Half-year report card for the oceans

The ocean-monitoring networks published their half-year assessment this week, and the numbers carried the familiar mixture of the alarming and the surprising that the oceans have come to specialize in. The Pacific kept its record sea-surface temperatures through the spring; the Atlantic ran cooler than the models expected, which is the kind of surprise that matters because it is the kind that the models get wrong; and the coral bleaching season arrived a month early in the Indian Ocean and ended a month early, which is the pattern that the monitoring groups describe as the new normal. The oceans are no longer the slow-moving baseline of the climate system; they are its fastest-changing part, and the report cards now arrive with the frequency and the urgency of a patient under observation.

The temperature numbers are the headline, and they deserve their place. The global ocean heat content, the measure of how much energy the sea has absorbed, set a record for the ninth consecutive month, and the rate of the increase has flattened the curve that the previous decade had established. The Pacific’s warmth is the result of the El Niño that faded last year leaving its heat behind, and the Atlantic’s coolness is the result of a current reorganisation that the monitoring buoys tracked moving south, and the two together mean that the globe’s two great heat reservoirs are out of phase in a way that the forecasters are still learning to read. The ocean’s heat is the climate’s memory, and the memory is being rewritten faster than the climatologists can annotate it.

The biology of the report is the part that I keep returning to. The bleaching season that arrived early and ended early was, on its own terms, a relative mercy: the corals of the central Indian Ocean were stressed for a shorter window than the models had projected, and the surveys report that the shallow reefs will mostly recover. The mercy is the kind that the climate system has been meting out, a temporary reprieve that arrives within a longer trend of rising stress, and the scientists are careful to say what the good news does not mean. The reefs are living on the borrowed time of a slightly cooler season, and the borrowing has been going on for years.

The monitoring itself is the quiet achievement of the report. The networks that produced these numbers, the buoy arrays, the profiling floats, the ships of opportunity, the satellites that measure the sea level and the surface temperature from orbit, are the largest instrument system ever built, and they have been running long enough to distinguish the signal from the noise, which is the thing that the oceans’ variability always made difficult. The half-year report is possible because the infrastructure was built, and the infrastructure was built because the oceans finally acquired the status that the climate system has always had: a system that deserves to be watched.

The report card’s most useful function is its honesty about the uncertainty. The Atlantic’s surprise, the early coral season, the out-of-phase heat reservoirs, are the ocean’s way of telling the climatologists that the models are still catching up, and the catching up is the real research programme of the decade. The oceans are the part of the climate system where the consequences arrive first and the understanding arrives last, and the half-year reports are the record of the gap. The numbers this week are a snapshot; the trend is a verdict; and the verdict, for the ninth year running, is that the patient is running a fever and that the treatment is not yet keeping pace with the symptoms.