In a couple of weeks’ time humanity can celebrate the 25-year anniversary of the Montreal Protocol, our greatest environmental success story.
In the margins of this good news there are however also a few new ozone concerns. Climate change may slow down some of the recovery of the ozone layer – and it may also lead to the formation of dangerous new ozone holes over inhabited areas.
And apparently it can do so through two different routes, new research by a group of scientists of Harvard University shows.
What do we actually mean with ´climate change´?
The climate changes because increasing concentrations of greenhouse gases in the troposphere trap more heat. A more accurate term for this climate change would therefore be ‘tropospheric warming’ – which has far-reaching climatological (and therefore meteorological) consequences.
Now the troposphere is the lowest and densest layer of the Earth’s atmosphere, with an average thickness of some 17 kilometers, more in the tropics, less over the poles. It is not only where 80 percent of the atmospheric mass is contained, also where 99 percent of aerosols float around, where we live and practically all of the weather and biology happens.
This not only means the troposphere is important, but also that ‘tropospheric warming’ almost equates to climatic warming in general.
GHG-induced climate change means tropospheric warming, but also stratospheric cooling
But there is good reason not to ignore ‘the rest of Earth’s atmosphere’ – most notably the lower stratosphere, which harbours that all-important ozone layer, our planetary ultraviolet radiation filter.
We’ve learned that another consequence of increased solar heat absorption in the troposphere is that the above-lying atmospheric layer, the stratosphere, becomes more isolated from the heat source of Earth’s surface.
Therefore a consequence of the main current climate change is not only tropospheric warming, but also stratospheric cooling, which is also measurable – and which disproves the solar hypothesis many climate skeptics still cling to.
We’ve also learned that ozone breakdown in the stratospheric ozone layer is promoted by low temperatures and there is actually a critical (extremely cold) temperature threshold to pass before CFC and other ozone killers can start of the catalytic reaction – which is why the scientific community has first witnessed ozone holes over Antarctica (in early spring, because sunlight is another requisite).
It is also why, as the boreal stratosphere seems to be cooling now, science witnesses the formation of ozone holes over the Arctic as well – an increasing phenomenon.
Arctic ozone holes can be quite dangerous as they can potentially migrate south to reach for instance populated areas of northern Europe.
The thunderstorm ozone connection
With – thanks to Montreal – atmospheric chlorofluorocarbon concentrations now slowly decreasing it seems safe to say that at least part of the recurring ozone depletion is caused by global warming – along the route of stratospheric cooling, the first route it seems.
Now another climatological consequence of elevated GHG levels we´ve recently paid attention to at Bits of Science is the increase of thunderstorms.
More and larger thunderstorms accelerates warming, but that’s not all there is to them – they also create stronger convection, convection that may break through the tropopause to reach stratospheric heights – and add water to the stratosphere.
And next to cold air, sunlight and CFCs the presence of water is another requirement for catalytic ozone breakdown the Harvard team warns – something they’ve actually observed over the US. That is however in the classical ozone breakdown reaction, which requires the CFCs, the cold air, and water as ice crystals, in stratospheric polar clouds. At lower latitudes this may not be the main concern. Here stratospheric water can also directly damage the ozone layer, perhaps also in seasons when we would most want its UV protection.
Water vapor injected into the stratosphere by powerful thunderstorms converts stable forms of chlorine and bromine into free radicals capable of transforming ozone molecules into oxygen, the Harvard release simply puts the mechanism.
And in their Science publication the researchers state the following: “The observed presence of water vapor convectively injected deep into the stratosphere over the United States fundamentally changes the catalytic chlorine/bromine free radical chemistry of the lower stratosphere by shifting total available inorganic chlorine into the catalytically active free-radical form, ClO. This chemical shift markedly affects total ozone loss rates and makes the catalytic system extraordinarily sensitive to convective injection into the mid-latitude lower stratosphere in summer.”
Increased risk of ozone loss equates to increased risk of UV damage
If indeed continued tropospheric warming would increase thunderstorm formation over the US then “increased risk of ozone loss and associated increases in UV dosage would follow”.
For various reasons skin cancer rates have been increasing in recent decades and years in many western countries. If climate change has not yet been one of these factors thus far, it may still further amplify that development.
© Rolf Schuttenhelm | www.bitsofscience.org