ALL THE TROUBLE IN THE WORLD: The Lighter Side of Overpopulation, Famine, Ecological Disaster, Ethnic Hatred, Plague and Poverty, by P.J. O’Rourke, Atlantic Monthly Press, 344 pages, $22.

Humor falls into several categories. First comes amusing, second is the slightly humorous, and then there is fall-down, laugh-out-loud funny. P.J. O’Rourke fits into the third, rare group.

The globe-trotting foreign affairs editor for Rolling Stone magazine has described life behind the Iron Curtain in “Holidays in Hell” and the Gulf War in “Give War a Chance,” each filled with his conservative irreverence. This time O’Rourke has compiled pieces about the post-Cold War world, poking holes into ideas about environmentalism and ethnic wars.

Somehow, even in the poorest Third World countries, O’Rourke finds humor in his pursuit of answers to world problems. He first visits Bangladesh, learning about the country’s troubles, its poverty and deforestation, and gets more information fron his guide, Abdur.

“Abdur had traveled a bit in the United States, and while we drove through the Bangladesh countryside he held forth on the scenic grandeur of the Lake Erie area,” O’Rourke writes. “`So beautiful a place,’ said Abdur, “`trees on both sides of the road!”‘

As in “Parliament of Whores,” which discussed the federal government, O’Rourke cites many studies and surveys. The book hits a few dry spots since angst-ridden books about the future of the planet tend to have a certain grimness to them, despite O’Rourke’s jabs. His criticizing of current ecological theories comes in handy when he describes the 1992 Earth Summit, a response to global worries. One American woman shows off her attempt at energy saving by using a solar cooker, which fails to broil hot dogs, only warming them. The cooker seems to act as a metaphor for O’Rourke; the theory precedes the practice.

He aims his strongest barbs at whatever allegedly more knowledgeable group he finds, which usually turns out to be Americans. Half of the Global Forum’s participants were “handsome, privileged young people groomed and dressed in that affectation of homely poverty which has been with us for a quarter of a century now.” They seemed to follow a reverse astrology, O’Rourke says, since instead of believing heavenly bodies affect every aspect of their lives, they believe they affect the universe around them.

O’Rourke writes his best when he observes the sights around him and lets the people he meets unintentionally perform, such as during a return trip to his old college in Ohio. When he sits in on an anti-pornography group and suggests they order out for pizza, it’s no surprise everyone there except him is a vegetarian, but it is still funny.

In the former Yugoslavia, he describes “multiculturalism in practice.” “There, all manners of diverse cultural groups are fully empowered — with guns. I watched as Serbian Chetnik nationalists tried to take the village of Golubic from Bosnian Muslims. The unspellables were attacking the unpronounceables.”

Other chapters include ones on plague and famine, which includes a stop in Somalia, and learning about”Economic Justice” in Vietnam. Even that blossoming capitalist country has a few things to learn, especially about music. “Unfortunately, Asia is the continent rhythm forgot,” he writes.

A devoted student of the environmental movement or Asian rock fans might disagree with O’Rourke, but readers can laugh and dissent at the same time.

For all the heralded changes in the world since O’Rourke’s first book, there is a common thread in much of it aside from the problems governments bring — his suspicion of people with all the answers, and the superiority of the market economy. He likes to make people laugh at their fears.

Once he journeyed into Soviet-controlled territory to discover that Polish high school students danced awkwardly and dressed in American fashions. Now he mocks worries about the decline of the environment and lukewarn wieners as he once did the Red menace. Anxieties about nuclear weapons falling from the sky have changed to wondering if chlorofluorocarbons make a big hole in it. Whatever the current monster under the bed might be, P.J. O’Rourke should stay around to cut it down to size.