NATURE'S END: The Consequences of The Twentieth Century. By Whitley Strieber and James Kunetka. Warner Books. 418 pp. $17.95.

NATURE'S END is a novel about what might happen if the jeremiads of conservationists go unheeded. The year is 2025, and the earth is hurting from overpopulation and environmental collapse. The sugar maple is extinct, and most other tree species are in jeopardy. Denver is still reeling from a lingering smog that killed 80,000 people in 2021. The Amazon Basin may never recover from a fire that destroyed an area the size of Texas. The burning of wood and fossil fuels has compromised the atmosphere almost beyond the point of rehabilitation.

Indeed, the Depopulationists believe that point has already been passed. Within 30 years, they assert in The Depopulationist Manifesto, human breath alone will add more carbon dioxide to the atmosphere than it can assimilate. "Combined with industrial pollution and the exhalations of insects," the manifesto claims, "the fatal overbalance will occur by 2035. Uncontrolled atmospheric overheating will then end life on this planet."

Their solution is a variation on the theme of Shirley Jackson's famous short story, "The Lottery." "At a given moment, the entirety of humanity will take a single oral dose. One third of the doses will be lethal." This draconian measure has gained enough support in the United States to give the Depopulationists control of the House of Representatives and a near-majority in the Senate. Their leader, Gupta Singh of Calcutta, is well on his way to becoming the most powerful man on earth.

To counteract the Depop movement, co-authors Strieber and Kunetka muster a crew that seems pitifully inadequate: John and Allie Sinclair, who took up the cause after the death of their son Tom, an environmental scientist, in the Denver sky-plague; and Scott Harper and Bell Evans, who were Tom's best friends. All of them are persuaded that a massive switchover to solar energy can arrest atmospheric decline and that the final solution would not be random -- the Depop leaders would choose who lives and dies.

The four dissidents have an unexpectedly formidable weapon in Delta Doctor, a computer program originally developed to profile human personalities for intelligence purposes. By law the program is also accessible to licensed "convictors," of whom there are only five in the world. John Sinclair is one of these, the only one who acts for the public good.

Conviction is a computer-aided process for ferreting out a human soul. Sinclair used it to expose the previous U.S. president as a fraud (he feigned evangelical piety but in his heart wasn't even a Christian), thereby driving him to suicide. On the run from the dreaded Tax Police, who are pursuing them on trumped-up charges, the quartet holes up now and then to plug in their terminal and escalate the conviction through another of its laborious levels. Their goal is to reveal the dark secrets of Singh's troubled psyche. He is, they suspect, a man in love with death for its own sake.

Whitley Strieber and James Kunetka have written an earlier novel together called Warday. On his own Strieber is also the author of Wolfen, a werewolf story for adults. Their strengths include atmosphere and action. A flashback to the Denver die-out is chillingly credible, and the cross-country chase that finally brings the protagonists face-to-face with Singh keeps the pages turning.

THE CHARACTERS in Nature's End tend toward the well-roundedness of cardboard, and some of the plotting devices verge on the trendy- silly. I could have done without the utopian hideaway where children and chimpanzees romp together, building a new tomorrow in -- where else? -- norther California. But on balance the novel is entertaining and intelligent.

Perhaps its most disturbing feature, though, is one much closer to hand than environmental Armageddon: a talking in-car computer, hooked up to police headquarters, that ticks off your violations and fines you on the spot. Compared to that overbearing method, the jolt of finding a ticket on your windshield is almost a pleasure.