WASHINGTON—Painting a stark portrait of a phenomenon that appears to be irreversible, a report published Thursday by the American Historical Association has found that the past is currently expanding at an alarming rate.

The comprehensive 950-page study, compiled by a panel of the nation’s most prominent historians, warns that the sum total of past time grows progressively larger each day, making it unlikely anything can be done to halt, or even slow down, the relentless trend.

“We believe the past is larger now than it’s ever been before,” said College of William and Mary professor Timothy Gibbon, lead author of the report, observing that whole generations of people have already become a part of history, and that if nothing changes, an untold number more can expect the same fate. “Many things that are in the past today were, during our parents’ and grandparents’ time, still in the present—or even the future. Based on precise measurements of its size, we believe the past has subsumed every single person and event that has ever existed.”

“It’s shocking to contemplate, but in the relatively short stretch since 1984, when I first began tracking its growth, the past has expanded by more than 30 years,” he added.

The report predicted this disturbing pattern will only continue, with one occurrence after another becoming part of a “colossal” historical record that, by all indications, appears intent on seizing absolutely everything without any discrimination. To date, nothing, no matter how significant, has been able to escape the past, which historians say has taken hold of episodes as momentous as the invention of the printing press, the execution of Louis XVI, numerous ice ages, the westward expansion of the United States, and the year 1995, among billions of others.

“This massive, unrestricted accrual of time is quickly becoming unmanageable—it’s growing bigger and bigger even now as I speak,” said Gibbon, who confirmed the past grew by more than six months in the time it took to research and write the new report. “Presidential administrations, extinct species, ancient empires—all have been claimed by a relentless past. There was some speculation that World War II would end history, but it didn’t.”

“Neither did the moon landing, the signing of the Magna Carta, the formation of Pangaea, the extinction of the dinosaurs, the fall of the Ming dynasty, the breaking apart of Pangaea, or the discovery of the Higgs boson,” he continued. “Indeed, these events have only served to make the past even larger.”

The report went on to state that concerns over the expanding past were outweighed only by fears about the future, which is assumed to become vanishingly small with each passing moment.