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| Climate change can cause ozone layer depletion |
By Henry Fountain, The New York Times, July 26, 2012
Strong summer
thunderstorms that pump water high into the upper atmosphere pose a threat to
the protective ozone layer over the United States, researchers said on
Thursday, drawing one of the first links between climate change and ozone loss over populated
areas.
In a study published online by the journal Science,
Harvard University scientists reported that some storms send water vapor miles
into the stratosphere — which is normally drier than a desert — and showed how
such events could rapidly set off ozone-destroying reactions with chemicals
that remain in the atmosphere from CFCs, refrigerant gases that are now banned.
The risk of ozone damage, scientists said, could increase if global
warming leads to more such storms.
“It’s the union between ozone loss and climate change that is really
at the heart of this,” said James G. Anderson, an atmospheric scientist and the
lead author of the study.
For years, Dr. Anderson said, he and other atmospheric scientists were
careful to keep the two concepts separate. “Now, they’re intimately connected,”
he said.
Ozone helps shield people, animals and crops from damaging ultraviolet
rays from the sun. Much of the concern about the ozone layer has focused on Antarctica,
where a seasonal hole, or thinning, has been seen for two decades, and the
Arctic, where a hole was observed last year. But those regions have almost no
population.
A thinning of the ozone layer over the United States during summers
could mean an increase in ultraviolet exposure for millions of people and a
rise in the incidence of skin cancer, the researchers said.
The findings were based on sound science, Dr. Anderson and other
experts said, but much more research is needed, including direct measurements
in the stratosphere in areas where water vapor was present after storms.
“This problem now is of deep concern to me,” Dr. Anderson said. “I
never would have suspected this.”
While there is conclusive evidence that strong warm-weather storms
have sent water vapor as high as 12 miles — through a process called convective
injection — and while climate scientists say one effect of global warming is an
increase in the intensity and frequency of storms, it is not yet clear whether
the number of such injection events will rise.
“Nobody understands why this convection can penetrate as deeply as it
does,” said Dr. Anderson, who has studied the atmosphere for four decades.
Mario J. Molina, a co-recipient of a Nobel Prize for research in the 1970s that
uncovered the link between CFCs and damage to the ozone layer, said the study
added “one more worry to the changes that society’s making to the chemical
composition of the atmosphere.” Dr. Molina, who was not involved in the work,
said the concern was “significant ozone depletion at latitudes where there is a
lot of population, in contrast to over the poles.”
The study, which was financed by the National Aeronautics and Space
Administration, focused on the United States because that is where the data was
collected. But the researchers pointed out that similar conditions could exist
at other midlatitude regions.
Ralph J. Cicerone, an atmospheric scientist and the president of the
National Academy of Sciences, who reviewed the study for Science, also called
for more research. “One of the really solid parts of this paper is that they’ve
taken the chemistry that we know from other atmospheric experiments and lab
experiments and put that in the picture,” he said. “The thing to do is do field
work now — measure moisture amounts and whether there is any impact around it.”
“The connection with future climate is the most important issue,” Dr.
Cicerone said.
Large
thunderstorms of the type that occur from the Rockies to the East Coast and
over the Atlantic Ocean produce updrafts, as warm moist air accelerates upward
and condenses, releasing more heat. In most cases, the updrafts stop at a
boundary layer between the lower atmosphere and the stratosphere called the
tropopause, often producing flat-topped clouds that resemble anvils. But if
there is enough energy in a storm, the updraft can continue on its own
momentum, punching through the tropopause and entering the stratosphere, said
Kerry Emanuel, an atmospheric scientist at the Massachusetts Institute of
Technology.
When Dr. Anderson
produced data about five years ago clearly showing these strong injections of
water vapor, “I didn’t believe it at first,” Dr. Emanuel said. “But we’ve come
to see that the evidence is pretty strong that we do get them.”
At the same time,
he added, “we don’t really understand what determines the potential for
convection in the atmosphere,” so it is difficult to say what the effect of
climate change will be.
“We’re much further along on understanding how hurricanes respond to climate change than
normal storms,” Dr. Emanuel said.
The use of CFCs, or chlorofluorocarbons, was phased out beginning in
the late 1980s with the signing of an international treaty called the Montreal
Protocol, but it will take decades for them to be cleansed fully from the
atmosphere. It is chlorine from the CFCs that ultimately destroys ozone,
upsetting what is normally a balanced system of ozone creation and decay. The
chlorine has to undergo a chemical shift in the presence of sunlight that makes
it more reactive, and this shift is sensitive to temperature.
Dr. Anderson and his colleagues found that a significant concentration
of water vapor raises the air temperature enough in the immediate vicinity to
allow the chemical shift, and the ozone-destroying process, to proceed rapidly.
“The rate of these reactions was shocking to us,” Dr. Anderson said.
“It’s chemistry that was sitting there, waiting to be revealed.”
Dr. Anderson said that if climate change related to emissions of
greenhouse gases like carbon dioxide and methane led to more events in which
water was injected well into the stratosphere, the effect on ozone could not be
halted because the chemistry would continue. “It’s irreversible,” he said.
If CFCs had not been banned, the ozone layer would be in far worse
shape than it is. But by showing that CFC-related ozone destruction can occur
in conditions other than the cold ones at the poles, the study suggests that
the full recovery of the ozone layer may be further off than previously
considered.
“The
world said, ‘Oh, we’ve controlled the source of CFCs; we can move on to
something else,’ ” Dr. Anderson said. “But the destruction of ozone is far
more sensitive to water vapor and temperature.”
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