(Inside Science) -- As humans pump more and more carbon dioxide into the atmosphere, the ocean is hard at work absorbing it, buying us more time to stave off the worst effects of climate change. Since the beginning of the Industrial Revolution in the mid-18th century, the ocean has absorbed roughly 39% of all human emissions. This phenomenon, called the "ocean carbon sink," is driven by the difference between carbon dioxide levels in the atmosphere and in the ocean.
New research suggests that changes in atmospheric carbon dioxide levels can strengthen or weaken the entire ocean carbon sink on very short time scales.
Larger differences in the levels of carbon dioxide in the atmosphere and in the ocean pump more carbon dioxide into the ocean's surface layer where it is dissolved and stored. And as the rate of carbon dioxide emissions increases, the ocean carbon sink strengthens in response, absorbing more and more of the greenhouse gas. Scientists have long recognized this trend over the past few decades. But at smaller scales, the relationship is much more complicated.
A big wave in the research community
Climate scientists know the sink strengthens and weakens over decades or even year to year but they haven't yet been able to pin down the exact mechanisms behind its variability. Data from the 1990s has had scientists' heads spinning for years. The climate data from this decade, on average, displayed a global weakening of the ocean carbon sink. But when researchers zoom in on the year-by-year data, they notice some erratic behavior. The early '90s showed an increasingly stronger carbon sink. But from 1993 to 2000 the ocean carbon sink significantly weakened.
Previously, changing ocean circulation patterns and winds had been proposed as the main forces at play. But a new study published in AGU Advances by a team of climate scientists led by Galen McKinley, an oceanographer at Columbia University in New York, points to additional factors. The researchers place the majority of the blame on two other forces that emerged during the early years of the decade: The 1991 eruption of Mount Pinatubo in the Philippines, and the slowing growth rate of atmospheric carbon dioxide.
The team combined a variety of data and models to isolate the most significant forces.
The scientists suggest that the eruption caused widespread cooling of the ocean, thus increasing the ocean's ability to dissolve carbon in its surface layer. This effect lasted for the first couple of years after the eruption until it faded away, leaving an atmosphere that had accumulated less carbon dioxide from earlier in the decade. These lower carbon dioxide levels occurred partly as a result of a slowdown in the growth of fossil fuel usage during the late 1980s and before the Soviet Union dissolved. But the most significant influence likely stemmed from a stronger "land carbon sink" in the 1980s and 1990s. Vegetation and soil probably absorbed more carbon than usual for reasons that are still poorly understood by scientists.
This, combined with the higher amounts of dissolved carbon dioxide in the ocean from the eruption, lowered the difference between the amounts of carbon dioxide in the atmosphere and in the ocean enough to weaken the global ocean carbon sink for the rest of the decade.
The researchers believe the swiftness of that response may have important implications for future efforts to slow climate change.
Reckoning with the sink's ally
Since 2001, the ocean carbon sink has continued on its pre-1990s path, getting stronger every year as rates of carbon dioxide growth in the atmosphere climb. But fluctuations in this growth rate are inevitable, so it is important to know how quickly the ocean will respond.
"The changes in the growth rate of atmospheric carbon dioxide will have real and immediate impacts on the ocean carbon sink," said study co-author Amanda Fay. "As we as a global community reduce our emissions, the ocean carbon sink will respond by slowing as well."
This may make the future of climate change mitigation look more difficult, but it also must be reckoned with as the world attempts to roll back the rapid devastation currently occurring from two important consequences of greenhouse gases, ocean acidification and warming.
Scientists have long known that variations in atmospheric CO2 levels cause changes in the movement of CO2 from air to sea. But the full scale of the relationship as described by McKinley and the team forces scientists to reconsider the relative importance of all the factors that influence the ocean carbon sink.
"What surprises me is the magnitude of their modeled impact," said Nicolas Gruber, a climate scientist at ETH Zurich who was not involved in the study. "We obviously now need to go back and check again."
In the next few years, researchers will seek more data and refine models as they work to reflect the true variability of the ocean carbon sink. This back-and-forth exchange is essential for policymakers to make the most informed decisions on climate change mitigation.
The ocean carbon sink will likely weaken in response to decreasing carbon dioxide emissions. But unpredictable events such as volcanic eruptions -- or global pandemics -- can change carbon dioxide levels and have rapid impacts on the ocean carbon sink. And as climate models continue to improve, the true sensitivity of the balance in the air-sea system will become clearer.
"People have to understand that the earth system has these very long time scales, and we are perturbing it on very short time scales," said McKinley.