快猫短视频

STANDING by the side of a snow-covered road, just before sunset, I know what
to look for. 鈥淭hey鈥檙e like the inside of an abalone shell,鈥� I鈥檇 been told.
鈥淏eautiful, lens-shaped clouds. But you have to look carefully. You have to get
your eye in.鈥�

In the event, there is no mistaking them. As the Sun dips over the horizon, a
mass of tear-shaped clouds appears from nowhere. They are petrol blue and green,
rimmed with vibrant pink鈥攍urid colours that have no business in a sunset.
Against the monochrome backdrop of snow and forest, they are shocking.

At 20 kilometres or more above the ground, the clouds lie far above the
world鈥檚 normal weather patterns. You鈥檒l find them only at the highest latitudes,
like here in Swedish Lapland over the tiny, frozen town of Kiruna, 200
kilometres north of the Arctic Circle. These clouds are natural鈥攂ut they
are also dangerous. At the other end of the world it鈥檚 clouds like these that
trigger the infamous Antarctic ozone hole each southern spring.

So far, the much more populous northern hemisphere has been spared a similar
fate. The ozone layer thins out each spring but so far it has never disappeared.
You would think, then, that the north should now be safe. Broadly speaking we
have curbed the use of chlorofluorocarbons (CFCs), the chemicals that do the
damage. Surely things can only get better.

Unfortunately, this isn鈥檛 the whole story. By a strange twist of fate, it
seems the ozone layer could be under attack from that other great environmental
bugbear, greenhouse warming. 鈥淲hen I tell people I work on ozone, everyone
always asks me `Is that related to the greenhouse?'鈥� says Ross Salawitch from
NASA鈥檚 Jet Propulsion Laboratory in Pasadena. 鈥淚 always have to say, `No, it鈥檚 a
separate problem.鈥� But we鈥檙e now realising that there are subtle reasons why
they might be connected.鈥�

It鈥檚 fear of this environmental double whammy that has prompted the European
Commission and NASA to set up a joint study of the ozone layer. NASA has stumped
up $20 million and a similar amount has come from the EC. It is the
largest ever field campaign for stratospheric ozone: since December 1999 more
than 330 scientists from 19 countries have passed through Kiruna.

Back in the car, I鈥檓 heading for Esrange, a launch facility 35 kilometres
from town, which belongs to the Swedish Space Corporation. It鈥檚 here that the
researchers launch their balloons, in a frozen square field surrounded by a dark
forest of birch, spruce and pine. This afternoon鈥檚 balloon is already hanging
limply in the air, thirty metres above the snow, with tens of metres of material
still draped along the ground. Over to the left, the gondola, bristling with
instruments, is slowly being raised by a small secondary balloon. Without this
floating start it would risk being dragged along the ground and smashed to
pieces when the main balloon goes aloft.

Those iridescent clouds are still visible, continuing proof that the air
overhead is intensely cold. Unlike the sodden troposphere鈥攚hich is where
we live, and where clouds and weather patterns usually form鈥攖he next layer
up, the stratosphere, is as dry as a bone. But make it cold enough and you can
squeeze water vapour out of its arid air to make clouds.

It is these polar stratospheric clouds (PSCs) that are responsible for most
of the ozone depletion. They are where harmless, stable chlorine compounds
derived from CFCs are transformed into their rapacious ozone-destroying forms.
Even in polar regions, PSCs only appear in winter, when the cold air sinks and
is whipped around the pole like a giant tornado. Furious winds at the edge of
this vortex seal off the air inside, isolating it from warmer influences. If the
temperature in the stratosphere falls to 鈥�78 掳C or so, the PSCs
appear, and their tiny uniform particles scatter sunlight into dazzling
colours.

Arctic ozone depletion is so much less serious than in the Antarctic simply
because the northern stratosphere is not as cold. The Arctic vortex is much more
disturbed and 鈥渓eaky鈥� than its southern cousin, allowing the temperatures within
it to rise. This disturbance is caused by giant atmospheric waves called
planetary waves, created when air flows over mountains or when it experiences a
sudden temperature difference between land and sea. Once formed, the waves move
upwards to perturb the stratosphere.

鈥淚t鈥檚 a bit like water waves moving up a beach,鈥� says Alan Plumb, an
atmospheric modeller from the Massachusetts Institute of Technology who is a
member of the American contingent at Kiruna. 鈥淎s the water gets shallower the
energy of the waves is concentrated into a smaller amount of water and so they
become steeper and break.鈥� Similarly, as planetary waves rise into the thin air
of the stratosphere they become bigger and bigger until they eventually break,
dumping their energy and tearing at the vortex. In the southern hemisphere,
there are fewer continents and mountains to create planetary waves, so the
Antarctic vortex is left relatively undisturbed. 鈥淎ll the ozone in a 5-kilometre
height range is destroyed,鈥� says Neil Harris, head of the European Ozone
Coordinating Unit in Cambridge, who is organising the Esrange launches. 鈥淲e鈥檙e
not at that stage in the Arctic鈥攜et.鈥�

But if cold is bad and warm is good, how can global warming make things
worse? The problem is that greenhouse gases like carbon dioxide only warm the
troposphere. 鈥淭he CO₂ acts as a blanket to trap the heat in the lower
atmosphere,鈥� says Salawitch. 鈥淭o balance the books, the stratosphere has to
肠辞辞濒.鈥�
(See 快猫短视频, 1 May 1999, p 28.)

Climate modellers began to suspect there was a link between greenhouse
warming and ozone depletion in 1992, when John Austin of Britain鈥檚
Meteorological Office in Bracknell and his colleagues published a paper in
Nature (vol 360, p 221) warning that extra CO₂ could herald an
Arctic ozone hole. However, their model runs called for massive quantities of
CO₂to be present in the atmosphere鈥攁mounts we鈥檙e unlikely to
reach before we have reduced the concentrations of chlorine compounds in the
atmosphere to safe levels.

More worrying was a paper published in 1998 by Drew Shindell and colleagues
at NASA鈥檚 Goddard Institute for Space Studies in New York (Nature, vol
392, p 589). They found that the tropical warming produced by more realistic
amounts of CO₂ would, according to their model, divert the planetary
waves in the north, preventing them from disturbing the vortex. The upshot: a
colder and more stable vortex above the Arctic that could trigger an
Antarctic-style ozone hole.

Could this be happening now? The best we can say so far is 鈥減erhaps鈥�. When
Salawitch was asked to write a comment about Shindell鈥檚 paper in the same issue
of Nature (p 551), he drew up a graph comparing temperatures in the
Arctic with the amount of ozone loss over the previous 18 years. 鈥淲hen I first
did this plot, I was shocked at the way it went,鈥� he says. 鈥淲ith data only to
1997, it looks like we鈥檙e falling off a cliff.鈥� Both the stratospheric
temperatures and ozone seemed to be plummeting, just as Shindell had predicted.
But the pattern didn鈥檛 continue: 1998 and 1999 were unusually warm and had very
high ozone levels.

This year, however, looks set to be another bad one. The PSCs have persisted,
temperature and ozone have plummeted, and though the satellite data show that
total ozone still hasn鈥檛 dropped as low as 1997 levels, March isn鈥檛 over
yet.

One thing these changes from year to year show is that the Arctic seems to be
much more sensitive to temperature changes than its southern counterpart. 鈥淭he
Antarctic vortex is a sledgehammer,鈥� says Jim Anderson, a Harvard University
chemist. 鈥淭he subtleties are just obliterated. The Arctic is a profoundly more
subtle system. And it reacts so fast it鈥檚 like a cobra in a box.鈥� In 1997, for
instance, ozone loss was as high as 50 per cent at some altitudes whereas in
1998 there was almost no loss. The only difference was a few degrees of
cooling.

This suggests that even if Shindell鈥檚 mechanism produced just a small amount
of stratospheric cooling, it would still make a big difference. That鈥檚 why
researchers need to learn more about what鈥檚 happening high above the Arctic.
They need to know what chemicals are present, how fast the reactions run, and
under what conditions the PSCs form. That is why they are coming to Kiruna with
balloons, satellites and planes: to probe the Arctic stratosphere as it鈥檚 never
been probed before.

The balloon that鈥檚 about to fly will measure everything it can about the PSCs
overhead. At a signal, the balloon wobbles upwards like a translucent sea
creature, its tail a giant tentacle flailing behind. By the time it reaches the
thin air of the stratosphere its scrawny envelope will have filled out into a
sphere a hundred times its current size.

Aboard the balloon is a device designed by a team from the Max Planck
Institute in Heidelberg, which funnels PSC particles into the detectors. For the
next three hours it will measure the sizes, compositions and chemistry of the
PSCs overhead. The researchers are eager to find out how big the particles can
grow, because bigger particles could increase the levels of harmful chlorine
present by removing nitrate from the stratosphere.

Disappearing act

It works like this. PSCs do their damage by ripping open harmless chlorine
nitrate and hydrochloric acid to release active, ozone-eating forms of chlorine.
But the ozone-destroying reactions require light, so the chlorine can鈥檛 do
its dirty work until the sunlight returns in the spring. If the clouds vanish
before the Sun reappears鈥攕ay the vortex breaks up early and the
temperature rises鈥攖hen there鈥檚 plenty of nitric acid around that can
recombine with the chlorine and lock it safely away. That鈥檚 what happened in
1998 and 1999. But what if the nitric acid disappears? What if the particles in
the PSCs grow so large that they sink under their own weight, taking the nitric
acid dissolved in them down as well? The result would be disastrous: the
liberated chlorine would remain in its active form, ready to attack the ozone
come springtime, even if the clouds disappear before then.

In the ultra-cold Antarctic this happens every year. 鈥淥nce you lose the
nitrogen, you鈥檙e on a collision course to very large ozone loss,鈥� says Anderson.
But in the northern hemisphere, things are more delicately balanced. 鈥淭he Arctic
sits right at the threshold for chlorine activation and denitrification,鈥� he
says. The colder stratospheric temperatures that result from greenhouse warming
could lead to larger PSC particles, pushing the Arctic over the threshold.

There is some evidence that denitrification may already be happening in the
Arctic, but the jury is still out. Results from the balloon flight will help the
researchers reach a verdict. As will experiments being performed out of the
Arena Arctica, a cavernous hangar at Kiruna airport that houses the scientists鈥�
fleet of research planes. David Fahey of the National Oceanic and Atmospheric
Administration鈥檚 Aeronomy Laboratory in Boulder, has an instrument aboard NASA鈥檚
ER2 plane, which can fly to altitudes of 20 kilometres, right through the heart
of the ozone layer. 鈥淚t鈥檚 unique,鈥� says Fahey. 鈥淵ou just don鈥檛 get other rides
like that into the stratosphere.鈥� He wants to know exactly how the nitrogen
compounds are distributed among the different sizes and types of PSC particles
at different temperatures. If the temperature drops, and all the particles grow
by the same small amount, it鈥檚 unlikely that any will be big enough to fall out
of the stratosphere. 鈥淏ut what if the growth is limited to just a few
particles?鈥� he says. Then they could grow large enough. 鈥淚f there are a very few
large particles containing a lot of nitric acid, this instrument will find
them,鈥� says Fahey.

At mid-afternoon in the hangar, word comes in that the ER2 is on its way back
from a sortie. A cousin of the U2 spy plane, it carries a suite of instruments.
The wingspan is enormous compared with its slender fuselage, making the ER2 an
awkward beast to land. Already, there鈥檚 a truck checking the runway, to warn the
pilot of any iciness. The plane finally arrives with a roar, echoed by a
relieved cheer from the frozen spectators. As it glides to a halt, ground crew
grab the wings on either side to prevent it tipping. The pilot emerges, removes
his pressure suit helmet and accepts a beer. He鈥檚 earned it. He鈥檚 been sitting
rigidly in the single cockpit for the past eight hours. Climbing down the steps,
he raises his bottle to acknowledge the applause.

Back in the hangar, Anderson has yet another take on the dangers of global
warming. Temperature is not the only problem, he says. There could also be a
change in the amount of stratospheric water vapour. The stratosphere is normally
very dry because of an invisible barrier that traps water vapour in the
troposphere below. This is the result of a temperature minimum that occurs at
the tropopause, where the two layers meet. As air rises through this cold point,
most of the water freezes out. 鈥淭he tropopause temperature is like a throttle on
the stratospheric water vapour,鈥� says Anderson. 鈥淚f you change the tropopause
temperature because of global warming, you鈥檙e going to change the stratospheric
water vapour.鈥� This additional weapon in global warming鈥檚 arsenal was outlined
by Anderson last November in a paper in Nature (vol 402, p 399).

Rough ride

More water vapour could have several adverse effects. It could make it easier
for PSCs to form, for instance, and it could enhance the creation of large cloud
particles that strip the stratosphere of its protective nitric acid. Add these
effects to the cooling predicted by Shindell and things begin to look rough for
Arctic ozone in the decades to come. 鈥淭hese are all possibilities,鈥� says Darryn
Waugh, an atmospheric modeller at Johns Hopkins University in Baltimore. 鈥淭he
worrying thing is we don鈥檛 know which will happen. None of them could happen,
all of them could happen. To predict the future, we have to understand all of
the individual processes much better.鈥�

Direct measurements of the clouds will be the key. And this mission will
provide plenty of these. Though European balloons have been flying from Esrange
for years, this is the ER2鈥檚 first visit to Kiruna. In previous campaigns, based
further south, it had to fly for hours before it had any hope of encountering a
PSC. Here it can take off directly into the vortex. A single flight has already
spent four hours or more flying through PSCs. The plane made it almost all the
way to the North Pole. Everyone is talking about it.

On its latest sortie, the ER2 was not alone. Accompanying it was a NASA DC8
plane, which returns to Kiruna a few hours later. From the outside it looks much
more conventional than its partner, but inside it鈥檚 a far cry from the
commercial airliner it鈥檚 based on. You can hardly move for instrument panels and
electronics boxes. And while the seats for the 40-odd scientific crew are plush
leather鈥攆irst class鈥攁nyone leaving their coffee in the wrong place
on the floor will come back to find it frozen solid. This is not a comfortable
ride and the researchers tumbling out are exhausted. It鈥檚 been a success though.
The plane flew beneath the ER2, using lasers to probe the PSCs its high-flying
partner passed through. Put together with the data from the balloon flights,
this information will help to pin down how colder temperatures affect the
PSCs.

Back in town, I鈥檓 treated to another extraordinary iridescent display in the
sky. Some of the locals are convinced the clouds are pollution. But the PSCs
were here long before anyone had the bright idea of putting CFCs into
refrigerators. It鈥檚 our pollution that鈥檚 reacting with the clouds and causing
the problem. And perhaps our CO₂ that will make the clouds more
prevalent. If so, the spectacular sunsets will be a harbinger of something much
more sinister. 鈥淭hat鈥檚 the sad thing about it,鈥� says Harris. You come up here
and it鈥檚 beautiful. You see the clouds and they鈥檙e gorgeous and then you
think鈥攗gh.鈥� I know just what he means.