Another human-induced perturbation of the atmosphere involves the thinning of the layer of ozone high in Earth’s atmo- sphere (10 to 35 km above the surface). This layer absorbs ultra- violet photons from the Sun, which are lethal to plant and animal life near the surface. The natural process of ozone destruction and formation involves the break up of ozone (O3) molecules by sunlight to form atomic and molecular oxygen (O and O2, respectively), and reformation of O3in a chemical chain involv- ing oxygen, water, and sunlight. Certain reactive elements such as chlorine can accelerate the break up of ozone by acting as a catalyst: a single atom of chlorine (Cl) reacting with ozone pro- duces ClO and stable O2. The ClO is also reactive and quickly combines with another oxygen atom to produce atomic chlorine and an O2 molecule. The chlorine atom is now free to attack another ozone molecule.
Eventually the chlorine may react with other molecules in the atmosphere to produce more stable compounds such as HCl or ClNO3 that are removed from the stratosphere – but only after a large number of cycles in which the chlorine has destroyed ozone. Human-produced refrigerants called chloroflu- orocarbons, or CFCs, contain chlorine. These compounds drift up into the stratosphere, where the chlorine is released by solar ultraviolet radiation. Since the mid-twentieth century this source of chlorine has exceeded natural sources such as sea salts,
which is why CFCs are accelerating the loss of ozone from the stratosphere.
Monitoring by satellites and aircraft now provides a world- wide picture of stratospheric ozone variations. Since the mid- 1970s an ozone hole has appeared over Antarctica, indi- cating enhanced destruction of ozone there. Apparently the cold winter temperatures and sluggish atmospheric circula- tions above the southern polar region encourage the forma- tion of water-ice clouds in the stratosphere. The ice parti- cles act as chemical sites where the HCl and ClNO3 react with each other to form Cl2. As spring approaches and sun- light hits the Antarctic atmosphere again, the Cl2 is broken apart into atomic chlorine, Cl. Thus, instead of being removed from the atmosphere, the chlorine is recycled into its active catalytic form again, enhancing the destruction of ozone and producing the deep springtime ozone hole around the south pole.
The particular climate conditions over the Antarctic conspire to make that region more sensitive than other parts of the globe to the introduction of chlorine into the stratosphere. However, this does not mean the rest of the world’s ozone is immune to the effects of enhanced chlorine. Ozone depletion is seen at all latitudes in the northern and southern hemispheres. Because the residence time of active chlorine in the stratosphere is decades, enhanced destruction of ozone worldwide will continue even though international agreements forced the phasing-out of CFCs beginning in the late 1980s.
Summary
Excellent data from ice cores and measurement of atmospheric CO2since the middle of the twentieth century shows that this greenhouse gas has increased by 40% since the time prior to beginning of the Industrial Revolution; most of this increase is the result of human activities. Other greenhouse gases have increased as well. The record of temperature, averaged over the year and over the surface of the Earth, is more difficult to interpret because of the urban heat island effect and the vari- able quality of instruments over time, but the global average temperature seems to have increased about 1 degree Celsius since the nineteenth century, and the increase became steeper after 1970. The results are consistent with what is expected in a simple, physically sound model of the balance between infrared and visible radiation in a cloud-free atmosphere – that is, the Earth’s lower atmosphere is warming up due to increase in greenhouse gases. Complications in the exact relationship between the increase in greenhouse gases and temperature include the role of water as vapor and clouds, atmospheric convection, and the role of the oceans in absorbing and releas-
ing heat and greenhouse gases. Detailed predictions of the role of the oceans and of changes to regional weather pat- terns associated with global warming are obtained though gen- eral circulation models of the Earth’s atmosphere and oceans.
Such models do well in predicting certain effects, like increased overall precipitation, but other aspects of changing climate are more uncertain. Crucial to the behavior of Earth’s climate in the next century is the role of ocean circulation, for example, potential changes to the frequency and intensity of the El Ni ˜no phenomenon, and more speculative possibilities such as reduc- tion in the thermohaline circulation of the North Atlantic as accelerated glacial melting and added rainfall change the sur- face saline content of the ocean. As we grapple with global warming, another human-caused impact on the environment, stratospheric ozone depletion, appears to have been resolved by international protocols on prohibition of industrial chemi- cals known to accelerate destruction of this molecule essential for reducing the amount of solar ultraviolet radiation reaching Earth’s surface.
Questions
1. Consider how you, as a policy maker, would weigh the eco- nomic consequences of various responses to global warming predictions. Would you take aggressive action now or a wait- and-see attitude?
2. How would you seek to eliminate the urban heat island effect from data sets used to construct global average temperature over time from the mid-nineteenth century to the present?
3. What might the response of climate be to the oceans if, hypo- thetically, they extended no more than 100 meters deep (as opposed to the actual, deep-ocean situation on Earth)?
4. Do a literature search to collect evidence for shrinkage of ice over the last century from glaciers, mountain tops and the Earth’s polar regions. What evidence is there that this shrinkage is not part of a normal cyclical waxing and waning?
General reading
National Research Council. 2010.Advancing the Science of Climate Change. National Academies Press, Washington DC.
Peixoto, J. P. and Oort, A. H. 1992.Physics of Climate. AIP Press, New York.
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23
Limited resources: the human dilemma
Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it.
HELEN KELLER
Introduction
Only in the last century has humanity’s command of technology and energy made it possible to feel a sense of security unknown for most of human history. And yet, ironically, this new sense that we can obtain and control what we need to make life lengthy and of high quality comes just as we find ourselves depleting what were once thought to be virtually inexhaustible
resources. As our numbers grow, how will humankind con- tinue to sustain the industrialized civilization that has made high standards of living – or the aspiration for such standards – a staple theme of the last century? This chapter addresses briefly the issues of future supplies of food, energy, and material resources.