The hole in the ozone layer over the Antarctic has begun to shrink as a result of a ban on some pollutants, scientists believe.
A study found that the hole has reduced in size by 1.5 million square miles (four million square kilometres) since the year 2000.
The reduction - an area the size of India - came as a surprise to experts as they thought the hole was getting bigger.
It became apparent after they took into account the damage to the ozone layer caused by a volcanic eruption.
Susan Solomon, the study's lead author, said: "It's a big surprise. I didn't think it would be this early."
The hole in the ozone layer - which protects the earth from ultraviolet radiation coming from space - was discovered in the 1950s.
But it was only after the effect on it of some compounds in aerosols and refrigerators that it was decided to take action.
The Montreal Protocol on Ozone Depleting Substances was agreed in 1987 to phase out the types of chemical that cause the most damage - CFCs and HCFCs.
It appeared that the hole was getting smaller until the hole reached a record size in October 2015.
But a team of US and British scientists have now measured changes in the hole taking into account the April 2015 eruption of the Chilean volcano Calbuco.
Ms Solomon said: "We can now be confident that the things we've done have put the planet on a path to heal."
The Montreal Protocol is viewed as a landmark achievement, with nearly 200 countries coming together to make a binding agreement to help protect the planet, in contrast to failed attempts to reach other environmental settlements.