Since it was discovered in 1985, the Antarctic ozone hole has been a potent symbol of humankind’s ability to cause unintended environmental harm. But now comes a glimmer of good news: The void in the ozone layer is shrinking. “It’s a big surprise,” says Susan Solomon, an atmospheric chemist at the Massachusetts Institute of Technology in Cambridge. “I didn’t think it would be this early.”

Although the hole will not close completely until midcentury at the earliest, the healing is reassuring to scientists who pushed for the Montreal Protocol. The 1987 international agreement phased out the industrial production of chlorofluorocarbons (CFCs): chlorine-containing chemicals that help trigger the destruction of stratospheric ozone, which screens out cancer-causing ultraviolet light. “You want to be sure that the actions we’ve taken have had the intended effect,” says Solomon, who led the study published online by Science this week.

Layers of depleted ozone open up over both poles just as winter gives way to spring. During the wintertime cold, nitric acid and water condense out of the atmosphere and form wispy clouds. The surfaces of the cloud particles host chemical reactions that release chlorine that came from CFCs. The chlorine, in turn, goes on to destroy ozone—but only in the presence of light. That is why, over Antarctica, ozone loss doesn’t get going in earnest until September, the beginning of the southern spring, when light returns to the pole. Peak losses are usually in October, and that is when researchers have typically taken stock of year-to-year changes in the hole.

Solomon and her colleagues found that the healing trend was more apparent in the month of September. Using a combination of measurements from satellites, ground-based instruments, and weather balloons, her team found that, since 2000, the September hole has shrunk by 4 million square kilometers—an area bigger than India.

To determine whether declining pollutants deserve credit for the recovery, the researchers used a 3D atmospheric model to separate the effects of the chemicals from those of weather, which can affect ozone loss through winds and temperature, and volcanic eruptions, which deplete ozone by pumping sulfate particles into the upper atmosphere. The sulfate can play the same role as cloud particles, activating chlorine.

The model helped explain why scientists saw a record ozone hole in October 2015, a glaring exception to the shrinking trend. Solomon and some other researchers at first wondered whether the recovery might be behind schedule. But the model showed that it was a fluke due to the eruption of the Calbuco volcano in southern Chile 6 months earlier, and it confirmed that declining levels of chlorine and its chemical cousin bromine were indeed responsible for the longer term healing trend. “To me, it’s the first time that has been shown unequivocally,” says Neil Harris, an atmospheric scientist at the University of Cambridge in the United Kingdom who was not involved in the work.

Solomon’s study follows earlier claims of healing, including a 2011 study that got some attention. But those studies separated out the effects of natural variability using relatively simple statistical techniques, and many researchers questioned the assumptions that went into them. Using a 3D model to tease out the underlying trend is a “much more sophisticated way to do the attribution,” says Ross Salawitch, an atmospheric scientist at the University of Maryland, College Park.

Still, Paul Newman, who runs NASA’s Arctic Ozone Watch website at Goddard Space Flight Center in Greenbelt, Maryland, is puzzled by Solomon’s finding that only half of the 4-million-square-kilometer shrinkage trend was due to a reduction in chlorine and bromine. The other half appeared to be due to weather. Weather effects ought to cancel out on average, resulting in no trend, he says. “If we can’t explain half the signal, then can we really explain the 50% of the signal we think we know?” Newman says the finding could point to a problem with Solomon’s model. Or, he says, it may reflect a real shift in polar weather, driven by climate change. “She’s uncovered a real scientific puzzle here,” he says.

Regardless, the result is a satisfying full-circuit ride for Solomon, who in 1986 led the study that first identified stratospheric clouds as the chlorine reaction sites, and who played important roles in the scientific assessments for the Montreal Protocol and subsequent status reports. “The fact that we’ve made a global choice to do something different and the planet has responded to our choice can’t help but be uplifting,” she says.