Investors led by Bill Gates and Kleiner Perkins have poured more than $500 million into carbon-capture tech. Meet the 5 buzziest startups that are turning harmful emissions into fuel and other products.

• To limit global warming to 1.5 degrees and avoid some of the most catastrophic effects of climate change, experts say we have to go beyond reducing emissions and start pulling carbon dioxide directly out of the air.
• A growing number of startups promise to do that, and they're backed by major VCs including Bill Gates, Khosla Ventures, and Kleiner Perkins, in addition to oil and gas giants like Chevron and ExxonMobil.
• Companies like Carbon Engineering suck carbon dioxide (CO2) out of the air and turn it into fuel. Others inject the greenhouse gas into concrete, saving concrete producers money and storing CO2 indefinitely.
• Here are the startups garnering the most attention, based on interviews with more than half a dozen investors.
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Here's the bad news: We've spewed so much carbon dioxide (CO2) into the air that to limit planetary warming to 1.5 degrees and prevent some of the worse impacts of climate change, we can no longer just limit emissions.

Climate scientists say we need to suck greenhouse gases out of the atmosphere, too.

The good news is that CO2 isn't just a greenhouse gas. It also has plenty of beneficial uses, like making bubbly sodas, low-emissions fuels, and even helping to strengthen concrete. Once you figure out how to extract it from the air (or from a power plant), you have a near-endless supply.

"You can either capture CO2 from a waste stream or use direct air capture technologies and then create a value-add product, whether it's a synthetic fuel, which you can use as jet fuel, or maybe a new polymer," Louis Brasington, an analyst at the research firm Cleantech Group, told Business Insider.

That's exactly what a trove of new startups is trying to do. Companies like Carbon Engineering and CarbonCure are betting they can turn CO2 into value - by turning it into fuel or injecting it into concrete.

They're pursuing this technology even absent of a high price on emissions, though these businesses could become even more lucrative if more countries place taxes or limits on emissions.

Some of the world's top clean-energy investors are betting on the tech, too. They include Bill Gates, Kleiner Perkins, and Khosla Ventures, in addition to oil and gas giants like BP, Chevron, and ExxonMobil. In the last two years, they've poured half a billion dollars into carbon capture, storage, and utilization startups, according to PitchBook.

"If we're going to hit a 2-degree target, and look at the pathways we're on, direct-air capture is very likely going to have to be part of the equation," said Jim Cabot, a managing director at Breakthrough Energy Ventures, a fund spearheaded by Bill Gates. "That would be an area where I expect you're going to see a lot more action in the coming years."

Here are the top startups where that action is taking place, according to half a dozen investors in the energy industry. They're listed by how much funding they've raised from least to most.