Suppose for a moment that human beings suddenly vanished from Earth and our cities, buildings and homes were left vacant. What would the Seacoast look like after a few years without human inhabitants?

According to author and journalist Alan Weisman, it wouldn’t take long for nature to reclaim its property. Streets would flood as plastic bags clogged the sewers. When the waters subsided, leaf litter would gather on the streets and plants would germinate in it, warping and shattering the pavement with their expanding roots.

“It’s amazing how little plants and wildflowers can crack open a sidewalk,” Weisman said. “You don’t have to wait until the concrete and the asphalt goes for the plants to colonize it. It’ll all happen much faster than people would believe.”

As new trees grow and drop deadwood on the ground, lightening strikes would spark fires that ignite nearby buildings, and without firefighters to extinguish the flames, whole neighborhoods would go up in smoke.

White pine trees would spread through farmlands and fields, and other vegetation would grow in the shade beneath them. Farm animals would find themselves in competition with wild animals, and most would fall to predation. Domestic cats would run wild and proliferate, feeding on area birds and rodents. Pet dogs would not be so lucky, as growing populations of coyotes, wolves and cougars would out-compete them.

As years passed, New England houses would gradually crumble, leaving only brick chimneys to mark their former locations. “Most of the houses in Portsmouth, 100 years after we’re gone, will be mostly lying on the ground, except for the chimneys and some mortared walls, but the mortar will all be crumbling,” Weisman said.

For a more detailed account of how your home would fall apart if you weren’t there to maintain it, read the second chapter of Weisman’s 2007 book, “The World Without Us.” Published by Picador, the New York Times bestselling book paints an intriguingly vivid picture of what would happen to Earth if humans weren’t around.

“I’ve got one paragraph in which I sort of dispense with the human race,” Weisman said.

He described a few unlikely but conceivable ways that people could quickly be wiped off the planet—an airborne virus that only afflicts humans, perhaps, or a nefarious biological weapon that sterilizes sperm.

For the purposes of the book, it doesn’t really matter how or why the human population makes its exit. Weisman uses his thought experiment to examine what traces of humankind would disappear, and what human relics, if any, would remain through the eons. Though sprouting from a fictional premise, his examination is based on factual scientific research that ultimately yields insights into how our behavior impacts the planet. 

Weisman will discuss “The World Without Us” at Old South Church in Newburyport, Mass., at 7 p.m. on Thursday, Sept. 4. The following night, at 7:30 p.m. on Friday, Sept. 5, he will visit South Church in Portsmouth. The free event is part of Seacoast Local and RiverRun Bookstore’s “Making the Connection” speaker series.

A Massachusetts resident, Weisman’s journalistic writings for Harper’s, The New York Times magazine, The Atlantic Monthly, The Los Angeles Times magazine, Vanity Fair and other publications have brought him all around the world. He has found that all his work relates to the environment in one way or another. “Every story, I’ve learned, ultimately is an environmental story,” he said.

The author of four previous books, Weisman had long wanted to write an environmental book that connected his experiences without creating a frightening, doomsayer’s treatise. He got his opportunity when the editor of Discover magazine called in summer 2003 and asked him to write an article about what the Earth would be like without humans. His research for the story convinced him that the idea warranted an entire book, and that it was the book he’d been waiting to write for years.

“By pretending that we’re already dead, I immediately disarm the fear factor that keeps so many people from reading environmental books,” Weisman said. “In this book, nobody worries about how we’re going to die, because we’re gone.”

Weisman traveled to Europe, Asia, Africa and Central America and spoke to sources ranging from astrophysicists and marine biologists to art conservators and subway workers. The book he eventually published has now been translated into more than 30 languages and was named Best Nonfiction Book of 2007 by Time Magazine and Entertainment Weekly.

In order to illustrate a future without people, Weisman began by studying what the Earth was like before it was populated by hominids. In pre-human times, enormous mammals roamed the planet, including several species of giant sloths. One Argentinean sloth, Weisman said, was taller than a mammoth. In those days, North America was home to three times as many species that weighed more than a ton as Africa has today. That began to change when homo sapiens spread to the continent and began hunting big game.

“There’s a coincidence of the arrival of human beings and the disappearance of a lot of big animals,” Weisman said.

Since that time, human beings have driven numerous species to extinction, and those animals will never come back, even if humans ceased to exist. But new species would evolve to fill the niches left by those that have disappeared, Weisman said. “Gradually, little things crawl out of the sea and they start over, and it takes millions of years, but nature recovers.”

Some dramatic changes of a world without people would manifest almost immediately. Weisman devotes one chapter of his book to describing post-human New York City. With no one to operate the pump facilities that keep underground tunnels free of water, the subway system would flood within two days. After anther 20 years, the steel columns that support streets above the subways would buckle and the ceiling would collapse, turning Lexington Avenue into a river. Eventually, Manhattan would return to the forest it once was.

Weisman spoke to a bridge maintenance worker in New York who described his struggle to prevent nature from taking over the structures. The conversation illustrated how quickly nature could conquer human constructs if given the chance.

“A bird can fly over one of his bridges, poop out a little seed, and that seed will lodge between two steel plates. There’s just enough bird poop on it for the seed to germinate and then suddenly he’s got a tree that is pushing apart two-inch thick plate steel,” Weisman said. “There’s no question that nature will eventually win this battle.”

Another example of nature’s resiliency comes from Cyprus Island, where Weisman explored an old resort that was abandoned during a war in 1974 and was never reoccupied. “Nature’s just completely eaten it to shreds,” he said. “The buildings are still standing, but none of them are salvageable at this point. There are trees growing right out of the roof. The concrete streets are filled with fields of flowers.”

Weisman also visited sites that were abandoned because of contamination, including Chernobyl, which “looks very lush these days.” He went to a pond in Colorado where people once made nerve gas. Considered one of the most contaminated places on Earth, the pond reportedly killed ducks and geese within minutes of contact and rapidly corroded the bottoms of boats. Even there, Weisman said, nature has found a way.

“It’s now a national wildlife center since we stopped making nerve gas out there, because all these animals just colonized it,” he said. “No matter how bad we whack nature, nature comes back. It always finds something that will grow, even in some field that we have bombarded with chemicals.” 

So, which human contributions to the world would nature fail to eliminate? In Portsmouth, Weisman said, the city’s oldest buildings would probably remain standing the longest.

“Look for the stuff that is made out of granite block, good New Hampshire or Massachusetts or Vermont granite that was quarried right there. Those buildings are going to last much longer than the modern ones made out of steel and glass or reinforced concrete,” he said.

In fact, the human dwellings that would survive longest without us are some of the first ones built, including South American caves and Spanish homes built from earth. These buildings were designed with nature in mind and are therefore much more efficient and sturdy than modern constructs. Unlike American buildings with western exposures that require massive amounts of electricity to cool, early humans strategically built homes that were naturally cool in summer and warm in winter, Weisman explained.

Some human developments will continue to cause problems long after we are gone. Weisman devotes a chapter of “The World Without Us” to the lasting effects of plastics. Although it has only been widely used for about 60 years, humans have already produced more than 1 billion tons of plastic, and none of it will go away for at least 100,000 years. Although plastic has a variety of important uses, the polymers used to make it do not biodegrade. “Its very virtue—indestructibility—is also its bane,” Weisman said.

Enormous quantities of plastic refuse wind up in the ocean, where it threatens marine life up and down the food chain. But Weisman believes microbes will eventually evolve to ingest plastic and remove it from the sea.

Another lasting human creation that has even more frightening implications is nuclear power plants. Despite all their safety precautions, Weisman believes all nuclear reactors will eventually be exposed to the elements, including the one you can see on your way to Hampton Beach. “Seabrook—it’s gonna fall apart some day,” Weisman said.

Nuclear plants in the United States have emergency generators that would turn on automatically if people were not there to operate them. But the diesel supplies that fuel many of those generators would run out after about a week. The cooling water that circulates around the reactor core would then start to evaporate, causing a meltdown. Cooling water is also needed for ponds where spent rods are stored. What’s more, the concrete containment dome that encloses plants like the one in Seabrook would eventually corrode.

“None of that stuff lasts forever, and it requires constant human maintenance to make sure that the water doesn’t evaporate out of the cooling ponds, which it would do because of the heat of those things,” Weisman said.

Even if we learn to live in harmony with nature, our species will not live forever, and human products like nuclear waste will still be around after we’re gone. “Every species eventually goes extinct, just the same way every human being eventually dies. It just happens. And when we do, we’re gonna leave behind some radioactive waste that has a half-life that is longer than the Earth has got left to exist,” Weisman said.

But there are some species that would miss us if we were gone, and they’re not just domestic farm animals and pets. Head and body lice would become extinct within a year or two, and cockroaches would no longer be able to survive in cities with cold winters.

There are also positive human contributions that would live on without us, including some artwork. Bronze sculpture would still be recognizable after millions of years, and music would continue to ride radio waves into outer space forever.

“There are some beautiful things about our legacy that will outlive us, but they’re also testimony to the fact that we add considerable beauty to this planet, and I would like to think that we could keep doing that,” Weisman said.

Although the author’s journalistic pursuits began because he was concerned about the future of the Earth, the focus of his mission has changed. “I’m not worried about the planet anymore. The planet’s gonna do fine. It’s been through worse than this,” Weisman said, noting that nature has bounced back from mass extinctions in the past.

Weisman is now alarmed about the planet’s ability to sustain humankind. We have already stretched beyond the limits of our resource base, he said, and populations of crucial pollinators like bees, bats and birds are crashing. Clearly, it’s we, and not the Earth, that are in trouble.

“We’re gonna have to learn how to reestablish some harmony and equilibrium with the natural ecosystem,” Weisman said. “If not, we’re just gonna eat ourselves into extinction. Other species have done it before us, and we are clearly not exempt from the rules.”

The only way some people have suggested that humans can ultimately escape extinction is by uploading our brains to computers. As reassuring as that sounds, Weisman doesn’t think it would work. Every machine requires physical human bodies for maintenance. “It ain’t gonna happen. Robots are not gonna save the human race,” Weisman said.

But there’s good news. Each of Weisman’s stories about nature’s resiliency also demonstrate how quickly Earth would recover from human damage if we allow it to heal. Weisman believes that humans have just as much right to exist as every other species; his book is not meant to suggest that the world would be better off without us.

“What I hope is that when people see how beautifully nature could come back and how wonderfully well it would function, that they would ultimately ask, ‘Oh God, isn’t there some way that we can be a part of that, but in harmony with nature and not in mortal combat with it?’”

Weisman’s book ends with an unusual suggestion on how humankind can carry on. He has discussed the book with not only environmental groups and media members, but with business leaders and industrial groups all over the world.

“Everybody’s concerned about the environment. We all know that our future and our descendants’ future all depend on this thing.”