Throughout the history of life on Earth, there’s been a little-noticed helper: a thin but important layer of ozone in our planet’s stratosphere. Transparent to visible light, this trioxygen molecule isn’t the type you breathe, but rather successfully absorbs incoming high-energy ultraviolet light. Without the ozone layer, this light would propagate down to the surface, where it’s capable of breaking organic bonds and working to counteract the natural life processes we hold so dear. Inadvertently, the widespread rise in chlorofluorocarbons (CFCs) and their use in aerosol cans began to destroy the protective ozone layer, and some 30 years ago, humanity banded together to virtually eliminate CFC use. We thought the hole would close and the problem would solve itself. But a new study, surveying a part of the ozone layer that hadn’t been examined before, shows that the overall problem hasn’t improved in 20 years.
Ultraviolet radiation is known to be dangerous, and our stratospheric ozone is our first line of defense. With the widespread adoption of and compliance with the Montreal Protocol, atmospheric ozone stopped decreasing, and measurements of the upper stratosphere indicated that ozone levels were recovering. The recovery was so significant that the leading models predicted a 100% recovery at most human-populated latitudes by the year 2100. But one unknown had yet to be investigated to the required level of scrutiny: the concentration of ozone at lower altitudes. Contrary to expectations and with no explanation for how it’s happened, the lower stratosphere appears to be losing ozone, so much so that the total amount of ozone over the most densely populated areas isn’t increasing at all.
In what promises to be the first unexpected result from atmospheric sciences in 2018, a team of researchers gathered four different datasets that have been monitoring the higher altitudes of Earth’s atmosphere, and analyzed them for changes in ozone concentrations. While the upper stratosphere showed the same increases in ozone densities, the lower stratosphere, carefully analyzed for the first time, showed the opposite effect. This is something none of the best ozone-layer models, successful as they are for other applications, were able to predict. According to Will Ball, the lead author on the new study,
The reason for the continuing decline is not fully understood, but could be a result of our changing climate, increases in unregulated short-lived chlorine species, or some as yet unknown factor, but chemistry climate models do not reproduce the current changes we find.
In fact, if you quantify the amount that ozone concentrations, overall, have changed, you find that the amount that ozone in the lower stratosphere has decreased virtually cancels out the increases seen in the other layers. This is an unexpected puzzle, since we understand how ozone is naturally produced in the stratosphere: by the same two ingredients — oxygen and ultraviolet light — that have always produced it. When ultraviolet light strikes an oxygen molecule, it breaks it up into two individual oxygen atoms. Each one can then react with another oxygen molecule, producing ozone molecules, which ought to remain in the stratosphere: where production peaks.