Is There a Connection between the Ozone Hole and Global Warming?

ozone hole over antarctica

Image: NASA

Ozone (O3) high in the atmosphere absorbs ultraviolet radiation from the sun, thereby protecting living organisms below from this dangerous radiation. The term ‘ozone hole’ refers to recent depletion of this protective layer over Earth's polar regions. People, plants, and animals living under the ozone hole are harmed by the solar radiation now reaching the Earth's surface—where it causes health problems from eye damage to skin cancer.

The ozone hole, however, is not the mechanism of global warming. Ultraviolet radiation represents less than one percent of the energy from the sun—not enough to be the cause of the excess heat from human activities. Global warming is caused primarily from putting too much carbon into the atmosphere when coal, gas, and oil are burned to generate electricity or to run our cars. These gases spread around the planet like a blanket, capturing the solar heat that would otherwise be radiated out into space. (For more detail on the basic mechanism of global warming, see carbon dioxide FAQ.)

Both of these environmental problems do, however, have a common cause—human activities that release gases into and alter the atmosphere. Ozone depletion occurs when chlorofluorocarbons (CFCs)—formerly found in aerosol spray cans and refrigerants—are released into the atmosphere. These gases, through several chemical reactions, cause the ozone molecules to break down, reducing ozone's ultraviolet (UV) radiation-absorbing capacity.

Because our atmosphere is one connected system, it is not surprising that ozone depletion and global warming are related in other ways. For example, evidence suggests that climate change may contribute to thinning of the protective ozone layer.

Figure 1.  Seasonal thinning of the ozone layer above Antarctica.

Source: NASA 

Ozone in the upper atmosphere

Ozone (O3) is found in two different parts of our atmosphere. Ground level ozone, a human health irritant and component of smog, is found in the lower atmosphere (troposphere) and has nothing to do with the "ozone hole." However, ozone in the stratosphere—the layer of atmosphere above the troposphere (see Figure 2)—accounts for the vast majority of atmospheric ozone. Stratospheric ozone is protective of human health as it absorbs ultraviolet radiation from the sun, preventing the radiation from hitting Earth's surface and harming living organisms from this biologically dangerous radiation. 

Stratospheric ozone is produced from reactions that occur with the energy from the sun (photochemical reactions). Although ozone is created primarily at tropical latitudes, large-scale air circulation patterns in the lower stratosphere move ozone toward the poles, where its concentration builds up.

In addition to this global motion, strong winter polar vortices are also important to concentrating ozone at the poles. During the continuously dark polar winter, the air inside the polar vortices becomes extremely cold, a necessary condition for polar stratospheric cloud (PSC) formation. These clouds generally last until the sun comes up in the spring setting up conditions for drastic ozone destruction.

What is the ‘ozone hole’ and what causes it?

In the 1980s, scientists discovered [4] that the ozone layer was thinning in the lower stratosphere, with particularly dramatic ozone loss—known as the "ozone hole"—in the Antarctic springtime (September and October). This is caused by increasing concentrations of ozone-depleting chemicals in the stratosphere that come from spray cans and refrigerants. These long-lived chlorofluorocarbons or CFCs (chemical compounds with chlorine and/or fluorine attached to carbon) remain in the atmosphere for decades to over a century depending on the CFC.  At the poles CFCs attach to ice particles. When the sun comes out again in the polar spring, the ice particles melt, releasing the ozone-depleting molecules from the ice particle surfaces. Once released, these ozone-destroying molecules do their dirty work, breaking apart the molecular bonds in UV radiation-absorbing ozone. [1, 2, 3]

Stratospheric ozone also has natural processes that remove it from the atmosphere. Tiny sulfate particles (aerosols) blasted into the stratosphere by volcanic eruptions Pinatubo (1991) and El Chichon (1982) caused measurable decreases in ozone for several years following the eruptions. Because of this phenomenon, there are worries about the possible consequences on the protective ozone layer if 'climate engineering' (also called 'geoengineering ’) is pursued in an attempt to slow global warming.  Some proposed climate engineering ideas include injecting artificial particles (aerosols) into the lower stratosphere to mimic the temporary global cooling effect of volcanic eruptions (see aerosols FAQ.)

Does the "ozone hole" contribute to global warming?

Stratospheric ozone absorbs energy from the ultraviolet part of the solar spectrum, heating the lower stratosphere. This part of the spectrum accounts for less than one percent of the total solar energy reaching our atmosphere. [1] Stratospheric ozone is important because it prevent dangerous ultraviolet rays from harming plants and animals on Earth's surface, but reductions in the amount of radiation absorbed does not have a measurable impact on temperatures below.

Does climate change have an impact on the stratospheric ozone layer

Structure of the atmosphere. Image: NOAA

The thickness of the polar stratospheric ozone layer depends on the rate of production of ozone in the tropical stratosphere, the movement of ozone from the tropics to the poles, the amount of ultraviolet radiation from the Sun, the polar stratospheric cloud cover, and the chemical reactions between the ozone and ozone- depleting substances. Each of these factors might be affected by climate change. [6]

Poleward motions in the stratosphere, which increase polar concentrations of ozone, as well as the strength of the polar stratospheric vortices, which decrease ozone via PSC formation, are both expected to increase as temperatures rise in the lower atmosphere (see Figure 2).

Yet temperatures in the lower stratosphere are decreasing as a result of increased carbon and other heat-trapping emissions [1]. The reason for this apparent paradox—increasing temperatures at the Earth's surface and decreasing temperatures in higher parts of the atmosphere—can be explained using the blanket analogy. Carbon dioxide and other heat-trapping gases rise into the atmosphere, spread around the globe, and act like a blanket holding in heat around Earth. This blanket also protects the warm surface of the Earth from the cold air above it. As heat-trapping gas concentrations increase, the blanket thickness also increases. This further warms the Earth’s surface; heats the blanket itself; and traps more heat in the lower atmosphere. Heat that normally (i.e. before blanket thickening) would escape the lower atmosphere and enter the stratosphere no longer does so, leaving the stratosphere cooler. Cooling of the lower polar stratosphere enhances PSC formation, and thus contributes to ozone loss. It appears unlikely that the decrease in ozone-depleting substances will lead to restabilization of the pre-1980 stratospheric ozone layer because of the competing and uncertain effects of further climate change. [6]

References

[1] Staehelin, J., et al. (2001). Ozone Trends: A Review. Reviews of Geophysics 39, 231-290.

[2] Solomon, S. (1999) Stratospheric Ozone  Depletion: A Review of Concepts and History. Revs. Geophys. 37, 275-316.

[3] World Meteorological Organization: Scientific Assessment of Ozone Depletion, 2002:http://ozone.unep.org/Publications/index.asp) (2003)

[4] Farman, J. et al (1985)  Large Losses of Total Ozone in Antarctica Reveal Seasonal ClOx/NOx  Interaction. Nature 315,  207.

[5] Molina, M. and F. Rowland (1974). Stratospheric Sink for Chlorofluoromethanes….Nature 249, 810-812.

[6] Weatherhead, E. C. and S. B. Andersen (2006):  The Search for Signs of Recovery of the Ozone Layer. Nature 441/4 doi:10.1038.

Summary prepared by M. Baker (University of Washington) and reviewed by B. Ekwurzel, N. Cole, and S. Shaw (UCS).

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