May 13, 2009
The year is 2065. Nearly two-thirds of Earth’s ozone is gone—not just over
the poles, but everywhere. The infamous ozone hole over Antarctica, first
discovered in the 1980s, is a year-round fixture, with a twin over the North
Pole. The ultraviolet (UV) radiation falling on mid-latitude cities like Washington,
D.C., is strong enough to cause sunburn in just five minutes. DNA-mutating UV
radiation is up more than 500 percent, with likely harmful effects on plants, animals,
and human skin cancer rates.
2064 without (above left) and with (above right) the effects of international
agreements to curb ozone-destroying chemicals in the 1980s and 90s.
(NASA images by the GSFC Scientific Visualization Studio.)
Such is the world we would have inherited if 193 nations had not agreed to ban
ozone-depleting chemicals, according to atmospheric chemists from NASA’s
Goddard Space Flight Center, the Johns Hopkins University, and the Netherlands
Environmental Assessment Agency. Led by Goddard scientist Paul Newman,
the team used a state-of-the-art model to learn “what might have been” if
chlorofluorocarbons (CFCs) and similar chemicals had not been banned
through the 1989 Montreal Protocol, the first-ever international agreement
on regulation of chemical pollutants.
“World Avoided.” His team envisioned what the Earth would have looked
like with high concentrations of ozone-destroying chemicals in the atmosphere.
(Photograph courtesy Paul Newman.)
“Ozone science and monitoring have improved over the past two decades,
and we have moved to a phase where we [scientists] need to be accountable,”
said Newman, who is serving as a co-chair for the latest “state of the science”
assessment report required by the terms of the Montreal Protocol. “We are at
the point where we have to ask: Were we right about ozone? Did the regulations
work? What kind of world was avoided by phasing out ozone-depleting substances?”
Ozone Chemistry
Ozone is Earth’s natural sunscreen, absorbing most of the incoming UV radiation
from the sun and protecting life from DNA-damaging radiation. The gas is naturally
produced and destroyed by sunlight-driven chemical reactions in the stratosphere,
between about 10 and 50 kilometers above the Earth’s surface. Ozone is made
when oxygen molecules (O2) absorb ultraviolet light and split into individual atoms
(O), which join with other O2 molecules to make O3—ozone. Ozone is destroyed
when molecules containing nitrogen, hydrogen, chlorine, or bromine catalyze reactions
that pair a single O atom with ozone (O3) to make 2 molecules of O2. It is a system
with a natural balance.
But chlorofluorocarbons—invented in the early 1890s, and first used in the 1930s as
refrigerants and propellants for chemical sprays—upset that balance. While CFCs
are not reactive at Earth’s surface, they become quite destructive when they are
exposed to ultraviolet light in the upper stratosphere. There, CFCs and their
bromine-based counterparts break up into elemental chlorine and bromine that
repeatedly catalyze ozone destruction. Worst of all, such ozone-depleting chemical
can reside for several decades in the atmosphere before breaking down.
Units) developed in the ozone layer above Antarctica. British researchers stationed
on the ice of Halley Bay, Antarctica, discovered the hole with ground-based
measurements (red). NASA satellites corroborated the discovery (blue) and
mapped the extent of the hole. (NASA graph by Robert Simmon, based on data
from the British Antarctic Survey and GSFC Atmospheric Composition Team.)
The chemical phenomenon opened up a springtime hole over Antarctica in the
1980s. Each winter, stratospheric temperatures are cold enough to form clouds,
even though the air is very dry. Chemical reactions on the surfaces of the cloud
particles convert chlorine from a relatively unreactive form into highly reactive
form. The September sunrise over Antarctica triggers ozone-destroying reactions
by these reactive kinds of chlorine, and the ozone concentration over the South
Pole drops from about 300 Dobson Units to as low as 100 Dobson Units.
(See “What is a Dobson Unit?”) By late spring, the rising temperature stops
the ozone destruction cycle. The ozone layer rebounds over summer and fall.
The ozone hole phenomenon opened the eyes of the world to the effects of
human activity on the atmosphere.
What Might Have Been
In the new analysis, Newman and colleagues set out to predict ozone losses
as if nothing had been done to stop them. The team started with the Goddard
Earth Observing System Chemistry-Climate Model, an earth system model
of atmospheric circulation that accounts for variations in solar energy, atmospheric
chemical reactions, temperature changes and winds, and interactions between
the stratosphere, where ozone is found, and the troposphere, the layer of
atmosphere closest to Earth. Their “world avoided” simulation took months of
computer time to process.
The researchers let global emissions of CFCs and similar compounds in the model
world increase by 3 percent per year, the rate at which they were growing before
regulation in the late 1980s. Then they let the simulated world turn from 1975 to 2065.
With continued production of CFCs, ozone levels worldwide would have dropped
to dangerously low levels. (NASA images by the GSFC Scientific Visualization Studio.)
By the simulated year 2020, 17 percent of global ozone is destroyed, and an ozone
hole forms each year over the Arctic as well as the Antarctic. By 2040, the ozone
“hole”—concentrations below 220 Dobson Units—is global. The UV index in
mid-latitude cities reaches 15 around noon on a clear summer day (10 is considered
extreme today).
Computer models predict that global average ozone will return to levels above
300 Dobson Units by 2064 (the reference future, blue line). In contrast, sustained
increases in the level of CFCs and other ozone-destroying chemicals would have
reduced global ozone below 100 Dobson Units (world avoided, red line). (Graph
adapted from Newman et al., 2009.)
Global reduction in ozone levels would lead to a huge increase in dangerous
ultraviolet (UV) radiation, with summer noontime UV index values at mid-latitudes
rising to 30—three times the level currently considered extreme. (Graph adapted
from Newman et al., 2009.)
said Richard Stolarski, a member of the NASA team and pioneer of atmospheric
ozone research. (NASA photograph by Robert Simmon.)
Surprising Collapse of Tropical Ozone Layer
In the 2050s, something strange happens: ozone levels in the stratosphere over
the tropics collapse to near zero in a span of six years. According to Goddard
scientist and study co-author Richard Stolarski, who was among the pioneers
of atmospheric ozone chemistry in 1970s, the rapid, near-total ozone destruction
is similar to what happens over Antarctica today.
Seeing a similar process occur over the tropics was surprising, says Stolarski,
“because we hadn’t expected the tropical stratosphere would get cold enough
to form stratospheric clouds.” The dramatic cooling appears to be the result of
two processes. “Ozone absorbs UV energy, which causes the surrounding
atmosphere to warm,” explains Stolarski. “So, by itself, loss of ozone leads
to cooling, which is something we expected to see.”
More surprising, says Stolarski, is that the temperature change intensified the
stratosphere’s large-scale, slow-moving circulation pattern. In that circulation, air
from the lower stratosphere rises into the upper stratosphere at tropical latitudes,
spreads toward the poles, and sinks. As air rises, it cools. As the circulation
strengthened, the amount of cooling increased, allowing stratospheric clouds—
today confined to polar latitudes—to form over the tropics. Runaway ozone
destruction followed.
By the end of the model run in 2065, global ozone drops to less than 110 DU,
a 67 percent drop from the 1970s. Year-round Arctic polar values hover between
50 and 100 (down from 500 in 1960). The intensity of UV radiation at Earth’s
surface doubles; at certain shorter wavelengths, intensity rises by as much as
10,000 times. Skin cancer rates would soar.
“Our world avoided calculation goes a little beyond what I thought would happen,”
said Stolarski. “The quantities may not be absolutely correct, but the basic results
clearly indicate what could have happened to the atmosphere. And models sometimes
show you something you weren’t expecting, like the precipitous drop in the tropics.”
“We simulated a world avoided,” said Newman, “and it’s a world we should be
glad we avoided.”
The Real World
The real world has been somewhat kinder. Production of ozone-depleting substances
was finally halted in 1992, though their abundance is only beginning to decline because
the chemicals can reside in the atmosphere for 50 to 100 years. The peak abundance
of CFCs in the atmosphere occurred around 2000, and has decreased by roughly 4
percent to date.
increased steadily (red line) if the Montreal Protocol and later agreements limiting CFCs
and other chemicals had not been adopted (gray lines). Observations (black line) agree
with the modeled results based on the latest restrictions. Ozone-destroying chemicals
should approach pre-industrial levels around 2064. (Graph adapted from Newman et
al., 2009.)
Stratospheric ozone has been depleted by 5 to 6 percent at middle latitudes, but has
somewhat rebounded in recent years. The largest recorded Antarctic ozone hole was
recorded in 2006, with holes of slightly smaller size since then. Newman, Stolarski, and
other colleagues have used their model to simulate how the real world ozone layer will
recover as well. Because of climate change from greenhouse gases, they say, the ozone
layer will probably not look exactly like it did in the 1970s.
peaked in the 2000s, is expected to shrink markedly by 2064. International agreements
successfully mitigated the threat posed by CFCs and other ozone-destroying chemicals.
(NASA images by the GSFC Scientific Visualization Studio.)
“I didn’t think that the Montreal Protocol would work as well as it has, but I was pretty
naive about the politics,” Stolarski added. “The Montreal Protocol is a remarkable
international agreement that should be studied by those involved with global warming
and the attempts to reach international agreement on that topic.”
Reference
- Newman, P. A., Oman, L. D., Douglass, A. R., Fleming, E. L., Frith, S. M.,
- Hurwitz, M. M., Kawa, S. R., Jackman, C. H., Krotkov, N. A., Nash, E. R.,
- Nielsen, J. E., Pawson, S., Stolarski, R. S., and Velders, G. J. M. (2009).
- What would have happened to the ozone layer if chlorofluorocarbons (CFCs)
- had not been regulated? Atmospheric Chemistry and Physics, 9(6), 2113-2128.
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