Tango in the Atmosphere: Ozone and Climate Change
“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.

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  Photograph of Tents in Antarctica

“I was looking the other way when it hit me,” says Drew Shindell, physicist at the NASA Goddard Institute for Space Studies (GISS). “I was aware that the stratosphere was cooling, and I was wondering about the effects that cooling was having on the chemistry of the atmosphere, especially on ozone depletion.” That is, initially he was wondering about the effects that climate might be having on ozone, not the effects that ozone itself might be having on climate.

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.

Shindell says, “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.


A midnight sun shines across the frigid Antarctic landscape, and makes dark triangles of researchers’ tents. Low temperatures in the stratosphere above both the Antarctic and the Arctic contribute to ozone depletion. Since ozone depletion itself contributes to decreasing temperatures, a feedback loop may be in operation. (Photograph courtesy Alberto Behar, NASA JPL/National Science Foundation)

  Diagram of Ozone Destruction Feedback Loop

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.

  Photograph of Polar Stratospheric Clouds lit from Below  

Thin clouds made of mixtures of ice, nitric acid, and sulphuric acid form in the upper atmosphere (stratosphere) over the poles when temperatures drop below -78°C (-109°F). Ozone depletion occurs in such polar stratospheric clouds. This particular cloud formation appeared over Iceland at an altitude of about 22 km on February 4, 2003. Its beautiful colors result from the very small size of the ice crystals it contains. These tiny particles bend and scatter the sun’s rays much like a prism. (Photograph courtesy Mark R. Schoeberl, 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.”

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Graph of Stratospheric Temperature and Ozone in the Arctic


“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.


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)

  Artist's Rendering of Aura Satellite

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.”


NASA’s Aura satellite, to launch in 2004, 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 Jim Hansen, at the NASA Goddard Institute for Space Studies (GISS). (Image by Jesse Allen)


The Variable Arctic





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  Photograph of Arctic Mountains above the Cloud Layer







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.

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  Photograph of Building Cumulus Clouds

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.

  • References:
  • Hansen, James E. 2002: Air Pollution as a Climate Forcing. Proceedings of a Workshop.
  • Network for the Detection of Stratospheric Change (NDSC). 2003: Connections between Climate Change and Ozone Depletion. Accessed on August 15, 2003
  • Ramaswamy, Venkatachala, et al. 2001: Stratospheric Temperature Trends: Observations and Model Simulations. Reviews of Geophysics, Vol. 39, Pages 71-122.
  • Shindell, Drew. 1999: Solar Variability, Ozone, and Climate. Accessed on January 2, 2003
  • Shindell, Drew and Grewe, Volker. 2002: Separating the influence of halogen and climate changes on ozone recovery in the upper stratosphere. Journal of Geophysical Research, Vol. 107.
  • Stockwell, W.R., et al. 1996: Ozone Formation, Destruction and Exposure in Europe and the United States. Forest Decline and Ozone: A Comparison Of Controlled Chamber and Field Experiments. Springer Verlag. Pp. 1-38.
  • Thompson, Anne M. et al. 2002: Strategies for Observing and Modeling Pollution. EOS, Transactions, American Geophysical Union, Vol. 83. Pp. 575, 581.

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)