Something in the air
In September, the world celebrated the 25th anniversary of the signing of the Montreal Protocol, a global accord to protect the ozone layer. Dave Hansford visits NIWA's Lauder Atmospheric Research Station, where much of what we know about this vital safety blanket was discovered.
Dr Richard McKenzie has one hell of an office. It's maybe 30 kilometres of placid verdure to the north wall, where the Hawkdun Mountains rise, glinting white, then glide gently west, melding seamlessly, it appears, into the St Bathans Range.
From there, a quarter-turn introduces the clean lines of the Dunstans – a crane of the neck to see all the way down the Manuherikia Valley, back towards Alexandra. And above it all, a classic, crystalline Otago sky – so big you have to look to find the sun.
After 30 years, you might think McKenzie would have stopped noticing it all, but that vast firmament is why he's here, at NIWA's Atmospheric Research Station near Lauder. I look up and see blue: the Atmospheric Scientist Emeritus sees the physical essence of the heavens – cocktails of gases – nitrogen, oxygen, argon, neon, helium. Molecules in endless pas-de-deux. Boundaries – hard yet diaphanous – set by subtleties like temperature, pressure, density. He knows that seeming blue serenity is just a trick of the light. It's a maelstrom up there: shrieking stratospheric winds, withering radiation, extreme chemistry.
But mostly, he's focussed on a sparse scattering of molecules some 20 or 30 kilometres above our heads. When ultraviolet (UV) light strikes an oxygen molecule (O2), it can cleave the two oxygen atoms apart. These estranged atoms float alone until they encounter another intact oxygen molecule. When they bind to that molecule, they create ozone (O3).
But the union is fleeting. When struck by UV, the ozone molecule splits once more, back into an O2 molecule, and an atom of atomic oxygen. This eternal seesaw of fusion and nihilism maintains the ozone layer, and you should be very glad it does. Around the immensity of the earth's atmosphere, the ozone layer is but a sheet of glad wrap – stratospheric ozone concentrations comprise around 10 parts per million – but without it, those lethal UV rays would kill pretty much everything.
McKenzie only has to glance at his computer to confirm that this mid-September day, above Lauder, the ozone layer is exactly 370 Dobson Units thick. It's a good solid shield – the global average is 300, which, if you brought all those ozone molecules down to ground level, at standard pressure and temperature, represents just three thin millimetres of armour between you and blistering harm.
But in just a few weeks, that shield will thin over Antarctica to less than 100 Dobson Units, as the ozone layer approaches its annual minimum. Most of us know that the ozone hole yawns wide over Antarctica each spring, torn open by chlorinated fluorocarbons (CFCs) emitted from our (and our parents') refrigerators, spray cans, air conditioners, foaming agents and other chemical conveniences. We might recall that politicians got together back in the eighties and signed some agreement to ban the CFCs, and that now the hole is slowly healing again.
What we don't generally appreciate is that McKenzie and his colleagues – taking meticulous measurements every day for thirty years, launching balloons, firing lasers, crunching numbers, analysing spectrograms, writing reports and papers, making presentations – have helped save hundreds of thousands of lives. One of them might have been yours.
Lauder scientists contributed to one study which concluded that, by 2030, the Montreal Protocol will prevent two million new cases of skin cancer worldwide each year.
The Protocol not only got chlorines and other ozone-killing substances banned, but it's done a good job of enforcing compliance, so that global skin cancer rates are now projected to peak around the middle of this century, then decline.
Without the international agreement, says McKenzie, "by 2030, the incidence rates [of skin cancer] would have increased, so that instead of approximately 300 people dying each year in New Zealand, it would have been 350."
But that's only the start of the salvation, he says, "because UV levels would have really rocketed up in the latter part of the century. By 2065, at the end of our model runs, UV levels in New Zealand would have been three times higher than they are now." By then, he says, serious skin damage could have been done in the time it took you to cross the street.
Beyond that, "hundreds, or even thousands, of extra deaths per year would have followed the exposures they got in 2065."
Eye on the sky
Instead, thanks to a complicated relationship between ozone and climate change, ozone may even recover beyond its original levels – in middle and high latitudes at least. That means that, by the end of this century, skin cancer cases could drop to below 1960s rates.
McKenzie came here to Lauder late in 1979, to study auroras. He meant to stay three years, until his son started school. "Andrew's now 36," he grins, as he shows me around a rooftop array of technologia. Domed all-sky cameras stare at the sun, blinking as they automatically record cloud cover at timed intervals. There's a quiet whir of servos as pyranometers, pyrheliometers, pyrgeometers and sunphotometers track the sun like robotic sunflowers, measuring radiation – diffuse, direct, longwave, filtered – as part of the international Baseline Surface Radiation Network.
Not all these devices are NIWA's. Science agencies from around the world have placed instruments at Lauder, because of the unsullied clarity of the skies here. "You can see things in the stratosphere here that you don't see at other sites," brags McKenzie. "Because the troposphere (the lowest 10–15 kilometres of sky) is so clean, the light from the sun through the atmosphere carries the imprint of the stratosphere, rather than the troposphere."
If Lauder's taught us one thing, it's the value of observational data – the sheer wisdom of watchfulness. When McKenzie tired of aurora nearly three decades ago and started measuring nitrogen oxides in the atmosphere, he had no idea of the eventual value of that work.
"The atmosphere is about 80 per cent nitrogen," says McKenzie, "and 20 per cent oxygen, so nitrogen oxides are important." After the ozone hole was discovered in 1985, theories pointed fingers at various suspects, including nitrogen oxides. "Our measurements here and in Antarctica," recalls McKenzie, "showed that theory was wrong."
Crucially, that turned the spotlight instead onto chlorine and bromine – components of CFCs. "Scientists then realised that the ozone hole had been getting bigger and bigger, as the concentrations of chlorine had been getting bigger and bigger."
McKenzie immediately grasped the implications. "We already had the world's highest death rate from melanoma. If ozone does go down, UV is going to go up. That became the focus of my research."
Over the long dark of the Antarctic winter, ozone is largely inert: without light, explains McKenzie, there's no UV radiation either, so there's a temporary ceasefire. But once the sunlight returns each spring, ozone-killing chemical reactions rekindle, playing out on the ice crystals that form polar stratospheric clouds. The intense cold drives the conversion of unreactive chlorine to reactive forms at breakneck speeds, opening a hole that reaches maximum width each October. Fierce, spinning polar winds contain the hole in what is essentially a giant, closed reaction vessel.
Ordinarily, such conditions only occur over Antarctica, but in 2011 it happened over the Arctic too.
But in one perverse sense, the ozone hole actually works in our favour here in New Zealand. As long as those polar winds are whirling, they keep ozone out as effectively as they trap air in. So when ozone produced in the tropics is dragged south by atmospheric circulation, it starts piling up at higher latitudes – right over New Zealand, even as the ozone layer hangs in tatters over Antarctica (our ozone minimum occurs instead in the autumn).
CFC production ended in 1996, and scientists have watched the ozone layer respond ever since. Lauder's Programme Leader of the New Zealand Regional Atmosphere Programme, Dr Olaf Morgenstern, says levels over Antarctica are projected to heal to 1980 levels "sometime between 2050 and 2070. In the Northern Hemisphere, that return might come 20 years earlier or so." But, he says, "It won't recover to the state that we have seen it in before, because the atmosphere has changed since then."
Mathematical models, fed a 'business as usual' scenario without the Montreal Protocol, says Morgenstern, have shown that we would have "fallen off a cliff, essentially." All the extra CFCs emitted, "would rival CO₂ as the leading climate agent." Their stratospheric cooling effect, exacerbated by runaway ozone depletion, would have seen polar-stratospheric clouds billow across the tropics, causing what Morgenstern calls "precipitous loss of ozone ... sufficient to condemn us to oblivion."
Around Lauder, the UV index typically peaks at 12. Untrammelled CFC production, he says, could have seen it hit 50 by the end of this century. "Under such conditions, you couldn't move out of the house any more, and you probably couldn't grow any plants."
Climate of uncertainty
Morgenstern's job is to predict what ozone recovery might mean for climate change, and vice-versa, and it's far from simple. Greenhouse gases trap warmth inside the Earth's atmosphere, but prevent it escaping into the stratosphere, which is duly getting colder – scientists say temperature changes there could be twice the magnitude of those near the Earth's surface: a one-degree increase down here equals a two-degree decrease in the stratosphere. The risk, then, from global warming is that those polar-stratospheric clouds might yet extend beyond the poles, undoing some of the ozone recovery that has begun.
"Over the last 50 years or so," says Morgenstern, "we've seen some quite dramatic climate change over Antarctica. The interior has cooled, but the Antarctic Peninsula is one of the fastest-warming regions on the planet. People now think that ozone depletion is the cause of that sharp contrast." Recent papers have suggested that effect may even influence climate all the way to the Equator.
As ozone levels are restored, he says, the trends in familiar climate traits – prevailing winds, the timing of rains, etc. – could alter, or even reverse. The cooling effect could switch, and the Antarctic continent might begin melting apace, as the Arctic is already doing.
"At the same time, of course, we're releasing more and more greenhouse gases into the air. They also have an effect," but it remains to be seen, he says, which one will predominate in the next few decades.
That's perhaps because most contemporary climate models don't explicitly include ozone chemistry – which may, suggests Morgenstern, limit their power to capture the detail of regional climate change. His quest is a mathematical climate model that includes – 'understands' – atmospheric ozone chemistry. Such a model could predict changes in trace gas concentrations and how they might affect the Earth's radiation balance – and show us what that could mean for temperatures, winds and transport processes.
NIWA's Fitzroy supercomputer now gives us the means, says Morgenstern, to make that leap. "At the moment, I'm running a chemistry climate model over the 19th century. It has comprehensive chemistry and a fully coupled ocean in it."
He looks out over the array of instruments, quietly taking the pulse of the sky above, checking its vital signs. "Many people don't understand why we take all these measurements," he says, "but they're the bedrock on which we rest our knowledge.
"People really want to know how their world is going to change. What's the prospect for agriculture in this country? Is it going to get drier or wetter? Warmer or colder? They want to know about extreme weather. In order to get that, you need better models – more comprehensive physics, chemistry, higher resolution.
"But you can't verify a model if you don't have data. If you can't establish that your model is doing alright, you can't say anything about the quality of your forecasts. So we have to continue with the measurements here."
McKenzie agrees. "The world's full of surprises. The ozone hole was completely unexpected. If there hadn't been people on the ground making all those measurements, it might have been too late."
There aren't many bits of land at these latitudes, he points out, so data from New Zealand is prime intelligence. "We represent a big chunk of the globe, and we can't rely on other people solving this problem for us." Meticulous monitoring might not be fashionable, he says, "but it's necessary."
The elephant in the atmosphere
It might have stretched over 1.1 million square kilometres at the time, but the ozone hole proved difficult to spot in the early eighties. We'd have known about it much sooner, had scientists just believed their own eyes. NASA satellites had been measuring atmospheric ozone since 1979, faithfully reporting incremental losses. However, the data came in faster than the scientists could analyse it, so they set up a processing programme which filtered out any measurements above - or, crucially, below - what they considered instruments could accurately process.
Says Dr Richard McKenzie, the satellites recorded losses so large, NASA's algorithms rejected them as bad data. "We all do this", he points out. "We all have rejection criteria in our data. They felt there must be something wrong, so they chucked out what they considered to be bogus data. It was very embarrassing for them."
Then, he says, the very first mention of the discovery may well have been lost in translation. "It was actually first reported at a conference in 1982 in Greece," he recalls. McKenzie missed the presentation in which Dr Shigeru Cubachi, of Japan's Meteorological Research Institute, reported low ozone measurements over Syowa, Antarctica. But he says that Cubachi's heavily-accented English may obscured the true significance of what he had witnessed. "So no one picked it up at the time, but you've got to be very careful to acknowledge that Japanese work."
Meanwhile, British Antarctic Survey scientist Joseph Farman had been collecting atmospheric data from Halley Bay, near the South Pole, since 1957. In 1982, his ozone readings dipped dramatically, by some 40 per cent. Like NASA, he figured the data must be dubious, and blamed his aging, temperamental spectrophotometer. Besides, he reasoned, if ozone levels had dipped that much, surely NASA's satellites would have detected the trend? Nevertheless, he ordered a new instrument. Next season, he and his colleagues found that ozone levels had fallen still further.
When they went back through old data, they realised that the slump had begun back in 1977. To make sure it wasn't a purely local phenomenon, they shifted the spectrophotometer the next year to a new site, 1000 kilometres from Halley Bay, where they recorded the same persistent trend. They decided to publish.
In may 1985, Farman and his colleagues Brian Gardiner and Jonathan Shanklin described their observations in Nature. They went on to point out the connection to increasing atmospheric CFC levels. NASA went back through their deleted satellite data, which soon corroborated the British observations. Farman et al.'s paper ranks among the most cited articles of all time.
The ozone hole was official.
Un-natural born killers – chlorine and bromine
In 1971, James Lovelock, proponent of the Gaia hypothesis, noted that the bulk of CFCs created since 1930 were still present in the atmosphere.
In 1995, Sherwood Rowland, Paul Crutzen and Mario Molina won the Nobel Prize for showing that convective air currents could carry CFCs all the way to the stratosphere, where, split by high-energy photons from sunlight, they could free their chlorine atoms.
Unleashed, chlorine – along with bromine (from the halons used in some old fire extinguishers, and from the fumigant methyl bromide, still used in New Zealand), and other accomplices like nitrous oxide destroy ozone on contact through a series of catalytic reactions. A single chlorine or bromine atom can persist for a century, destroying many thousands of ozone molecules.