The next big billion-dollar cannabis markets investors are rushing to

The accelerating pace of marijuana legalization in the United States has certainly grabbed headlines around the world in the past few months. Four states (and the District of Columbia) already allow the use of recreational marijuana. And next month five more will vote on similar legalization — California, Nevada, Maine, Massachusetts and Arizona. Meanwhile, medical marijuana has been given the green light in half of all U.S. states.

Legal cannabis sales in the United States jumped 17 percent, to $5.4 billion, in 2015 and are expected to grow by 25 percent this year, to $6.7 billion, according to Arcview Market Research. By 2020 legal cannabis sales in the United States are projected to hit $21.8 billion.

But America's hardly the only place rethinking its stance on cannabis. As states rewrite the rules on the legality of the drug, other parts of the world are making their own changes.

Globally, $150 billion was spent last year on marijuana, said Scott L. Greiper, president of Viridian Capital Advisors. Almost all of that was done illegally. But as legalization efforts grow, angel and venture capital investors are getting interested in companies specializing in weed.

The Arcview Investor Network, for instance, has put $84 million into 130 companies in the cannabis sector since 2013, with most of those investments coming in the past 1.5 years. And a growing number of those investors are looking beyond U.S. borders. There are a number of countries where there are exciting opportunities, say experts in this sector.

Of all of those, there's none more intriguing to investors than Canada. The Great White North was the first country to legalize medical marijuana on a federal level in 2001, and it's widely expected to legalize recreational use within the next year. (Prime Minister Justin Trudeau made legalization one of the cornerstones of his campaign and has kept up the push for it since being elected.)

Canada's stance to limit the number of licenses for growers, distributors, etc., is of special interest to investors. That practice by the state opens up the possibility to create large, robust companies. And venture firms are happy to back those. The three leading Canadian cannabis companies — Aurora Cannabis, Aphria and Canopy Growth — have each raised nearly $30 million from investors, Greiper noted.

"We don't see that size of investment activity anywhere in the world for a single company in the cannabis marketplace," he said. "Canada is the most mature of the legal cannabis marketplaces in the world."

Israel is another country that has been on investor radars. Medical marijuana has been legal there since 2007, which has allowed the medical research-and-development community to begin digging deep into the DNA of cannabis to look for pharmaceutical products the plant can yield beyond smoking it or infusing it into edible products.

"Israel has been at the forefront of medical cannabis research for years now," said Troy Dayton, CEO and co-founder of The Arcview Group. "They've been creating a fertile environment for medical research."

Australia is a newcomer to the field, having federally legalized marijuana for medical and scientific purposes earlier this year, but the country has quickly become an area of interest. So far, roughly 10 public companies have emerged in the sector, including MMJ PhytoTech and MGC Pharmaceuticals, says Harrison Phillips, an analyst at Viridian. And most have partnered with Canadian firms to spur growth.

"Australia has moved to the forefront of medical marijuana by leveraging Canadian expertise," he said.

Likewise, Spain is still at the beginning of pot reform, but it's showing a lot of potential. The country hosts Spannabis, Europe's largest cannabis trade show. And though it's still a quasi-legal substance (to legally partake, you must join a private smoking club to purchase it), the country has been called the "New Amsterdam" by enthusiasts (who previously flocked to the Netherlands capital, which has made a tourist industry of legal marijuana since 1976).

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"It looks a lot like the California industry did in the late '90s and early 2000s, where you have a quasi-legal situation happening, where the public is fine with it and the government is OK with it, but it's not yet taxed and regulated," said Arcview's Dayton. "There is an industry there that's ripe for growth."

For other countries, it's still too early to say if they'll be attractive areas for cannabis-focused investors. In Uruguay, where the drug has been legal since 2013, the government limits the price to around $1 per gram, muting business opportunities for now. Jamaica may have a rich cannabis history — Bob Marley can lay claim to one of the drug's cultural icons — but the country is still rolling out its programs, and no major companies have emerged yet. Chile, similarly, where at-home consumption has been decriminalized, is still an immature market.

Now Mexico is just starting to consider the idea of legalization, though Dayton says the results of the ballot issues in California and Arizona (where voters will decide this November whether to legalize recreational marijuana usage) could sway that argument one way or another. If the U.S. border states OK recreational pot, he says, it's likely only a matter of time before Mexico follows suit.

— By Chris Morris, special to CNBC.com