The research team analyzed two decades of deep sea oceanographic data collected by observational mooring programs to show that a critical piece of Earth’s global system of ocean currents in the North Atlantic has weakened by about 12 percent over the past two decades.

“Although these regions are tens of thousands of miles away from each other and abyssal areas are a few miles below the ocean surface, our results reinforce the notion that even the most remote areas of the world’s oceans are not untouched by human activity,” said the study’s lead author Tiago Biló, an assistant scientist at the Rosenstiel School’s NOAA Cooperative Institute for Marine and Atmospheric Studies.

As part of the NOAA-funded project DeepT (Innovative analysis of deep and abyssal temperatures from bottom-moored instrument), the scientists analyzed data from several observational programs to study changes over time in a cold, dense, and deep water mass located at depths greater than 4,000 meters (2.5 miles) below the ocean surface that flow from the Southern Ocean northward and eventually upwells to shallower depths in other parts of the global ocean such as the North Atlantic.

This shrinking deep-ocean branch — that scientists call the abyssal limb — is part of the Atlantic Meridional Overturning Circulation (AMOC), a three-dimensional system of ocean currents that act as a “conveyer belt” to distribute heat, nutrients, and carbon dioxide across the world’s oceans.

This near-bottom branch is comprised of Antarctic bottom water, which forms from the cooling of seawater in the Southern Ocean around Antarctica during winter months. Among the different formation mechanisms of this bottom water, perhaps the most important is the so-called brine rejection, a process that occurs when salty water freezes. As sea ice forms, it releases salt into the surrounding water, increasing its density. This dense water sinks to the ocean floor, creating a cold, dense water layer that spreads northward to fill all three ocean basins — the Indian, Pacific, and Atlantic oceans. During the 21^st century, the researchers observed that the flow of this Antarctic layer across 16°N latitude in the Atlantic had slowed down, reducing the inflow of cold waters to higher latitudes, and leading to warming of waters in the deep ocean.

“The areas affected by this warming spans thousands of miles in the north-south and east-west directions between 4,000- and 6,000-meters of depth,” said William Johns, a co-author and professor of ocean sciences at the Rosenstiel School. “As a result, there is a significant increase in the abyssal ocean heat content, contributing to local sea level rise due to the thermal expansion of the water.”

“Our observational analysis matches what the numerical models have predicted — human activity could potentially impose circulation changes on the entire ocean,” said Biló. “This analysis was only possible because of the decades of collective planning and efforts by multiple oceanographic institutions worldwide.”

In their findings, published in the journal PNAS today, the researchers show how they were able create a multi-layer soft structure and an artificial fluidic system to mimic the musculature and mucus structures of biological suckers.

Suction is a highly evolved biological adhesion strategy for soft-body organisms to achieve strong grasping on various objects. Biological suckers can adaptively attach to dry complex surfaces such as rocks and shells, which are extremely challenging for current artificial suction cups. Although the adaptive suction of biological suckers is believed to be the result of their soft body’s mechanical deformation, some studies imply that in-sucker mucus secretion may be another critical factor in helping attach to complex surfaces, thanks to its high viscosity.

Lead author Tianqi Yue explained: “The most important development is that we successfully demonstrated the effectiveness of the combination of mechanical conformation — the use of soft materials to conform to surface shape, and liquid seal — the spread of water onto the contacting surface for improving the suction adaptability on complex surfaces. This may also be the secret behind biological organisms ability to achieve adaptive suction.”

Their multi-scale suction mechanism is an organic combination of mechanical conformation and regulated water seal. Multi-layer soft materials first generate a rough mechanical conformation to the substrate, reducing leaking apertures to just micrometres. The remaining micron-sized apertures are then sealed by regulated water secretion from an artificial fluidic system based on the physical model, thereby the suction cup achieves long suction longevity on diverse surfaces but with minimal overflow.

Tianqi added: “We believe the presented multi-scale adaptive suction mechanism is a powerful new adaptive suction strategy which may be instrumental in the development of versatile soft adhesion.

“Current industrial solutions use always-on air pumps to actively generate the suction however, these are noisy and waste energy.

“With no need for a pump, it is well known that many natural organisms with suckers, including octopuses, some fishes such as suckerfish and remoras, leeches, gastropods and echinoderms, can maintain their superb adaptive suction on complex surfaces by exploiting their soft body structures.”

The findings have great potential for industrial applications, such as providing a next-generation robotic gripper for grasping a variety of irregular objects.

The team now plan to build a more intelligent suction cup, by embedding sensors into the suction cup to regulate suction cup’s behaviour.

While they say in their comment article that consistently measuring subsidence is a great achievement, they argue it is only the start of finding solutions. Predicting future subsidence requires models that consider all drivers, including human activities and climate change, and how they might change with time.

The research paper, published in the same issue, considers 82 cities with a collective population of nearly 700 million people. The results show that 45% of the urban areas that were analysed are sinking, with 16% falling at a rate of 10mm a year or more.

Nationally, roughly 270 million urban residents are estimated to be affected, with nearly 70 million experiencing rapid subsidence of 10mm a year or more. Hotspots include Beijing and Tianjin.

Coastal cities such as Tianjin are especially affected as sinking land reinforces climate change and sea-level rise. The sinking of sea defences is one reason why Hurricane Katrina’s flooding brought such devastation and death-toll to New Orleans in 2005.

Shanghai — China’s biggest city — has subsided up to 3m over the past century and continues to subside today. When subsidence is combined with sea-level rise, the urban area in China below sea level could triple in size by 2120, affecting 55 to 128 million residents. This could be catastrophic without a strong societal response.

“Subsidence jeopardises the structural integrity of buildings and critical infrastructure and exacerbates the impacts of climate change in terms of flooding, particularly in coastal cities where it reinforces sea-level rise,” said Prof Nicholls, who was not involved in the study, but whose research focusses on sea-level rise, coastal erosion and flooding, and how communities can adapt to these changes.

The subsidence is mainly caused by human action in the cities. Groundwater withdrawal, that lowers the water table is considered the most important driver of subsidence, combined with geology and weight of buildings.

In Osaka and Tokyo, groundwater withdrawal was stopped in the 1970s and city subsidence has ceased or greatly reduced showing this is an effective mitigation strategy. Traffic vibration and tunnelling is potentially also a local contributing factor — Beijing has sinking of 45mm a year near subways and highways. Natural upward or downward land movement also occurs but is generally much smaller than human induced changes.

While human-induced subsidence was known in China before this study, Profs Nicholls and Shirzaei say these new results reinforce the need for a national response. This problem happens in susceptible cities outside China and is a widespread problem across the world.

They call for the research community to move from measurement to understanding implications and supporting responses. The new satellite measurements are delivering new detailed subsidence data but the methods to use this information to work with city planners to address these problems needs much more development. Affected coastal cities in China and more widely need particular attention.

“Many cities and areas worldwide are developing strategies for managing the risks of climate change and sea-level rise,” said Prof Nicholls. “We need to learn from this experience to also address the threat of subsidence which is more common than currently recognised.”