Scientists recently reported that the ozone hole over Antarctica is showing signs of healing. This wonderful news comes almost 20 years after the Montreal Protocol banned the production and use of clorofluorocarbons (CFCs) in 1987. The decline means that CFCs are finally dropping in concentration in the atmosphere and are either breaking down high in the stratosphere or going into the ocean. Biologically inert, the CFCs in the ocean don’t harm any marine life, but they have proven very useful for oceanographers trying to understand circulation in the deep ocean.
CFCs were used in many industries from refrigeration to aerosols largely because they were cheap to manufacture, were biologically inert, and were excellent refrigerants. On top of these qualities that were desirable for industry, scientists found that they could measure CFCs in the atmosphere in extremely low quantities (parts per trillion) and the history of their manufacture and atmospheric release was very well known.
These same properties made CFCs an attractive compound for oceanographers to measure in the ocean. Since it has long been known that the ocean takes up gasses in the same mixtures as they are in the atmosphere, it made sense that these chemicals should be measurable in marine waters as well. Utilizing the known history of CFC release to the atmosphere, oceanographers have a clock, of sorts, that can let them know when that particular water parcel was last in contact with the atmosphere. For example, if you have a water sample with a CFC-12 concentration of 310 ppt, then that water was last in contact with the atmosphere in 1980!
Measuring CFCs in the ocean became part of the standard suite of measurements taken by scientific research cruises in the early 1990s. These cruises were primarily interested in collecting the data to provide a snapshot of the global ocean and a selection of the cruise lines are repeated each decade since that time.
The most recent cruise from the central Atlantic, along line A16, was in 2013 and when all of the CFC data are plotted together, it becomes possible to interpret the flow of CFCs into the ocean depths.
Higher CFC concentrations (green, yellow, and oranges) represent waters that have most recently been in contact with the atmosphere. While right at the surface along the entire north-south section of the Atlantic, these higher concentrations are only present at depth (greater than 500 m) towards the poles. That is because it is at the poles where conditions get cold and salty enough for waters to become denser than their surroundings and sink to greater depths (a brief primer on density in the ocean).
Depending on the temperature and salinity characteristics of the waters where they are mixed into the oceans interior, they sink to great depths (1500 – 4000 m) and slowly spread along the density surface towards the interior of the ocean basin. As seen in the figure above, the higher concentrations of CFCs have not made it much past ~40 degrees north or south along this line of longitude. The lack of CFCs (purple) in these areas means that those waters have not been in contact with the atmosphere since CFCs have been in production.
(Are you curious about those two blobs of higher CFC water at the equator? They didn’t magically sink to that depth at the equator, but rather they are different types of North Atlantic Deep Water (NADW) that have traveled south from the Labrador Sea (upper) and Denmark Strait (lower) by hugging the coast of North America in the deep western boundary currents. These waters are described in greater detail here. )
As said earlier, scientists can use the CFC concentrations to determine the ages of the water masses and the above figure shows exactly that. The upper panel shows the age of the waters on a surface of constant density (as opposed to depth) that hovers roughly at ~1500 m while the lower panel is roughly at a depth of 3500 – 4000 m. These two surfaces are chosen because those densities are most commonly associated with waters that regularly mix to specific temperatures and salinities and are formed in the same regions year after year, called a water mass. The Upper Labrador Sea Water (upper panel) and Nordic Seas Overflow Water (lower panel) are two of the major water masses that are formed in the North Atlantic and as the figure shows, it takes decades for these waters to reach the Equator as their journey along the Global Ocean Conveyor Belt begins.
It is worth noting in that figure that at any given latitude, the youngest waters are on the western side of the basin, which highlights the movement of the deep western boundary current compared to the relatively still interior of the basin. The positioning of this current is due the balancing of forces as the Earth rotates.
In part due to these findings, we know that the speed of circulation in the deep ocean is ~1 cm s-1 on average, or about the speed of a motivated slug. At that speed, scientists estimate that water sinking in these formation zones in subpolar regions will next be in contact with the atmosphere in ~1000 – 2000 years from now (depending on the exact path it takes) most likely somewhere in the North Pacific. For some perspective, that means that waters currently reaching the surface in the Pacific may have last been in contact with the atmosphere about the same time as your favorite religious figure was walking around and dispersing their teachings.
Bringing this story back to where we started, the healing of the ozone hole is the beginning of an environmental success story where scientists recognized a problem, galvanized public support, and global policy makers came together and instituted a global solution. While it may take another 30 years for the ozone hole to heal, and even longer for CFCs to leave the atmosphere, their trace of deep ocean currents and mixing will prove useful for generations of oceanographers to come.