• Broadly speaking, the world's three richest tech billionaires are all trying to develop new technologies that can reduce the world's carbon dioxide emissions.
• It's widely known that trees are among the most efficient carbon-capture machines on earth.
• Trees and reforestation, however, are relatively low down on the tech billionaire agenda list.

Climate change appears to be high on the agenda for tech billionaires like Elon Musk, Jeff Bezos and Bill Gates but some are questioning whether they're focusing their efforts on the right areas.

Broadly speaking, the three richest tech billionaires — who rank in the top five richest people on the planet — are all trying to develop new technologies that can reduce the world's carbon dioxide emissions.

Musk is largely focused on funding carbon capture technologies, Gates is particularly bullish on nuclear energy, and Bezos has created a dedicated "Bezos Earth fund." All of them believe that technology has a major role to play in tackling climate change and they're doing their utmost to ensure they're pushing the boundaries when it comes to climate tech.

"They basically think in the 'Iron Man way,' which is that we can build the technology to innovate ourselves out of it," Christian Kroll, the founder and CEO of search engine Ecosia, told CNBC on a video call, adding that they should be focusing more on planting trees.

"No technology will ever get there," he said in reference to trees. "And on top of that, you're getting so many things for free. You're getting fertile soil, you're doing something against the biodiversity crisis, and you're helping the water cycle so you have less droughts and less floods."

Global carbon dioxide emissions have soared over the last 100 years, leading to unprecedented global warming and climate change.

It's widely known that trees are among the most efficient carbon-capture machines on earth. They remove carbon dioxide from the atmosphere through a chemical reaction known as photosynthesis where they turn the gas into energy that they use to grow. Empress trees, for example, can absorb about 103 tons of carbon a year per acre.

Twelve of the top 20 climate solutions relate to either agriculture or forests, according to climate non-profit Project Drawdown, which is based in San Francisco.

Last week, Britain's Prince William underscored the importance of investing in nature to tackle climate change and protect our planet.

"We must invest in nature through reforestation, sustainable agriculture, and supporting healthy oceans, because doing so is one of the most cost effective and impactful ways of tackling climate change," he said.

"It removes carbon from the atmosphere, helps build more resilient communities, tackles biodiversity loss, and protects people's livelihoods. This is crucial if our children and grandchildren are to live sustainably on our precious planet."

Jack Kelly, the founder of Open Climate Fix and a former researcher at Alphabet-owned AI lab DeepMind, told CNBC that a mix of approaches is required. "I think we need a wide range of interventions, both tech and reforestation," he said. Open Climate Fix announced Tuesday that it has raised over £500,000 ($689,000) from Google.

Dave Waltham, a professor in the Department of Earth Sciences at Royal Holloway, University of London, told CNBC that "natural climate solutions" like tree planting can be viewed as "emergency first aid."

"They buy us time to develop permanent solutions," he said. "New forests, for example, absorb CO2 for 40 years or so and then reach an equilibrium. Buying time this way is immensely valuable as we still cannot produce completely climate-neutral food, steel, energy, and concrete."

Trees and reforestation, however, are relatively low down on the tech billionaire agenda list, according to Kroll.

While the tech billionaires wouldn't necessarily be able to "solve" climate change by planting more trees, they could have a "massive impact" if they dedicated more of their capital to the matter, he said.

According to the Bloomberg Billionaire Index, Amazon founder Bezos is worth $197 billion, Tesla founder Musk is worth $181 billion, and Microsoft founder Gates is worth $145 billion.

Representatives for Musk and Gates did not immediately respond to CNBC's request for comment, while a representative for Bezos declined to comment.

Forests or fusion?

There's no denying that tech billionaires are becoming increasingly interested in climate change.

In January, Tesla CEO Musk pledged to invest $100 million in new carbon capture technologies. Carbon capture is the process of trapping waste carbon dioxide either directly from the air, or just before it gets emitted from factories and power plants.

His investment in new carbon capture technologies dwarfs the $1 million he spent on trees in 2019 when he gave YouTuber Jimmy "MrBeast" Donaldson a donation to help him reach a $20 million tree planting target.  

Musk's stance on climate change is complicated, however. While he runs a relatively green electric vehicle company, he has also been criticized for his love of bitcoin, which is now one of the world's biggest CO2 emitters. 

Meanwhile, Gates thinks nuclear energy is the future and his TerraPower company, which he founded in 2008, is aiming to build a fully functional advanced nuclear reactor.

In his new book "How to avoid a climate disaster," Gates doesn't seem to be convinced that trees are worth investing in.

"It has obvious appeal for those of us who love trees, but it opens up a very complicated subject ... its effect on climate change appears to be overblown," he writes.

Gates argues that the most effective reforestation strategy is to stop cutting down so many of the trees we already have and says that "you'd need somewhere around 50 acres' worth of trees planted in tropical areas to absorb the emissions produced by an average American in their lifetime."

The Microsoft mogul clarified his stance on trees in a podcast interview with New York Times journalist Kara Swisher in February.

"If you're going to fund for 10,000 years constantly replanting it, then that's a legitimate offset," said Gates. "If you're just planting one generation of trees, it doesn't get you much. You know, I'm not saying it's a mistake or anything. But that will not make a significant dent in this problem."

Gates, who is now the largest owner of farmland in the U.S., added: "The idea that there's a place to plant a trillion trees, that's just wrong."

Elsewhere, Bezos created the $10 billion Bezos Earth Fund last February to provide financial support to scientists, non-governmental organizations, activists and the private sector.

So far, the Bezos Earth Fund has issued grants to several organizations that focus on reforestation including Eden Reforestation Projects, The Nature Conservancy, and The Natural Resources Defense Council.

Amazon, however, has been criticized for increasing pollution with its planes and vans, and for using excessive amounts of cardboard when packaging its products. Amazon says that its packaging is 100% recyclable and that it doesn't use plastic clamshells and wire ties.

Amazon Web Services, the company's cloud computing behemoth, and Microsoft also operate energy intensive data centers around the world.

Turning profits in plants

But Kroll thinks the tech billionaires are still relatively "obsessed" with dreaming up new technologies to take on the problem. 

His company, Ecosia, has made tree planting a major part of its identity.

Headquartered in Berlin, Ecosia donates 80% of its profits to charities that focus on reforestation. Essentially, if a person goes on the Ecosia search engine and performs a search, almost all of the money that the company makes from digital ads will be used to plant trees.

The company has partnered with over 60 tree planting organizations who have planted over 123 million trees, Kroll said, adding that they're mostly in developing countries in the tropics.

"Through our tree planting, each search is removing around 1kg of CO2 from the atmosphere," said Kroll. "I'm doing dozens of searches every day so thousands of searches every year. That's a few tons of CO2 removed from the atmosphere just by searching."

Kroll suggested that people should only be classed as billionaires when they remove a billion tons of CO2 from the atmosphere.

"All the others are just dollar billionaires," he said. "That's boring. We don't need that in a 21st century anymore."