The most successful environmental international treaty was the Montreal Protocol which banned CFC's, chlorofluorocarbons, which were once hailed almost universally as a godsend, sort of, um, like the car (and lately the electric or electric hybrid car).
Sigh...
Of course, this treaty was not an immediate ban on the production of CFC's, nor did it make all of the CFC's in automotive air conditioners, old refrigerators which remained operational over decades, discarded hair spray cans or other sources disappear. In fact much of the bulk of CFC's manufactured before the treaty began to take effect are still on the planet, and are still in the atmosphere. In some sense, children being born today are still dealing with that stuff. Nor should it be claimed that no one cheats on the treaty, nor that the treaty was truly comprehensive. It is still legal in some places to apply methyl bromide, for instance, to strawberry crops. Moreover the half-life of many CFC's is measured in centuries, and in some cases millenia, and they are catalytic destroyers of ozone, meaning that one molecule of a CFC can destroy hundreds of billions of ozone molecules before it is itself is destroyed by forming phosgene, fluorophosgene, fluorochlorophosgene or some other decay product.
Famously phosgene was a war gas widely used by both sides in the first world war. (It is also a very useful synthetic intermediate, but that's another question...)
A report in the current edition of the prestigious scientific journal Nature shows that for the first time, a measurable ozone hole has opened in the Northern hemisphere.
Previously...
...all observed "ozone holes" were observed in the Southern Hemisphere.
There are many reasons why this is the case, the most obvious being that the Southern Pole is on a continental landmass, resulting in colder temperatures there than are found in the Northern Pole.
But...
Well, I'll just produce some excerpts from the paper, Nature, , Unprecedented Arctic ozone loss in 2011 Volume: 478, Pages: 469–475 Date published: (27 October 2011)...
Chemical ozone destruction occurs over both polar regions in local winter–spring. In the Antarctic, essentially complete removal of lower-stratospheric ozone currently results in an ozone hole every year, whereas in the Arctic, ozone loss is highly variable and has until now been much more limited. Here we demonstrate that chemical ozone destruction over the Arctic in early 2011 was—for the first time in the observational record—comparable to that in the Antarctic ozone hole. Unusually long-lasting cold conditions in the Arctic lower stratosphere led to persistent enhancement in ozone-destroying forms of chlorine and to unprecedented ozone loss, which exceeded 80 per cent over 18–20 kilometres altitude. Our results show that Arctic ozone holes are possible even with temperatures much milder than those in the Antarctic. We cannot at present predict when such severe Arctic ozone depletion may be matched or exceeded...
Some technical stuff:
Trace gas profile measurements of HNO3, HCl, ClO, ozone and N2O (a long-lived tracer used to assess descent) are from Aura MLS45 version 3 retrievals; data quality screening is as recommended in the MLS data quality document56. MLS data are retrieved on pressure surfaces; potential temperature as a function of pressure from MLS DMPs52 calculated from GEOS-5 analyses is used to interpolate to isentropic surfaces. Vortex averages of MLS data are calculated using the 1.4 × 10−4 s−1 scaled PV contour to define the vortex edge, using PV values from the MLS DMPs52. Active chlorine is in the form of ClO mainly during the daytime, and thus measured ClO amounts vary with the solar zenith angle (SZA) at which the measurements are taken. Only daytime ClO measurements are used here. Northern high latitudes are sampled near midday local time, southern high latitudes are sampled in late afternoon, thus the SZA of Aura MLS Antarctic measurements is ~7° higher on average than that in the Arctic. Reactive chlorine partitioning shifts away from ClO at higher SZAs7, 12, leading to ~30% lower ClO measured by Aura MLS in the Antarctic than in the Arctic under fully activated conditions. MLS measurements are unavailable from 27 March through to 20 April 2011 because of an instrument anomaly. Upper Atmosphere Research Satellite (UARS) MLS measurements, used for analysis of 1995–96 and 1996–97, are sparse because of the UARS yaw cycle and other measurement gaps26. The time of day of UARS measurements varied through the yaw cycle, in the middle of which no daytime ClO measurements were obtained10; thus ClO values shown in 1995–96 and 1996–97 near those dates (including the mid-February 1996 measurements shown in Fig. 2g) are not representative of the degree of chlorine activation...
Actually N2O is not really a tracer gas, no matter what they say in the paper about it being a "long lived species unaffected by chemical processes."
It is, in its own right, a catalyst for ozone destruction. Moreover, its accumulation in the planetary atmosphere is not easily solved by, for instance, changing the technology of automotive air conditioners.
It is produced by something more difficult to ban, um, agriculture
Don't worry, be happy.
HCl by the way, represents the actual decay of a chlorofluorocarbon. Although it's corrosive, and is a constituent (albeit a minor one) of so called "acid rain" its presence in the atmosphere and conditions underwhich it rains out (before reforming a new CFC) is probably a good thing, since it represents the loss of a CFC molecule. Nevertheless it will be tens of thousands of years, if ever, before CFC levels return to those approximating their levels at the middle of the last century.
In fact, as a chemist who has worked with it, I never thought I'd be cheering for atmospheric phosgene, but I do in fact.
Have a great day tomorrow.