PARIS — Data from three European Earth observation satellites shows that the degradation of the Earth’s protective ozone layer has ceased in the last decade, and there are preliminary indications that it may be regenerating, the European Space Agency (ESA) announced Sept. 21.

The agency cautioned that it is too early to declare victory in the decades-long effort to protect the ozone layer by banning man-made bromine and chlorine gases that are known to destroy ozone.

But the ozone’s depletion of about 7 percent per decade observed in the 1970s and 1980s had been stopped by around 1997, according to the satellite data. In the decade since, a slight regeneration estimated at between 0.8 percent and 1.4 percent per decade has been observed.

The data for the years since 1997 were collected by the Gome ozone-monitoring instrument aboard ESA’s ERS-2 radar Earth observation satellite; the Sciamachy imaging spectrometer on the Envisat radar Earth observation satellite; and the Gome-2 instrument on the Metop-A polar-orbiting meteorological satellite operated by Europe’s Eumetsat organization.

Statistically speaking, ESA said, the slight regeneration in the ozone layer registered between 1997 and 2008 is not much different from a zero-change trend. It will take several more years of satellite observation to confirm it.

A 1987 international treaty called the Montreal Protocol on Substances That Deplete the Ozone Layer called for the gradual phasing out of substances thought to be responsible for ozone depletion.

Bromine is among the substances. Francois Hendrick, of the Belgian Institute for Space Astronomy, who with colleagues from Germany’s University of Bremen analyzed concentrations of bromine monoxide in the stratosphere, said the Montreal Protocol’s restrictions now are being felt in the stratosphere given the decline in bromine levels as recorded by satellite data.

Peter B. de Selding was the Paris bureau chief for SpaceNews.