The hole in the ozone layer above Antarctica reached 10.5 million square miles this year, the seventh largest in 48 years of monitoring, according to NASA and NOAA.
It hit that annual maximum on 18 September. Across the peak depletion season, from 7 September to 13 October, it averaged 9.6 million square miles, or 25 million square kilometres.
The ozone hole is not a hole in the ordinary sense. It is the area of the stratosphere in which ozone falls below 220 Dobson units, the threshold used to mark the severe seasonal thinning that forms over the pole each southern spring.
Depth is counted separately from width. NOAA's balloon-borne instruments at the South Pole Baseline Atmospheric Observatory recorded a minimum of 105 Dobson units directly overhead on 17 October. The record low, 92 Dobson units, was set in October 2006.
This year's hole was therefore wide without being especially deep, and the agencies were specific about why. The driver was meteorology rather than chemistry.
The polar vortex, the ring of winds that encircles Antarctica through the southern winter and creates the cold conditions in which chlorine and bromine from human-made compounds destroy ozone, was unusually large and strong. The planetary-scale weather waves that normally break it up were especially weak in August and September.
"That weak wave action kept the strongest polar vortex winds further from the pole by several degrees of latitude," said Amy Butler, a research scientist at NOAA's Chemical Sciences Laboratory.
The long-run picture has not changed. Stephen Montzka, a senior scientist at NOAA's Global Monitoring Laboratory, said chlorine levels in the Antarctic stratosphere from ozone-depleting substances have fallen by roughly a third from their peak, measured against 1980s levels. Controls agreed under the 1987 Montreal Protocol continue to drive recovery, which scientists expect to be complete by the 2070s.
"The annual ozone hole has been forming later in the season in recent years, which tells us the ozone layer is healing," said Paul Newman, a senior scientist at the University of Maryland, Baltimore County, who led NASA's ozone research team at Goddard Space Flight Center for many years. "But an unusually powerful polar vortex has extended ozone depletion in the Antarctic this season."
Newman added that if ozone-depleting substances were still as prevalent as they were at their peak in 2000, this year's hole could have been one to two million square miles larger. The largest hole observed since monitoring began reached its own maximum in September 2000.
It is worth being precise about which number is carrying the news. The hole's width this year was set by wind. Its depth was set by chemistry. Its health, on the evidence offered, is established by the date in the diary.
This newspaper finds that an improvement on the old arrangement. A quantity that can stand at ten and a half million square miles and still represent healing is a robust quantity. It cannot be embarrassed by its own size.
The logic runs in both directions, which is the mark of a sound indicator. A small, early hole would be the worse sign. A vast, late one is the better. Punctuality is the diagnosis and area is merely weather.
Our analysts therefore recommend that the Montreal Protocol be reread as a scheduling agreement. Its signatories did not undertake to make the hole smaller. They undertook to make it later.
On that measure the treaty is performing well. The hole opened late, reached 10.5 million square miles, and is expected to close roughly on time.
The Department of Unnecessary Research has asked the agencies to confirm the only figure that now matters. Recovery is due in the 2070s. Nobody has said how big the hole will be when it arrives.