But what we haven’t understood well, apparently, is how the produced ozone is redistributed or, potentially, destroyed by other processes. While there are certainly puzzles concerning what the ozone in Earth’s atmosphere is doing, the past 20 years of data have shown one thing definitively: it hasn’t simply remained constant. (At least, over the latitudes which have been carefully monitored.) From 1998 until 2011, the total ozone column density actually rose, only to fall again back to 1998 levels over the past few years. There is neither a good theory nor a good empirical model for why this is happening, but it will no doubt become one of atmospheric science’s biggest open questions.
The mystery deepens if we look at the lowest layer of Earth’s atmosphere: the troposphere. This layer, consisting of the closest few kilometers of atmosphere to our surface (and over 80% of the Earth’s atmosphere, by mass), has shown an increase in ozone density. Admittedly, the data has only been available with global coverage for approximately 12-13 years, but it’s quite convincing: it shows that the ozone density in the lowest layers is rising, just like it is in the upper stratosphere. This makes what’s occurring in the mid-to-lower stratosphere all the more puzzling.
There are five major conclusions to come out of this work, some (but not all) of which are promising:
- The Montreal Protocol continues to demonstrate its effectiveness for increasing ozone density in the upper stratosphere, as predicted.
- Mysteriously, the lower stratosphere has shown a larger-magnitude ozone decline over the same time period.
- Overall, the global mid-latitude ozone density in the stratosphere has slightly decreased, as the lower stratosphere effect has been slightly more powerful.
- If you add the tropospheric increases in, the total ozone density has only remained relatively constant.
- And finally, the state-of-the-art models do not reproduce the observed ozone levels in the lower layers of the atmosphere.
Although the study that drew these conclusions doesn’t have a surefire explanation for this result, there are two possible culprits. One is very short-lived substances (VSLS) that could be destroying portions of the ozone layer; research into that is ongoing. But the second possibility is one that no one’s happy about: global warming.