Clouding our Skies May Save the Planet, But ...
Why I don't like that idea ....
The plot below, buried deep in chapter 7 of the most recent IPCC Assessment Report (AR6), shows estimates of the various contributors to changes in global air temperatures since the start of the industrial revolution.
The thin and thick black lines nearly overlay each other, showing that the overall increase is attributable to mankind. A comparison of those with the purple line confirms that most of that 1.3C increase can be attributed to increases in carbon dioxide. Methane (CH4) and nitrous oxide (N2O) are also important - and highly relevant in a new Zealand context. But there are several other players too. Tropospheric ozone and the gases that destroy it in the stratosphere - including the ‘halogenated’ gases controlled by the Montreal Protocol - also have a significant warming effect. In fact, curbing their production in recent times has averted around 0.2C of warming (of course no warming from those would have occurred if those man-made gases hadn’t been produced in the first place!).
But here I want to focus on the effects of aerosols. Aerosols are just clusters of particles or molecules suspended in the air. Their significance here is they can scatter radiation - both incoming solar radiation and outgoing infrared radiation. The graph clearly shows that made-made aerosols from pollution (grey line) as well as volcanic aerosols (green line) can have significant cooling effects. That’s why the deliberate injection of aerosols into the stratosphere is being considered as way to limit global warming. Scarily, the geo-engineering option has even got its own name now - and an acronym to boot! It’s called Stratospheric Aerosol Injection (SAI).
I hate the idea. Wouldn’t it be better to fix the problem at source by reducing our dependence on burning fossil fuels?
Injection of aerosols of the stratosphere may divert enough sunlight to help slow global warming. But at what cost? And who pays that cost? The main source of the problem is pollution from the northern hemisphere, but the downside will be especially large in the southern hemisphere. Increased aerosol in the stratosphere will lead to ozone depletion, and just as in the past with the Antarctic ozone hole, effects will be largest in the southern hemisphere. But my main gripe is with the effect on light quality. Movie makers flock to New Zealand to take advantage of its stunning backdrops. But it’s not just their physical form that makes the landscapes so attractive. It’s also the light that bathes them. Our measurements at Lauder show that the air in rural New Zealand is ten times cleaner that what’s considered clean air at comparable latitudes in the northern hemisphere. That means a much larger fraction of the light we see comes directly from the sun without being scattered by aerosols. There’s much less diffuse light. And that adds drama by deepening shadows preserving colour vibrancy.
It has to be said that deep shadows also present challenges when televising sporting events like rugby from outdoor stadiums on bright sunny days because the exposure levels required for shaded areas of the pitch are so different from sunlit areas. But that’s a price I’m willing to pay. It’s much less of a challenge in Europe than for matches in New Zealand, Australia South Africa. But, just like their rugby, the spectacle in Europe is much flatter.
In passing I must add that shadows are always less deep in the UV region than for visible light. That’s because even clean air scatters light much more efficiently at those shorter wavelengths. It’s like living in a perpetual fog. There’s an important health consequence too. Your UV protection from shade is less than you’d guess from the sharp-edged shadow appearance you see. For good protection against UV, make sure most of the entire sky is also obscured.
The photo below was taken near Dunedin overlooking an isolated beach: just one of many on the beautiful undersold coastline of Otago.
The version below is my clumsy attempt to show how the same view might look after the aerosol injection. Flat and Lifeless in comparison. Actually, it would be worse. The contrast between the clouds and blue sky would be replaced by a milky grey blancmange. I know which version I prefer. And I know that version will bring the movie makers.
There’s an ethical question here. Who gets to decide whether my views will be spoiled? A faceless bureaucrat in Geneva or Washington DC doesn’t own the Aotearoa sky? Or worse?
Thanks for reading this. Previous posts on the intersection between Ozone, UV, Climate, and Health can be found at my UV & You area at Substack. Click below to subscribe for occasional free updates.