The world ocean plays a major role in regulating the climate, in part by absorbing more than a quarter of the billions of tons of heat-trapping greenhouse gas that humans put into atmosphere.

Scientists have suspected that even as these human-propelled emissions rise, the ocean's capacity to store them is maxing out. Research released this week in the journal Nature adds new strength to that argument.

In their study, Samar Khatiwala of Columbia University and colleagues developed an ingenious mathematical method for charting the history of how much CO2 the ocean has absorbed since the beginning of the industrial era in 1765: by tracing the amount of human-produced CO2 in water masses of different ages and different geographic origins.

According to their findings, the ocean currently holds around 150 billion tons of carbon. Just over a decade ago, their calculations suggest, it was around 114 billion tons -- a figure that closely aligns with research published in 2004 by NOAA marine chemist Chris Sabine and colleagues.

In the course of producing this data, Khatiwala and colleagues were also able to measure the rate of the water's uptake of CO2, currently about 2.3 billion tons of carbon a year. They discovered that while this amount is still increasing, this rate has slowed the proportion of total emissions absorbed by the ocean has decreased, by over around 10% since 2000.

"In absolute terms, this is not very large, and its important ... to recognize that there are uncertainties with all such estimates," said Khatiwala in email. "What's concerning however is the rather rapid change, and the long term implications of such a decline in the efficiency."

The findings have significant ramifications for how fast and far to cut our greenhouse gas pollution. As the ocean absorbs less and less carbon, climate conditions may change a lot faster than even the edgiest climate models have predicted. (This could be happening already, in fact).

It's a good (in a bad way) example of a "climate tipping point" that, once we hit it, will be all but impossible to back away from based on our current knowledge and technologies.

Khatiwala and his colleagues were also able to estimate the amount of CO2 being absorbed on land: by taking their estimate of ocean absorption, as well as the amount of carbon remaining in the atmosphere, and subtracting them from the known amount of emissions from burning fossil fuels.

They were surprised to find that at the moment, the land is absorbing more CO2 than it is giving off, a reversal of the historic trend. One reason may be that increased levels of atmospheric CO2 may be providing extra fuel for photosynthesis, allowing living plants to grow more quickly.

This probably shouldn't be taken as a panacea for the ocean's diminishing capacity to absorb carbon. The world ocean covers around 70% of the Earth's surface. So even if we were to stop the development of every remaining square acre of forest and grassland that remains, there simply isn't enough land to make up the difference.

Still, we're learning a great deal lately about the immense amount of carbon that forests can store -- particularly old-growth forests. So this new research about ocean sinks bolsters the many already-good reasons to include strong forest protection and restoration measures in economic and ecological plans for curbing global warming's worst impacts.