Nearly three decades after the world banned chemicals that were destroying the atmosphere’s protective ozone layer, scientists said Thursday that there were signs the atmosphere was on the mend.

The researchers said they had found “fingerprints” indicating that the seasonal ozone hole over Antarctica, a cause of concern since it was discovered in 1984, was getting smaller. Although the improvement has been slight so far, it is an indication that the Montreal Protocol — the 1987 treaty signed by almost every nation that phased out the use of chemicals known as chlorofluorocarbons, or CFCs — is having its intended effect.

Full recovery of the ozone hole is not expected until the middle of the century.

“This is just the beginning of what is a long process,” said Susan Solomon, an atmospheric chemist at the Massachusetts Institute of Technology and lead author of the study, published in the journal Science.

Ozone high in the stratosphere protects life on Earth by absorbing damaging ultraviolet rays from the sun. But ozone is destroyed by reactions with chlorine and other atoms that are released by CFCs and similar chemicals, which were used for decades as refrigerants and propellants.