Unchecked pollution, land mismanagement and population growth have pushed the world to the brink of its environmental Pearl Harbor: a global food shortage that could starve millions, the Worldwatch Institute says.

The environmental research group, which has surveyed the Earth's condition annually since 1984, has issued perhaps its grimmest report, "The State of the World 1989.""We are losing at this point, clearly losing the battle to save the planet," said the report's chief author, Lester R. Brown. The impending result, he warned, "will shake the world to its foundation."

Yet a glimmer of optimism lightened Brown's message at a briefing on the Worldwatch findings last week. With growing awareness of the hazards at hand, he said, a new political consensus is arising that may defeat them.

As Americans faced heat waves, drought and beach pollution last summer, "What people began to sense was that the planet might be changing," Brown said. "And that they might be responsible."

But with the atmosphere getting hotter, arable land disappearing and overpopulation continuing, time is short. "By the end of the next decade, the die will pretty well be cast," the report said.

Brown identified two critical concerns: Environmental degradation - from the loss of topsoil to a growing scarcity of water to apparent global warming caused by air pollution - that has cut farm output; and continuing overpopulation.

"The overall model suggests that we may be moving into a very difficult situation with food, one where food security may replace military security as the principal preoccupation of many governments in the world," Brown said.

Already the world is undergoing "a loss of momentum in the growth of food output." He cited this evidence:

- World grain production per capita has declined each year since 1984 - "a little in 1985 and 1986, quite a bit in 1987, a lot more in 1988."

- Last year's North American drought cut U.S. harvests by 30 percent, and other major producers could not make up the loss: China's harvest fell 3 percent, the Soviet Union's 9 percent.

- Croplands have shrunk 7 percent in China since 1978, mostly because of industrial development, and 13 percent in the Soviet Union, mostly through land mismanagement. The Earth is losing 24 billion tons of topsoil a year - as much as covers Australia's wheat belt - chiefly from overtilling.

"The world's farmers are now trying to feed 86 million more people each year and trying to do it with 24 billion fewer tons of topsoil," Brown said.