Shaded by rain

In its fortieth year, the Australian Institute of Marine Science went back to 121 reefs and counted. Hard coral cover in the north rose from 30.0 to 35.1 percent. In the center, from 28.6 to 31.6. In the south it slipped, 26.9 to 26.4. All three regions now sit above or just below their long-term average.

That is a better year than anybody had a right to expect.

The sea was warmer than average again. What changed the outcome was a late monsoon: cloud over the reef through the worst weeks of summer, and sea temperatures pulled down with it. The institute's own words are that this likely limited the severity and extent of bleaching.

A reef was saved by rain. Not by zoning, not by water quality policy, not by anything anybody did on purpose.

Cover is up in two regions of three and the reef is back around its long-term average. That is the measurement, it was taken the same way it has been taken for forty years, and no other quantity in this argument was counted at all.

On this reading the reef has demonstrated something real about itself. Given one cool summer, it grew back toward where it was, and that capacity is in the data rather than in a model.

Dr Mike Emslie, who leads the monitoring program, said something easy to read past: while many reefs did not lose coral cover, bleaching likely prevented the continued increases that would normally be expected.

Read that carefully and the good year turns into a smaller year than it should have been. The reef did not lose ground; it also did not gain what an unbleached summer would have handed it. That missing growth is real and it is not in the survey, because a survey counts what is there rather than what would have been.

One thing underneath the headline belongs to this reading, and it is in the survey. In the north — the region that posted the largest gain — crown-of-thorns starfish numbers are rising in a pattern the institute reads as the start of the fifth outbreak wave since the 1960s.

The disagreement is not about coral. Both readings accept every number in the survey. They disagree about the counterfactual — what this year would have produced without the heat — and no monitoring program measures a year that did not happen.

What would settle it is a run of summers cool enough to show the reef's unimpeded growth rate, which is the one experiment nobody can schedule. Failing that, the fifth outbreak wave will answer a different question fairly soon, and the answer will arrive in the same regions that just posted the gain.