an interview with Quartz.

And he’s betting on relatively fast progress. “Within the next 15 years,” Gates predicts in his letter, “I expect the world will discover a clean-energy breakthrough that will save our planet and power our world.”