The 1987 Montreal Protocol stands as a demonstration that sometimes, when faced with a threat to life, the world can stand together and act cooperatively to save ourselves. The protocol set deadlines for the phase-out of various ozone-depleting gasses, most famously chlorofluorocarbons (CFCs). Despite occasional backsliding, the Protocol has held. Right-wing predictions of harm to the economy have proven largely baseless, and the “ozone hole” over the South Pole in spring slowed, paused, and eventually reversed its growth. Now we've learned the downstream effects are following the same path.
Human-induced damage to the ozone layer not only exposed us to dangerous ultraviolet radiation but it also caused changes to high-altitude winds, which in turn affected rainfall and ocean salinity. However, the winds stopped shifting around the year 2000, and have shown signs of reverting to their previous state. Meteorologists have been unsure whether or not this recovery is a temporary aberration, but have now found evidence it is a consequence of reduced emissions of ozone-depleting gasses, and thus stands a good chance of continuing.
That does not necessarily mean that all the hole's consequences will pass quite as quickly. In complex systems like the atmosphere, a major shock can cause a shift into a different state that is not always reversed when the initial impetus passes. Dr Antara Banerjee of the University of Colorado led an effort to determine whether changes to Antarctic winds represent a signal or noise.
Through the 1980s and '90s, the Southern Hemisphere's mid-latitude jet stream shifted 2º towards the pole and the tropical circulation known as the Hadley Cell experienced greater overturning. These effects, and others, have been conclusively traced to the loss of ozone in spring over Antarctica, which allowed for greater penetration of ultraviolet light, warming the troposphere where we live and cooling the stratosphere above. This may seem distant from human affairs, but the Hadley Cell changes reduced rainfall in semi-arid areas, putting at risk the livelihood of millions of farmers struggling to grow crops on marginal land.
In Nature, Banerjee's team report that trends in the three most important meteorological consequences of the hole all stopped around the year 2000. There's been plenty of bouncing around since then (the years 2000-2005 saw a rebound on all three, which was almost perfectly erased up until 2010). However, we now have a long enough sample to smooth out short-term fluctuations and a pause is clear. Whether a bounce back has begun is less certain, but the authors write that “The consequences of the Montreal Protocol for tropospheric circulation should become more apparent in the future.”
However, the paper warns that greenhouse gasses also affect the same wind systems often in the same way as ozone depletion and emissions may induce shifts powerful enough to overwhelm any recovery. “The answer will depend on CO2 emissions trajectory,” they write. Just like everything else.