The ocean has been doing the work for billions of years, long before humans had any idea of climate or thermostats. It absorbs heat, moves it around, releases it gradually, and silently prevents the planet from swinging between extremes. Scientists occasionally find it difficult to explain how life on Earth depends on this system’s ongoing operation because it is so vast and fundamental. The ocean is now displaying indications that even its enormous capacity has limits after decades of absorbing the effects of industrial civilization.
The figures are truly astounding. Since the Industrial Revolution, the ocean has absorbed about 90% of the excess heat produced by human activity. More than 25% of the carbon dioxide that people emit into the atmosphere is absorbed by it. The average surface temperature on land would be significantly higher than it is now without that buffer. To put it another way, the ocean has been doing us a great favor. How much longer that can go on at the same rate is the question that keeps marine scientists up at night.
The ocean’s sheer volume and a circulation system known as the “global conveyor belt,” which is a slow, continuous flow of water that transports warm surface currents from the tropics toward the poles and returns cold, dense water along the deep ocean floor, are what make the ocean such an efficient temperature regulator. The most well-known component of this system is the Gulf Stream, which helps Western Europe experience comparatively mild winters by pushing warm Atlantic water northward. The same circulation distributes nutrients that are essential to entire marine food chains and prevents overheating in equatorial regions. It is a sophisticated, self-controlling system. Or it was, anyway.
Warming alters the conditions that initially cause this circulation, which is the issue that researchers have been documenting for years. The conveyor belt’s engine, thermohaline circulation, relies on cold, salty, dense water that sinks at high latitudes. Surface water becomes less dense and less likely to sink as it warms and glaciers continue to melt freshwater into the North Atlantic. The pump becomes weaker. It’s possible that the disruption will occur gradually enough to allow for manageable adaptation. It’s also possible that the changes will occur more quickly and sharply than current models predict.
Stratification is a related phenomenon that may be equally important but gets less attention. There is less mixing between layers because warmer surface water sits more clearly on top of cooler deep water. Fish populations and phytoplankton, which absorb carbon, are supported by this mixing, which brings nutrients and oxygen from the deep to the surface. The ocean’s capacity to cycle carbon and nutrients becomes less effective as stratification rises, raising concerns about whether the ocean’s ability to absorb carbon will decline at the exact moment when it is most needed.
In a warmer world, El Niño cycles—already a disruptive force—are predicted to intensify. El Niño events cause abnormally warm water in the central and eastern Pacific to alter weather patterns across continents, drying out rainfall-dependent areas and flooding others. When the catch disappears, Peruvian fishing communities have known for generations that something has changed in the Pacific. The larger context has changed, as these occurrences now take place against an already elevated baseline ocean temperature.
It’s difficult to ignore how little public discussion there is regarding the practical implications of the ocean’s changing behavior. It makes sense that people are concerned about coral bleaching and sea level rise. However, the ocean’s function as a temperature regulator—its gradual, massive impact on whether the climate system stays even somewhat stable—deserves far more attention than it usually receives. The damage has long been absorbed by the ocean. Scientists are beginning to feel that the ledger is due.
