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Ozone
and Climate Change
Tango in the Atmosphere:
Ozone and Climate Change
By Jeannie Allen, NASA
Earth Observatory, February 2004
"Ozone
chemistry is at the heart of atmospheric chemistry."
-- Bill Stockwell, Desert Research Institute
Ozone affects climate, and
climate affects ozone. Temperature, humidity, winds, and the presence of other
chemicals in the atmosphere influence ozone formation, and the presence of
ozone, in turn, affects those atmospheric constituents.
Interactions between ozone and
climate have been subjects of discussion ever since the early 1970s when
scientists first suggested that human-produced chemicals could destroy our ozone
shield in the upper atmosphere. The discussion intensified in 1985 when
atmospheric scientists discovered an ozone “hole” in the upper atmosphere
(stratosphere) over Antarctica. Today, some scientists are predicting the
stratospheric ozone layer will recover to 1980 ozone levels by the year 2050.
These scientists say we can expect recovery by that time because most nations
have been abiding by international agreements to phase out production of
ozone-depleting chemicals such as chlorofluorocarbons (CFCs) and halons. But the
atmosphere continues to surprise us, and some atmospheric scientists recently
demonstrated a new spin on the ozone recovery story that may change its ending.
Well before the expected stratospheric ozone layer recovery date of 2050, ozone’s
effects on climate may become the main driver of ozone loss in the stratosphere.
As a result, ozone recovery may not be complete until 2060 or 2070.
Ozone’s impact on climate
consists primarily of changes in temperature. The more ozone in a given parcel
of air, the more heat it retains. Ozone generates heat in the stratosphere, both
by absorbing the sun’s ultraviolet radiation and by absorbing upwelling
infrared radiation from the lower atmosphere (troposphere). Consequently,
decreased ozone in the stratosphere results in lower temperatures. Observations
show that over recent decades, the mid to upper stratosphere (from 30 to 50 km
above the Earth’s surface) has cooled by 1° to 6° C (2° to 11° F). This
stratospheric cooling has taken place at the same time that greenhouse gas
amounts in the lower atmosphere (troposphere) have risen. The two phenomena may
be linked.
Says Dr. Drew Shindell of the
NASA Goddard Institute for Space Studies (GISS), “I’ve long been aware that
chemistry and climate influence one another strongly. I started to ask how cold
the stratosphere might get because of increasing amounts of greenhouse gases. I
was wondering whether or not the cooling in the stratosphere would be rapid
enough that more ozone depletion would take place than we had previously
calculated. Would the cooling be so fast that even more ozone depletion would
occur before the impact of international agreements to limit ozone had time to
take effect?”
This would create a possible
feedback loop. The more ozone destruction in the stratosphere, the colder it
would get just because there was less ozone. And the colder it would get, the
more ozone depletion would occur.
The deepest ozone losses over
both the Arctic and the Antarctic result from special conditions that occur in
the winter and early spring. As winter arrives, a vortex of winds develops
around the pole and isolates the polar stratosphere. When temperatures drop
below -78°C (-109°F), thin clouds form of ice, nitric acid, and sulphuric acid
mixtures. Chemical reactions on the surfaces of ice crystals in the clouds
release active forms of CFCs. Ozone depletion begins, and the ozone “hole”
appears. In spring, temperatures begin to rise, the ice evaporates, and the
ozone layer starts to recover.
The graph above shows total ozone and
stratospheric temperatures over the Arctic since 1979. Changes in ozone amounts
are closely linked to temperature, with colder temperatures resulting in more
polar stratospheric clouds and lower ozone levels. Atmospheric motions drive the
year-to-year temperature changes. The Arctic stratosphere cooled slightly since
1979, but scientists are currently unsure of the cause. Future NASA missions,
starting with the Aura satellite, will improve our understanding of the links
between global climate change and ozone chemistry. (Graph based on data provided
by Paul Newman, NASA GSFC)
Reflections on a Possible
Delay
The concept that stratospheric
cooling due to ozone loss may lead to a delay in recovery of the ozone layer has
fallen on fertile ground. Scientists running different kinds of global models
are finding similar results. “That gives us confidence,” says Dr.
Venkatachalam Ramaswamy, at NOAA’s Geophysical Fluid Dynamic Laboratory. “We’re
confident in our assessment, because the models can help us to understand the
observed ozone and temperature changes on a global scale.”
Stratospheric cooling may have
been taking place over recent decades for a number of reasons. One reason may be
that the presence of ozone itself generates heat, and ozone depletion cools the
stratosphere. Another contributing factor to the cooling may be that rising
amounts of greenhouse gases in the lower atmosphere (troposphere) are retaining
heat that would normally warm the stratosphere. However, scientists hold varying
degrees of conviction about the nature of the link between tropospheric warming
and stratospheric cooling. “The warming of the troposphere and its potential
influence upon the stratospheric circulation is an important consideration,”
points out Ramaswamy, “though the quantitative linkages are uncertain. It is
possible that they may be interdependent only in a tenuous manner.”
“The problem is that we haven’t
had adequate data,” Ramaswamy continues. “Observations have been primarily
limited to only a very few locations in the stratosphere. We have only 20 years
of full global coverage from satellites. Of course radiosonde goes back 40 years
but that is not global coverage.”
Jim Hansen, of NASA’s Goddard
Institute for Space Studies, agrees with Ramaswamy on the need for data. “Climate
forcing by ozone is uncertain because ozone change as a function of altitude is
not well measured. Especially at the tropopause (where the troposphere meets the
stratosphere), we don’t know enough. The climate system is highly sensitive,
especially to changes in the tropopause region. We need exact temperatures and
ozone profiles at different altitudes and around the globe.” Hansen and others
look forward to the launch of NASA’s Aura satellite in 2004. A vital part of
NASA’s Earth Observing System, Aura will observe the composition, chemistry
and dynamics of the Earth’s upper and lower atmosphere, including temperatures
and ozone amounts. “What Aura will give us is quite exciting. There will be a
suite of instruments measuring in regions not well measured before,” says
Hansen.
In spite of large uncertainties
that remain, scientists express a sense of accomplishment with their
achievements so far. “I think one of the successes has to be the fact that we
can now explain the observed temperature trends in the stratosphere reasonably
well, states Ramaswamy. “There is actually a very strong indication that the
observed changes in radiative and chemical species are responsible for
globe-wide cooling of the stratosphere.”
The Variable Arctic
Although many global scale models
agree with each other and with observations on the future of ozone recovery,
most regional scale models do not agree. Atmospheric models show that the
cooling influence of ozone depletion accounts very well for observed cooling
winter-time temperature trends in the Antarctic, but not in the Arctic.
Differences among regions make
predictions about complex atmospheric chemistry problematic. The Arctic and
Antarctic regions, where low stratospheric ozone amounts are of great concern,
differ in significant ways. The complex topography of the high latitude Northern
Hemisphere, with its distribution of land masses and oceans, makes the Arctic
atmosphere more dynamic and variable.
The Antarctic is colder than the
Arctic. Antarctic winds form a relatively stable vortex for long periods of
time, and the vortex allows temperatures of the air trapped within it to get
extremely low. Shindell explains, "In the south, air masses just sit over
the pole and get colder."
Such stability makes the
Antarctic somewhat more predictable than the Arctic. Shindell says, "It's
so variable in the Arctic that we have to have better data to figure out what we
should believe and what we can have confidence in for the future."
These coastal mountains in
southeast Alaska are representative of the rugged terrain of the Northern
Hemisphere's high latitudes. High mountains and the contrast between large
continental landmasses and open ocean in the Northern Hemisphere disturb the air
over the Arctic, preventing the formation of a stable circulation pattern. In
part, it is the lack of a stable "polar vortex" that prevents the
Arctic from experiencing the extremely cold temperatures and dramatic ozone loss
seen above Antarctica. In spite of this, large ozone losses occurred in the
Arctic during the last several years. (Photograph courtesy NOAA Photo Library)
Although dramatic ozone depletion
did not occur in the Arctic in the 1980s when it occurred in the Antarctic,
times are changing. Very large ozone losses have occurred in the Arctic
recently, especially in the late 1990s. Ozone chemistry is very sensitive to
temperature changes. Since temperatures in the Arctic stratosphere often come
within a few degrees of the threshold for forming polar stratospheric clouds,
further cooling of the stratosphere could cause these clouds to form more
frequently and increase the severity of ozone losses.
The Arctic may be changing in
another way that differs from the Antarctic. With stratospheric cooling, the
differences in temperature between the stratosphere and the troposphere are
increasing. Differences in temperature creates winds, so stratospheric wind
speeds have been increasing. (The Antarctic isn't affected by increasing
greenhouse gases like the Arctic is because it's colder, and the polar wind
circulation over the Antarctic is already very strong.)
Shindell says that from both
observations and models, he has found increasing wind speeds not only at high
altitudes but also near the surface. "That's a large effect on
climate," he points out. "Changes in stratospheric ozone and winds
affect the flow of energy at altitudes just below, which then affect the next
lower altitudes, and so on all the way to the ground. That would be the most
intriguing aspect of all this, though it's still controversial."
Ozone and Climate at the
Surface
Interactions between ozone
and climate naturally occur not only in the stratosphere, but also at the
Earth's surface (troposphere). There are known chemical and physical aspects of
ozone formation we can watch carefully as climate changes. Ozone forms in the
troposphere by the action of sunlight on certain chemicals (photochemistry).
Chemicals participating in ozone formation include two groups of compounds:
nitrogen oxides (NOx) and volatile organic compounds (VOCs). In general, an
increase in temperature accelerates photochemical reaction rates. Scientists
find a strong correlation between higher ozone levels and warmer days. With
higher temperatures, we can expect a larger number of "bad ozone"
days, when exercising regularly outdoors harms the lungs. However, ozone levels
do not always increase with increases in temperature, such as when the ratio of
VOCs to NOx is low.
As the troposphere warms on a
global scale, we can expect changes in ozone air quality. Generally speaking,
warming temperatures will modify some but not all of the complex chemical
reactions involved in ozone production in the troposphere (such as those
involving methane). Because of the short-lived nature of these chemical
constituents and variations across space and time, the uncertainty is too large
to make predictions. Scientists can only speculate about specific kinds of
change, about the direction of change in a particular location, or about the
magnitude of change in ozone amounts that they can attribute to climate.
Some speculation involves VOC
emissions from natural biological processes. Certain kinds of plants such as
oak, citrus, cottonwood, and almost all fast-growing agriforest species emit
significant quantities of VOCs. Higher temperatures of a warming climate
encourage more plant growth, and therefore higher levels of VOCs in areas where
VOC-emitting plants grow abundantly. Soil microbes also produce NOx. Soil
microbial activity may also increase with warmer temperatures, leading to an
increase in NOx emissions and a consequent increase in ozone amounts.
A warming climate can lead to
more water vapor in the lower atmosphere, which would tend to produce more
ozone. But cloud cover can also diminish chemical reaction rates because of
reduced sunlight and therefore lower rates of ozone formation. Monitoring and
analyzing such interactions is the best way we can improve our predictive
capabilities. (Photograph courtesy Jeannie Allen, NASA GSFC/SSAI)
Another impact of climate on
ozone pollution in the troposphere arises from the probability that higher
temperatures will lead to greater demand for air conditioning and greater demand
for electricity in summer. Most of our electric power plants emit NOx. As energy
demand and production rises, we can expect amounts of NOx emissions to increase,
and consequently levels of ozone pollution to rise as well.
Water vapor is also involved in
climate change. A warmer atmosphere holds more water vapor, and more water vapor
increases the potential for greater ozone formation. But more cloud cover,
especially in the morning hours, could diminish reaction rates and thus lower
rates of ozone formation.
Understanding the interactions
between ozone and climate change, and predicting the consequences of change
requires enormous computing power, reliable observations, and robust diagnostic
abilities. The science community's capabilities have evolved rapidly over the
last decades, yet some fundamental mechanisms at work in the atmosphere are
still not clear. The success of future research depends on an integrated
strategy, with more interactions between scientists' observations and
mathematical models.
http://www.giss.nasa.gov/research/features/tango/
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