One of the most common mistakes that we have observed in discussions of climate and atmospheric change is confusion between the rather separate concepts of ozone depletion and global warming. This isn’t necessarily surprising given the scant information that most people pick up from the media. However, for many years meteorologists have been fighting a rearguard action to persuade people that the globe isn’t warming because there is more sun coming through the ozone hole. There are however important connections between the two issues that complicate potential actions that we might take to alleviate the different problems. This week, for instance, a new IPCC report was released that looked at the greenhouse warming potential of many of the replacement chemicals (HFCs and HCFCs) that were used to replace CFCs in aerosol cans and refrigeration units under the Montreal Protocol (and subsequent amendments).
The connections actually go both ways: Firstly, CFCs, HFCs etc. are greenhouse gases, while ozone is both a greenhouse gas and an absorber of solar radiation (in the UV range), and so changes to their concentrations affect the radiation transfer through the atmosphere. Secondly, the chemistry that controls ozone loss is very sensitive to the local temperature and humidity, and as that is affected by climate change, that will impact ozone depletion as well.
The original CFCs were powerful greenhouse gases (about 0.34 W/m2 forcing since 1850), and even allowing for a cooling due to the subsequent depletion in stratospheric ozone (-0.15 W/m2), they had a net warming effect. Therefore the ongoing phase-out will help both the stratospheric ozone problem and reduce the forcings leading to global warming. CFC concentrations are indeed now starting to level out and are expected to decrease further in coming decades. However, some of the replacement gases (for instance HFC-23) which are not as harmful to ozone, nonetheless have an significant greenhouse warming potential. The total forcing from these replacements is expected to be small compared to increases in CO2, but any reductions that can be easily made can potentially offset some increases in CO2. Thankfully, some other replacements exist (for instance ammonia) which neither affect ozone nor the greenhouse effect. The cure for ozone depletion has not turned out to be worse than the disease!
On the other hand, some of the climate change effects on ozone were discussed previously in connection with Arctic ozone levels. These effects are both chemical and dynamical. The chemical impacts relate mainly to increasing levels of methane and stratospheric water vapour directly affecting the local chemistry. Additionally, stratospheric cooling (caused by increasing CO2 as well) has an indirect effect on the rates of many of the ozone-destroying reactions (accelerating ozone loss). Dynamically, planetary and gravity wave activity, (related to convection and the jet streams, for instance) all affect the momentum balance in the stratosphere and control the Brewer-Dobson overturning circulation. Therefore changes to those can potentially affect the stratospheric circulation and thus change stratospheric winds and stability. These dynamic effects can often lead to local changes of temperature (particularly in high latitudes) much greater than any radiative change e.g through possibly changes to the strength of the polar vortex (Shindell et al, 1998).
So, as we learn more about stratospheric ozone and climate change, what were once two separate problems have become more and more entwined. It therefore appears unlikely that meteorologists are going to get a break anytime soon from explaining exactly how the two issues do, and don’t, connect.
Global Warming and Ozone Layer Depletion:
STS Issues for Social Studies Classrooms
James A. Rye, Donna D. Strong, and Peter A. Rubba
Thats depressing! remarked a preservice teacher about the likely consequences of China and India achieving the energy-intensive lifestyle of the United States. She had just been looking at the record of average surface temperatures of the Earth. The year 1998 was the warmest since reliable temperature records have been kept, and the twentieth year in a row in which the average global surface temperature was higher than the 119-year average. The next year, 1999, was the fifth warmest on record; the six warmest years on record have occurred in the last decade.1
If the global warming trend continues, the results could be depressing indeed: melting polar ice along with thermal expansion of the oceans could raise the sea level, flooding coastal cities, and many agricultural landscapes could dry out, becoming deserts. And yet, as the class studied the issue further, we learned that the news is not all bad. The nations of the world have already taken collective action to solve one global atmospheric problem: depletion of the ozone layer. Global warming, as we will discuss below, is a different and a bigger problem, but scientists have already come together to measure, understand the causes of, and set goals for reducing the rate at which it occurs.
The preservice teacher quoted above was attending our college course on the value of Science-Technology-Society (STS) education for middle and upper elementary students. Social studies teachers have called for meaningful learning in social studies that is issues-centered, connects to students lives, and provides opportunities for action taking.2 We have developed a unit of study on global atmospheric change for school students, in the hope that teachers will find it useful to include this STS issue in their curriculum.3 Global warming and ozone layer depletion exemplify the possibilities that await when teachers bring STS topics into the social studies classroom.
The STS movement, as it was advanced during the 1980s (largely due to Project Synthesis), challenged social studies teachers to prepare students to be scientifically literate as well as socially conscious.4 The issues of global warming and ozone layer depletion are central to Education for Citizenship in the 21st Century and correlate closely to four of the ten strands of the NCSS social studies standards: 3 People, Places, and Environment; 8 Science, Technology, and
Society; 9 Global Connections; and 0 Civic Ideals and Practices.5 The purpose of this article is to describe some of the basic science that underlies the issues of global warming and ozone layer depletion, elaborate the rationale for including these issues in social studies, and provide related suggestions for social studies instruction. This information has been synthesized from comprehensive analyses published by scientists working together in large collaborations, and from works by the authors.6
Global warming and ozone layer depletion are two different problems (see the sidebar Misconceptions about Global Atmospheric Change on page 95). Global warming is caused by the greenhouse effect, which is essential to life as we know it on planet Earth. Electromagnetic energy coming from the sun is absorbed by the Earth, which radiates some of this energy outward as infrared energy (heat). Some of this infrared energy escapes into space, but much of it is absorbed by greenhouse gases in the lower atmosphere (the troposphere) and is radiated back to the Earth as heat energy.
The greenhouse effect, then, is a warming of the Earths surface that makes it hospitable to life. Without the greenhouse effect, the surface of the Earth would be a frigid -100°F. Alternatively, a runaway greenhouse effect, like that found on the planet Venus, would result in a surface temperature of 900°F or more. Greenhouse gases, including water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), are produced as part of the natural system (for example, CO2 and H2O are by-products of respiration and combustion). These gases act much like the glass roof of a greenhouse, letting sunlight through, but keeping heat locked in.
Since the Industrial Revolution, humans have developed technologies (for example, motor vehicles) that produce large quantities of greenhouse gases. Also, humans have invented new molecules that are greenhouse gases, for example, chlorofluorocarbons (CFCs) and some CFC-substitutes that are used as coolants and solvents. Increased amounts of all types of man-made greenhouse gases released into the atmosphere enhance the greenhouse effect. The gas that is most responsible for enhancing the greenhouse effect is CO2, because human activities create so much of it and it has a long lifetime in the atmosphere (meaning that molecules are present in the air for a long time before being used by plants or being bound or broken down in other chemical events).
Over the past 300 years, atmospheric CO2 levels have increased by about 30%.7 How do we know this? Tree rings, pieces of old coral, and cores taken from glaciers and from mud at the bottom of the ocean provide scientists with samples of atmospheric changes that go back 150,000 years. This increase in CO2 comes largely from the burning of fossil fuels (coal, oil, and natural gas) in automobiles and electrical power plants. People burning tropical rainforests (biomass burning) to clear land also contribute to the problem of too much atmospheric CO2  This practice is doubly destructive because it also upsets the balance of the carbon cycle. Living trees and other green plants play a major role in taking carbon out of the atmosphere through photosynthesis, the process whereby plants use energy from the sun to make food; photosynthesis utilizes carbon from CO2. Accordingly, green plants are known as a carbon sink. The oceans also are a carbon sink because small plantsphytoplanktonin the water take up great amounts of carbon from atmospheric CO2.
Most atmospheric scientists are now convinced that human industrial activity has caused an enhanced greenhouse effect, leading to an increase in global warming: [T]he further accumulation of greenhouse gases commits the Earth irreversibly to further global climatic change and consequent ecological, economic, and social disruption.8 However, there continues to be debate in the scientific community over the best way to predict how much warming will occur, how much of the warming humans are responsible for, and what might be the best ways to decrease the production of greenhouse gases. To complicate matters, some scientists even speculate that the effects of global warming may perturb ocean circulation patterns so much as to throw us into another ice age.9 Despite these uncertainties, a growing scientific consensus is urging governments and industries to work together to reduce man-made greenhouse gases (see the sidebar Hot News on page 93). Such international cooperation is not just a dreamit happened recently with regard to another problem of atmospheric change: damage to the ozone layer.
Depletion of the Ozone Layer
An ozone molecule (O3) is composed of three atoms of oxygen. Ozone in the upper atmosphere (the stratosphere) is referred to as the ozone layer and protects life on Earth by absorbing most of the ultraviolet (UV) radiation emitted by the sun. Exposure to too much UV radiation is linked to skin cancer, cataracts, and depression of the immune system, and may reduce the productivity of certain crops. Accordingly, stratospheric ozone is known as good ozone. In contrast, human industry creates ozone pollution at the ground level. This bad ozone is a principal component of smog. The ozone layer is reduced when man-made CFC molecules (comprised of chlorine, fluorine, and carbon) reach the stratosphere and are broken apart by short-wave energy from the sun. Free chlorine atoms then break apart molecules of ozone, creating a hole in the ozone layer. The hole in the ozone layer over the Antarctic in 1998 was the largest observed since annual holes first appeared in the late 1970s.10
CFCs were once used in aerosol sprays and as foam blowing agents. Their manufacture is now banned by an international treaty, the Montreal Protocol, signed by 160 nations. But because CFCs have a long atmospheric lifetime (about 50 years), those manufactured in the 1970s continue to damage the ozone layer today. The amount of CFCs in the stratosphere is now peaking. The good news is that scientists forecast that the ozone layer will return to its earlier, stable size by the middle of the 21st centuryassuming that nations continue to comply with the treaty.
When the ozone hole was first detected, there was emotional debate in which many U.S. industries fiercely resisted a ban on CFCs. It took a few years for scientists to show conclusively that human activity was causing the damage. It did not take long for scientists to invent other chemicals that could replace CFCs for industrial and commercial purposes, but would not harm the ozone layer. CFCs used as propellants were first banned in the United States in 1978.
Why Discuss Global Climate Change in Social Studies?
Developing an awareness of STS issues is central to teaching the Science, Technology, and Society strand in the NCSS standards. Such instruction enables learners to describe examples in which science and technology have changed the lives of people (for example, motor vehicles allow us to get places quickly and vacation far from home) and led to changes in the physical environment (for example, these vehicles have also led to superhighways, huge shopping malls, and parking lotsall of which are drastic changes in our landscape). Studying STS issues also enables students to cite examples of policies that govern scientific and technological applications (for example, the Montreal Protocol and subsequent amendments that have outlawed the manufacture of CFCs).
The Civic Ideals and Practices strand is addressed as learners realize that there are multiple points of view of the same issue. For example, extra atmospheric CO2 may fertilize crops, but the severe storms and regional droughts that may accompany global warming will damage them. Understanding the environmental consequences of personal actions is central to the previous strands as well as People, Places, and Environments and Global Connections. The National Science Education Standards identify science and technology and science in personal and social perspectives as two of the major content standards for students. Thus, inclusion of STS issues in social studies instruction links what students are learning in social studies and science classes, providing an interdisciplinary focus.11
Global warming and ozone layer depletion will doubtless receive continuing attention from the media and professionals in the fields of science, economics, and social studies. Jane Lubchenkco, professor of zoology at Oregon State University, contends that humanity is entering the Century of the Environment and calls for A New Social Contract for Science, that would attend to societal problems and social justice issues resulting from human impacts on the ecological systems of the planet.12 People in developing countries stand to suffer disproportionately greater problems than those living in wealthier countries.13 If so, developed countries need to seriously consider the ethics of the status quo, in which the have nots must endure the consequences of lavish energy consumption by the haves. Citizens of the wealthy nations may be called upon to help achieve a more equitable system by, for example, reducing personal energy consumption, paying a carbon tax for the environmental costs of global warming, subsidizing energy-efficient technologies being brought into the marketplace, or subsidizing the transfer of such technologies to Third World countries. Americans will face important decisions as consumers, voters, and global citizens.
Ideas for the Classroom
Applying scientific knowledge and taking citizen actions toward solving societal problems are integral to social studies education. The STS Investigation and Action instructional model sets forth four categories of citizenship actions: ecomanagement, consumerism, political action, and persuasion.14 We developed an STS investigation and action unit of study that is now available online (www.ed.psu.edu/CI/Papers/sts/gac-main.html).15 Based on research with students in grades five and above, the unit can be used for interdisciplinary instruction in social studies and science.
Introductory and investigative lessons provide prerequisite knowledge for citizenship actions. Each lesson includes social studies and science concepts as well as activities for investigating relationships between the concepts. For example, students use colored transparency film and painted cans to learn about the transmission, absorption, and reflection of light energy. They compute their familys monthly electricity and gasoline consumption and then estimate their familys contribution to atmospheric CO2.
Several lessons in the STS unit on climate change are designed to help students appreciate the relationship between acquiring knowledge and using it to resolve problems that affect society. Further, opportunities for action provide good creative outlets for students. For example, students in grades 5 and 6 can (1) make educational presentations using a storyboard to civic and church groups; (2) develop a brochure entitled We are the Caretakers; (3) enact a debate for students that includes representatives from the lumber industry, a ski resort, a power company, a fishing family living on the coast, and so forth, and (4) make environmental posters that suggest possible solutions to the problem of global warming and display these in public places.16
Teachers can use a pop bottle model to simulate global climate change, which helps students visualize the fluxes (rise and fall) of atmospheric CO2. The model suggests possible impacts of human behaviors on global warming, and how citizen actions collectively might contribute to the common good. For example: (1) purchasing additional motor vehicles would increase the flux of greenhouse gases into the atmosphere, whereas carpooling would decrease it; (2) planting trees (green pop bottles are used for sinks) would increase the flux of CO2 out of the atmosphere; and (3) burning tropical rainforests would both increase the flux into and decrease the flux out of the atmosphere. To help them understand the model, students can decorate the bottles with magazine images to represent different sources of CO2. They might create cards with one side illustrating a change in a source or sink (for example, ride a bike instead of driving), and the other side stating how the fluxes should be adjusted (decrease the flux of greenhouse gases into the atmosphere).
In the coming years, there will likely be debates in the U.S. Congress about what might be the best ways to slow global warming. Banning a single chemical, as was done for CFCs, will not be adequate. Agriculture, manufacturing, power generation, and transportation are a few of the human activities that could be affected by policies to reduce global warming. Without taking sides, a social studies teacher can present some of the opposing viewpoints to the class while also listing the possible costs, benefits, and risks of various policy proposals. Critical thinking will be called for. Who are the major figures in these debates? What interests do they represent? What sources of information do they cite? With a basic introduction to some of the atmospheric science, students should be able to follow much of this news and get involved if they wish. We will all be affected by the outcome.
Finally, teacher modeling of responsible citizenship actions is another suggestion for social studies instruction that emerges from research on elementary teachers. Some teachers operate under the premise that if teachers show active interest and involvement in taking citizenship action, then students are more likely to do the same. One teacher simply put it this way: I think our best teaching is [by] example. We cant tell them to do things and not do them ourselves.17
1. Its Too Darn Hot, UCAR Quarterly 27 (1998): 12; National Climate Data Center, Climate of 1999 Annual Review (Washington, DC: National Oceanic and Atmospheric Administration, 2000). Online at www.ncdc.noaa.gov/ol/climate/research/1999/ann/ann99.html.
2. K. Rasmussen, Teaching Social Studies for Meaningful Learning, Curriculum Update (1999, Winter): 3, 6.
3. Our unit of study on global climate change is online at www.ed.psu.edu/
CI/Papers/sts/gac-main.html. For those who do not have web access, a sample Investigation lesson, Human-Produced Greenhouse Gases and the Enhanced Greenhouse Effect, can be ordered. Please send $1 (payable to West Virginia University) to cover copying and mailing to Dr. James A. Rye, P.O. Box 6122, West Virginia University, Morgantown, WV, 26506.
4. N. Harms and R. Yager, What Research Says to the Science Teacher (Washington, DC: National Science Teachers Association, 1982); Project Synthesis was an initiative of the National Science Foundation in the late 1970s in which experts critically examined the state of science education. Recommendations for the ideal state of science education were set forth in four goals: personal needs, societal issues, academic preparation, and career education.
5. National Council for the Social Studies, Science and Society Committee, Teaching about Science, Technology, and Society in Social Studies: Education for Citizenship in the 21st Century, Social Education 54 (1990): 189-193; National Council for the Social Studies, Expectations of Excellence: Curriculum Standards for Social Studies (Washington, DC: NCSS, 1994).
6. J. Houghton, L. Filho, B. Callander, A. Harris, A. Kattenberg, and K. Maskell, eds., Climate Change 1995: The Science of Climate Change (New York: Cambridge University Press, 1996); J. Rye, Understanding the Role of Chlorofluorocarbons in Global Atmospheric Change, Journal of Geoscience Education 46 (1998): 488-493; F. Mackenzie, Our Changing Planet: An Introduction to Earth System Science and Global Environmental Change, 2nd ed. (Englewood Cliffs, NJ: Prentice Hall, 1998); J. Rye and P. Rubba, Student Understanding of Global Warming: Implications for STS Education Beyond 2000, in D. Kumar and K. Chubin, eds., Science, Technology, and Society: A Sourcebook on Research and Practice (New York: Plenum, 2000): 193-230.
7. K. Taylor, Rapid Climate Change, American Scientist 87 (1999): 320-327.
8. Scientists Statement on Global Climatic Disruption, 1997 (Washington, DC: Ozone Action, 1997). Online at www.ozone.org/stateii.html.
9. K. McDonald, Debate Over How to Gauge Global Warming Heats up Meeting of Climatologists, The Chronicle of Higher Education (February 5, 1999); D. Bender and B. Leone, eds., Global Warming (San Diego: Greenhaven Press, 1997).
10. R. Kerr, Deep Chill Triggers Record Ozone Hole, Science 282 (1998): 391; M. Dameris, V. Grewe, R. Hein, C. Schnadt, C. Bruhl, and B. Steil, Assessment of the Future Development of the Ozone Layer, Geophysical Research Letters 25 (1998): 3579-3582.
11. National Research Council, National Science Education Standards (Washington, DC: National Academy Press, 1996).
12. J. Lubchenko, Entering the Century of the Environment: A New Social Contract for Science, Science 279 (1998): 491-497.
13. Intergovernmental Panel on Climate Change, Climate Change 1995: IPCC Second Assessment Report (Geneva, Switzerland: World Meterological Organization, 1995); P. Martens, How will Climate Change Affect Human Health? American Scientist87 (1999): 534-541.
14. P. Rubba and R. Wiesenmayer, Increased Action by Students, in R. Yager, ed., The Science, Technology, Society Movement: What Research Says to the Science Teacher (Washington, DC: National Science Teachers Association, 1993) 7: 169-175.
15. P. Rubba, R. Wiesenmayer, J. Rye, M. McLaren, K. Sillman, K.Yorks, D. Yukish, T. Ditty, V. Morphew, C. Bradford, D. Dorough, and K. Borza, Global Atmospheric Change: Enhanced Greenhouse Effect, Ozone Layer Depletion and Ground Level Ozone Pollution (University Park, PA: Pennsylvania State University, 1995); See also note 2.
16. M. McLaren, K. Yorks, D. Yukish, T. Ditty, P. Rubba, and R. Wiesenmayer, Taking Actions on Global Warming: What Middle School Students Have Done, Bulletin of Science, Technology and Society 14 (1994): 88-96.
17. P. Rubba and J. Rye, The Views of Four Elementary Teachers on the Importance of Modeling Responsible Citizenship Action, Journal of Elementary Science Education 9 (1997): 82-98.
James A. Rye is an assistant professor in Educational Theory and Practice at West Virginia University in Morgantown, Virginia. Donna D. Strong is a teacher at Thompson Middle School in Alabaster, Alabama. Peter A. Rubba is a professor, Director of Academic Programs, and Director of the World Campus at Pennsylvania State University in University Park.
This article was originally submitted to the Elementary department of Social Education, whose editor is Mary Haas.
Websites on Global Climate Change
Greening Earth Society, an organization sponsored by some utilities and energy companies, provides a contrarian point of view against apocalyptic predictions of climate change. www.greeningEarthsociety.org
Intergovernmental Panel on Climate Change (IPCC) provides reports on human-induced climate change based on peer-reviewed scientific literature. The IPCC also sets goals for developed and developing countries to meet for reducing the industrial production of greenhouse gases. The IPCC is an interesting experiment in international and scientific consensus building and government cooperation. Watch the news for congressional debates about whether the IPCC goals are fair and how nations might best achieve them. www.ipcc.ch
National Aeronautics and Space Administration (NASA) provides Total Ozone Mapping Spectrometer (TOMS) multimedia files that show the history of the ozone hole. For example, one can view a graph that tracks the size of the Antarctic ozone hole from 1979 to 2000, or a recent color image of it. jwocky.gsfc.nasa.gov/multi/multi.html
National Climatic Data Center (NCDC) provides surveillance on United States and global temperatures and produces reports on climate. www.ncdc.noaa.gov
National Environmental Education and Training Foundation (NEETF) provides Report Cards on Americans understandings of environmental issues. www.neetf.org
United States Environmental Protection Agency (EPA) provides sites on ozone depletion (www.epa.gov/ozone/resource/public.html), global warming (www.epa.gov/globalwarming), and related information and activities for students and teachers (www.epa.gov/students). See also The Sunwise School Program for grades K-8, which is about ozone depletion and ultraviolet radiation. www.epa.gov/sunwise
Misconceptions about Global Atmospheric Change
Our research with students (upper elementary and above) reveals that they hold some misconceptions about two global atmospheric change issues: ozone layer depletion and global warming.* Our findings, corroborated by others, have implications for a citizenry that needs to make informed actions on issues that affect our global society.**
One misconception is that aerosol sprays purchased today contain chlorofluorocarbons (CFCs) and are a major cause of ozone layer depletion. By international agreement, such sprays are no longer manufactured. The real concern is that CFCs from old refrigerators and air conditioners be properly disposed of, rather than continuing to leak CFCs into the atmosphere.
Another misconception is that ozone layer depletion causes global warming by letting in extra solar radiation. This idea connects two issues (global warming and depletion of ozone in the upper atmosphere) inappropriately, which diverts attention from the main cause of global warming: the combustion of fossil fuels. Ozone depletion allows more ultraviolet radiation to reach the Earths surface, which can cause cancer, but the amount of warming resulting from this radiation is negligible.
These misconceptions may be reinforced by the popular press and are not limited to children: a national survey reveals that almost one-third of adults think spray cans are the leading source of CFCs (they no longer are), and over one-half believe that the majority of our electricity is generated from sources that do not produce carbon dioxide; but power generation is indeed a major source of this greenhouse gas.***
* D. Dorough, J. Rye, and P. Rubba, Fifth and Sixth Grade Students Explanations of Global Warming and Ozone: Conceptions Formed Prior to Classroom Instruction (Paper presented at the National Association for Research in Science Teaching Annual Meeting, San Francisco, CA, April, 1995).
** E. Boyes and M. Stanisstreet, The Greenhouse Effect: Childrens Perceptions of Causes, Consequences, and Cures, International Journal of Science Education 15 (1993): 531-552; V. Christidou and V. Koulaidis, Childrens Models of the Ozone Layer and Ozone Depletion, Research in Science Education 26 (1996): 421-436; A. Hobson, Ozone and Interdisciplinary Science TeachingLearning to Address the Things that Count Most, Journal of College Science Teaching 23 (1993): 33-37.
*** National Environmental Education and Training Foundation, 1998 National Report Card (Washington, DC: 1998). Online at www.neetf.org.
Hot News: Climate Change Update
As we go to press, the Intergovernmental Panel on Climate Change (IPCC) is in the process of releasing their newest document on the science of climate change. A draft Summary for Policymakers is available at the IPCC website (see Climate Change 2001: The Scientific Basis at www.ipcc.ch). Therein, atmospheric scientists report that the average surface temperature of Earth has increased by approximately 0.6°C (1.1°F) over the past century. Further, there is now stronger evidence that human activitiesespecially the burning of fossil fuels and the subsequent increase in atmospheric levels of the greenhouse gas carbon dioxideare responsible for most of the warming over the past 50 years.
The IPCC now predicts that the average global temperature will increase by 1.5° to 5.8°C by the end of the 21st century, which is a higher estimate than that in the previous IPCC report and is made on the basis of more evidence. An increase of a few degrees Centigrade may not sound like much, but it probably would accelerate the retreat of glaciers and ice caps. Sea level could rise enough to flood coastal cities, especially if temperature increases are sustained beyond the next century. This rate of temperature increase is very likely to be without precedent during at least the last 10,000 years. The greenhouse gases that are emitted now will affect climate for centuries, in part because severa#151;most importantly, carbon dioxidehave long atmospheric lifetimes. The implications of these findings for society and public policy are great.