Stratospheric Ozone Depletion Ozone and LIfe In the absence of an ozone layer, life could only evolve under water Formation of O3 in the Atmosphere • • • • O2 + UV O + O O + O2 O3 O + O2 O3 Overall Reaction: 3O2 + UV 2O3 Atmospheric gradient Most of the ozone present in the atmosphere (~90%) is located in the stratosphere. Atmospheric concentrations of ozone average 12 parts per million (ppm). How Does Stratospheric Ozone Protect Us? • O3 “absorbs” UV-B radiation and “uses up” its energy, converting it to harmless heat • UV + O3 O + O2 • Some of the free oxygen atoms combine with other ozone to form oxygen: O + O3 O2 + O2 • Or, they recombine with existing oxygen to reform ozone: O + O2 O3 + heat • The ozone layer does not really “shield” us or “reflect” harmful UV-B radiation, but rather “uses up” its energy before it can reach earth Timeline of CFC Development • 1928: DuPont scientists first develop CFCs as a refrigerant and a propellant. • CFCs believed ideal: non-flammable, non-toxic, good insulator, inexpensive, stable (inert), light. • However, because CFCs are inert and light, they do not react in the lower atmosphere, and can be carried by updrafts to the stratosphere where intense UV radiation can break them apart. • It can take 5-15 years for a CFC molecule to migrate to the stratosphere. Timeline of CFC Development (cont’) • 1951: DuPont begins mass production of CFCs • 1971: Scientist James Lovelock first speculates that CFCs released to the atmosphere could still be there. (Where is Away??) • 1973: Atmospheric scientists Mario Molina and F. Sherry Rowland hypothesize that CFCs could be reaching the stratosphere, where they can be broken apart and a single chlorine atom could contribute to the destruction of 100,000 ozone molecules. • 1974: Molina and Rowland publish their theory in the journal Nature; the story is picked up by a science writer at The New York Times and published on the front page. • 1974: DuPont responds with their own study claiming that CFCs are safe in the troposphere; calls for more research. Timeline of CFC Development (cont’) • 1975: 200% increase in CFC use from 19681975, annual growth in consumption of 1020%/year. • 1979: The FDA and the EPA ban “nonessential” uses of CFCs in aerosol cans. THE FIRST TIME A SUBSTANCE WAS BANNED WITHOUT DEFINITIVE PROOF OF HARM. • 1982: 20 other nations join U.S. ban on nonessential uses. Early Warnings of Ozone Depletion • 1982: British scientific team in Antarctica using groundbased instruments announce 20% decline in O3 levels in the stratosphere (in their spring). • 1982: U.S. team using satellite-based Total Ozone Mapping Spectrometer (TOMS) reports no ozone loss over Antarctica. • 1983: British team reports a 30% decline in ozone levels, U.S. team contradicts. • 1985: British team reports a 50% decline in ozone levels. At first, U.S. team contradicts, re-check their instruments and then confirm the British findings. • Acknowledgement of “ozone hole” in the media creates public and political alarm. Early Warnings (cont’) • 1986: Australian government announces doubling of skin cancer cases in previous 10 years. • 1986: DuPont scientists speculate that tropospheric (“bad”) ozone will migrate to stratosphere to “fill” the hole. • 1987: Preliminary research indicates that loss of ozone over Antarctica due primarily to human emissions of ozonedepleting chemicals. • 1987: UN holds meetings in Montreal, and 45 nations sign the Montreal Protocol to reduce CFC use by 50% by the year 2000. Protocol is ratified in 1989. • 1988: DuPont continues to argue that CFCs are safe to use, and initially oppose the Montreal Protocol. However, by year end they announce an impending phase-out of CFC production. Monthly average ozone levels over Antarctica 1956-1994, for the month of October Newspaper cartoon from 1986 Major ozone-depleting substances developed in the past century O3 Chemistry UV + CFC releases “chlorine free radical” (Cl) Cl + O3 ClO (chlorine oxide) + O2 ClO + O Cl (free radical) + O2 Cl free radical destroys O3, creating ClO, which reacts with free atomic oxygen (O) to create another Cl free radical and start the process all over again. One CFC molecule can destroy an estimated 100,000 molecules of O3 Creation O + O2 O3 + heat Natural balance between ozone creation and ozone destruction Ozone Balance O3 creation = O3 destruction Destruction UV + O3 O + O2 O3 Destruction Creation O + O2 O3 + heat Human – made compounds add to natural rates of ozone destruction, resulting in a net loss of stratospheric ozone Ozone Destruction O3 creation < O3 destruction Destruction UV + O3 O + O2 Plus human – made compounds uv Protection The ozone layer absorbs and uses up most incoming UVB radiation before it can strike the surface of the earth. UV-B radiation can cause skin cancer, cataracts, and reduced immune system as well as harming plant life. Daily Dynamic of Ozone Layer What is a Dobson Unit? The Dobson Unit (DU) is the unit of measure for total ozone. If you were to take all the ozone in a column of air stretching from the surface of the earth to space, and bring all that ozone to standard temperature (0 °Celsius) and pressure (1013.25 millibars, or one atmosphere, or “atm”), the column would be about 0.3 centimeters thick. Thus, the total ozone would be 0.3 atm-cm. To make the units easier to work with, the “Dobson Unit” is defined to be 0.001 atm-cm. Our 0.3 atm-cm would be 300 DU. Yearly Dynamic of Ozone Layer • http://ozonewatch.gsfc.nasa.gov/ Consequences of high uv Skin Cancer Consequences of high uv Cataracts • Cataracts may be a more widespread health effect of ultraviolet-B radiation than skin cancers, because all populations will be affected. Cataracts occur when the lens inside your eye becomes increasingly opaque resulting in 'misty' or 'foggy' vision. Consequences of high uv Weakened Immune System • UV-B irradiation has suppressive effect on immune system • Skin is an immune organ - certain types of lymphoid cells found predominantly in skin • Skin-associated lymphoid tissue responds to antigens that enter the body by way of the skin. – This cutaneous immune surveillance system is vital to our ability to resist invasions by infectious agents. Consequences of high uvEcosystems • Ecosystems – Reduced yield for some crops • (e.g. wheat, oats) – Decreased forest productivity – Reduced surface phytoplankton • affects aquatic productivity Consequences of high uv Antarctic Food Chain uvb Reduces algal growth Krill Increased Smog • Atmospheric Chemistry – Increased smog (more uv to provide energy for atmospheric chemical reactions) Other greenhouse gases - CFCs Northern Latitudes? • Southern “hole” now greater than ever • “hole” migrates toward equator • hole now instead over Arctic – usually less (less land area – need frozen H2O) – but, more CFCs – Europe – O3 decline 8% in last 10 yrs. – winter decline over U.S. – 5% – hole never expected over U.S. – now observed – http://visibleearth.nasa.gov/view_rec.php?id=9556 Science and Politics of Ozone Depletion • 1990: Follow-up meeting in London leads to new goal of complete CFC phase-out in developed countries by 2000 and in developing countries by 2010. • 1992: Follow up meeting in Copenhagen calls for complete phase-out by 1996, DuPont promises to halt production by 1997. • 1992: Rush Limbaugh publishes book claiming that the ozone depletion “crisis” is a hoax. (see also http://www.nas.nasa.gov/About/Education/Ozone/controversy.html) • 1995: Congressional hearings also challenge ozone science. • 1995: Ironically, Rowland and Molina receive Nobel Prize in chemistry for their early work on ozone depletion. • 1996: Satellite study provides definitive confirmation of human role in stratospheric ozone depletion. Junk science continues. • 1996: CFC ban begins, but black market appears. TOMS Total Ozone Monthly Averages What’s Happening to the Ozone Layer Today? • Stratospheric ozone concentrations have stabilized, and concentrations of Cl in the stratosphere are nearing their peak. However, atmospheric concentrations of bromine compounds (another ozone depleter) are still increasing. • Stratospheric ozone levels over the mid-latitudes of the northern hemisphere are 3-6% below pre-1980 levels, with higher losses in the winter/spring and losses of 1-3% in summer/fall. • Levels of UV-B radiation reaching the earth’s surface continue to increase, although this trend is expected to reverse itself as the ozone layer is replenished. • Phase-out of CFC use is almost complete. CFC substitutes like HCFCs and HFCs also deplete the ozone layer, but at only a fraction (1-15%) of the rate of CFCs because they react in the lower atmosphere. • Recent years have seen record levels of ozone depletion over Antarctica due to atmospheric conditions (polar stratospheric clouds). Good News The Good News Bad News The Bad News We continue to use other ozone-depleting chemicals besides CFCs. Recently, the Bush administration sought to exempt U.S. farmers from a Montreal Protocol requirement to phaseout use of the pesticide methyl bromide. To the extent that CFCs and other ozone-depleting chemicals are phased out, the ozone layer should be able to replenish itself by the middle of this century. Cl and Br Sources Good Ozone v. Bad Ozone Remember! • Difference between good ozone and bad ozone. •Stratospheric ozone depletion is not the same as global warming. These are two different, albeit related, environmental problems. Lessons Learned from the Ozone Depletion Story • There is no away (TINA). • Scientific uncertainty is no excuse for inaction. Uncertainty will almost always be present. • Not all science is created equal. • Standard media ethics that call for “both sides of the story” can complicate coverage of some environmental issues. • The “burden of proof” typically rests with environmentalists. • Global problems require global solutions.
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