(CNN) — Think those lines at the airport are long? Fast-forward a few years and they're going to be waaaay longer according to new forecasts that say the volume of global air passengers is set to more than double over the next two decades.

By 2040, air passengers will be taking a total of 20.9 billion journeys, according to the Airports Council International, a global body that represents the planet's airports. That's a huge travel leap from from 8.2 billion passenger trips in 2017.

So where will these extra passengers be coming from? According to ACI, mainly from China , which is expected to outstrip the United States to become the No. 1 country of origin for air travelers.

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Chinese passengers will be making four billion air journeys in 2040, accounting for almost 19% of all global traffic, ACI says. US passengers will make up about 15% of the market, with India in third place with 6.3%.

The huge surge expected in passenger numbers is good news for the global aviation industry, which has been pouring billions of dollars into upgrading existing airport facilities and building ambitious new ones.

But ACI warns more investment will be needed if the planet's aviation networks are to avoid heavy congestion.

Angela Gittens, director general of ACI World, says the rise of Asia-Pacific markets is a positive development, but a global plan is needed for the years ahead.

"While this is welcome, surging air transport demand threatens to outstrip current and planned airport infrastructure in many regions around the world," she says. "These physical capacity considerations and potential bottlenecks in air transport infrastructure pose challenges to the global industry in accommodating the strong forecast future demand."

Geopolitical tensions

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Despite the rosy traffic forecasts, Gittens says cross-border disputes and nationalist trade policies such as those favored by the current US government may cause numbers to waver.

"Geopolitical tensions and protectionist policies that retreat from further economic integration and air transport liberalization could also have an adverse effect on the air transport industry," she says.

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"The aviation industry must come together to respond to these challenges and help to ensure communities continue to reap the social and economic benefits of air service growth. Policy at a national and global level should be focused on facilitating sustainable growth over the long term."

Other nations taking to the skies in increasing number over the next couple of decades will include Vietnam, Saudi Arabia, UAE and the Philippines, according to ACI's data covering 110 countries.

Fastest-growing countries for passengers 2017-2040

1. Vietnam -- 7.8%

2. India -- 6.8%

3. Saudi Arabia -- 6.2%

4. United Arab Emirates -- 5.7%

5. China -- 5.5%

6. Philippines -- 5.4%

7. Indonesia -- 5.4%

8. Colombia -- 5.4%

9. Mexico -- 5.4%