Bill Gates has a $1 billion fund, collected from a who's who of fellow billionaires, ready to invest in startups and research to solve climate change.

On Tuesday, in his annual letter with his wife, Melinda, he said that fixing climate change involves way more than renewable electric energy, though he's encouraged by progress there.

Manufacturing and agriculture are overlooked problem areas, he said.

He's looking for climate-friendly ways to make concrete and steel, as well as ideas on dealing with methane produced by cows "when they belch and pass gas."

"I wish more people fully understood what it will take to stop climate change," Bill Gates said in his annual letter, published on Tuesday.

Each year, the Microsoft founder and his wife, Melinda, publish a letter outlining the areas that cheer, worry, and surprise them.

The topics in the letter are taken from the couple's experiences trotting the globe to combat poverty, disease, and other problems.

Bill Gates is tackling climate change through programs funded by the Bill and Melinda Gates Foundation and through Breakthrough Energy Ventures, a $1 billion investment fund he's spearheaded that's backed by a who's who of billionaires, including Jeff Bezos, Marc Benioff, Richard Branson, Reid Hoffman, Jack Ma, George Soros, Tom Steyer, Meg Whitman, and Mark Zuckerberg.

BEV is looking to invest in startups and research tackling five areas, which Gates called "grand challenges": agriculture, buildings, electricity, manufacturing, and transportation.

Bill Gates, the Microsoft cofounder and philanthropist. REUTERS/Rick Wilking In the letter, Gates said there was promising progress with renewable energy for electricity.

But he said there are two areas accounting for 21% and 24% of worldwide greenhouse gas emissions that are sorely ignored: manufacturing of building materials, and agriculture.

Making building materials like steel and cement requires a lot of fossil fuels and processes that belch out carbon, he said.

And the world is on track to double its buildings by 2060, Gates said, adding that that is like building "an entire New York City every month for 40 years."

"We need to find a way to make it all without worsening climate change," he said.

Agriculture is another gas-producing culprit. Despite the pastoral image of cows grazing on rolling green hills, Gates noted that cattle "give off methane when they belch and pass gas."

"A personal surprise for me: I never thought I'd be writing seriously about bovine flatulence," he added.

He's not advocating a ban on cattle farming, nor is he saying we should end construction or transportation — he just wants the world to focus more on solutions to all the areas hurting the climate.

"Solar panels are great, but we should be hearing about trucks, cement, and cow farts too," he said.