Within a decade or two the Earth's stratospheric ozone will be destroyed by human-induced effects. Will this lead to a catastrophe for life-forms on the surface?
We are now entering the last third of this course on Earth Catastrophes, and will focus our attention on the surface environment, where atmospheric, oceanic, and solid Earth phenomena interact. This is of course the location of human existence, and our understanding of the complex systems operating at the surface of the planet is key to mitigating the hazards that arise. In the cosmological portion of the class we found that nuclear forces, fusion energy, supernovae, gravity and collisions played major roles in the history of the Earth. In the portion of the class dealing with internal solid Earth processes we found that temperature structure, rheology, melting, magma properties, strain accumulation and release, and convection played major roles. At the surface, solar power, gravity, and the action of water will be paramount, but chemical reactions play a central role as well.
We'll begin by considering the atmosphere, which a gaseous envelope on the Earth, largely expelled from the interior by volcanic emissions. The atmosphere is a very complex dynamical system, with myriad scales of chemical and dynamical processes that give rise to the effects of interest for this class. While typical awareness of research on the atmosphere extends no further than weather forecasting, there has been a huge research effort on atmospheric phenomena for many decades, and this continues today. The system is rich in complexity and challenging to understand to the level required for predicting the future behavior of the system.
The atmosphere is of interest for multiple scales, including large scale climatic patterns on the surface as well as short-term phenomena such as storms and tornadoes. We'll consider both climate (average atmospheric properties over time scales much longer than weeks or months) and weather (atmospheric events with durations from minutes to weeks) effects and their associated catastrophic influences on humanity. First, we must get a handle on what the atmosphere is made of and how it works as a system.
The atmosphere has a relatively simple bulk composition, with 79% nitrogen (N2) and 20% oxygen (O2), and almost 1% argon. That does not leave room for much else, does it? There are other gases in the atmosphere, such as carbon dioxide (CO2) which is 340 PPM and slowly increasing at present, and water (H2O) which varies from 1-4% in ephemeral component of the atmosphere. The oceans have more than 300 times more water than is present as water vapor in the atmosphere. Water cycles into and out of the atmosphere faster than other components (via evaporation/precipitation), and this is an essential part of the surface system, in that the water cycle is key to life on the Earth. Another trace gas in the atmosphere is Ozone (O3), which is 2-12 PPM 30 km high into the atmosphere. While such traces gases are negligible when considering the atmospheric volumes moving around in weather patterns, we will see that the trace gases have critical roles in heating the atmosphere and associated screening of electromagnetic radiation passing through the atmosphere.
The distribution of gases in the atmosphere is not uniform, as heavier molecules such as oxygen (O2) are concentrated low in the atmosphere. Even climbing a reasonable mountain will reinforce for you that oxygen content depletes with altitude. The variation of components of the atmosphere with altitude leads to a stratification of the atmosphere in terms of thermal structure and chemical processes. The atmosphere gradually thins and there is no abrupt demarcation of the end of the atmosphere, as there are still more gas molecules 200 km up than in deep space.
Energy from the sun arrives at the top of the atmosphere as solar radiation. The spectrum of the solar radiation is the relative power of different wavelengths of light. This is largely controlled by the surface temperature of the son, which is about 6100 degrees C. We all know that hot objects glow differently depending on their temperature. The specific temperature of the son causes the peak energy radiated to be near the visible light band, with diminishing ultraviolet and infrared radiation at shorter and longer wavelengths respectively. This is not a surprising fact, since human eyes evolved to exploit the primary energy available in the environment, which proves to be the 'visible' light wavelengths. Of course, our nomenclature is defined by human vision, which is the result of evolution with lots of light in the 0.4-0.7 micron range being available. The Earth is much cooler than the son, with a surface temperature of around 15 degrees C. Thus, the radiation from the earth is shifted way toward the infrared spectrum. This effectively means that there is an asymmetry of radiation input into the atmosphere, with shorter wavelength energy coming in from the solar radiation and longer wavelength energy coming up from the Earth. This is key to the Greenhouse effect which occurs on this, and all other planets with atmospheres.
There are three main mechanisms of heat transfer in any physical system; conduction, radiation and convection. We have seen that within the rocky Earth conduction is important within boundary layers such as the lithosphere, while convection is important within the bulk of the interior of the mantle and core. In the atmosphere radiation and convection are the most important processes of heat transfer. Incident energy, either from above or below the atmosphere can have a range of interactions: it can reflect, be absorbed, scatter or transmit through the atmosphere. Depending on the type of interaction, the energy can heat up the atmosphere producing differential pressures that induce convective motions. Let us consider the type of interaction.
An important interaction is reflection of incident energy, either from within the atmosphere or from the Earth's surface. Mainly reflection is caused by clouds and light-colored surfaces such as snow or ice (they are white, or light, because all wavelengths of light are reflected; remember white light results from a mix of all colors of light). The albedo is the ratio of outgoing to incoming radiation. For example, the highly reflective snow cover at the South Pole has an albedo of 0.96, so the energy does not heat the ice much because it is almost entirely reflected. When the Earth has a higher percentage of cloud cover, the same occurs, with less solar radiation penetrating to the surface. We actually can estimate the total reflection from the Earth, and hence its albedo, by the clever approach of looking at the moon. Even when only a sliver of the a new moon is visible (the moon is between the Earth and sun, so we do not see the illuminated, sunward side) we are able to see the entire circle of the moon (try it to make sure). How can we see the dark moon? Well the answer is that the moon's dark side is illuminated by energy reflected from the Earth. This so-called Earthshine, indicates and albedo for the atmosphere of about 0.33. Long-term changes in water content of the atmosphere can cause the albedo to vary, which suggests that the heat available to penetrate into the atmosphere or to the surface varies as well.
Absorption is a process by which molecules in the atmosphere interact with electromagnetic photons with different frequencies and either do or do not absorb the energy and become excited (heated). Absorption is a strong function of the wavelength of radiation that is available and of the atmospheric composition. In particular, the minor gases play a major role because unlike nitrogen and oxygen (O2) they can absorb extensive components of the solar radiation. This is important because absorption causes the atmosphere to heat up, and the layered chemical variations thus cause complex thermal structure with altitude into the atmosphere. The gases that preferentially absorb electromagnetic radiation in the infrared band are called Greenhouse gases. These gases are transparent to short waves but absorb long waves. Examples of important Greenhouse gases are carbon dioxide, water, methane and ozone. Water and carbon dioxide are particularly effective in absorbing the infrared band (remember, that is where the Earth's radiation is peaked), while ozone is very effective at absorbing ultraviolet radiation. The atmosphere actually allows visible light to pass with little absorption, which is a fortuitous result of chemistry and the particular composition of our atmosphere. This transparency to visible light, compounded by the fact that the sun preferentially radiates visible light has strongly affected life on Earth, in the very nature of photosynthesis and in the evolution of sensory organs that could exploit visible light. At the same time, we evolved in an environment shielded from ultraviolet and infrared radiation, so there are unpredictable effects of modifying the atmosphere's filtering behavior that need to be considered.
The other important process is scattering, which is another wavelength dependent phenomenon. The molecular structures in our atmosphere are such that they more strongly interact with shorter wavelength (blue) light. The sky appears blue, because an observer at any point on the surface senses light scattered from many positions in the atmosphere, with the blue light being systematically the more scattered component. Red light propagates with little scattering through the atmosphere, so sunsets and sunrises appear red, because the long oblique path through the atmosphere scatters away the blue component, leaving the red to be seen when you look toward the sun. The sun itself has a yellowish color because of the scattering of the blue light.
So, the effects of reflection, absorption and scattering control the balance of light energy penetrating through the atmosphere to the surface, or from the surface outward, as well as the internal heating of the atmosphere. If we consider the total power in incoming solar radiation, about 3% is absorbed in the stratosphere by ozone (preferentially UV light at that), 13% is absorbed by water and dust in the troposphere, 40% is absorbed by clouds (some of which is backscattered into space and some of which is scattered to the surface. Overall, at the surface about 47% of the incident power reaches the surface, 22% which is direct from the input radiation and 25% of which was absorbed and radiated by clouds on the way through. This is the energy affecting the lowermost weather systems, the diurnal heating of the surface, and the human environment. Some of the energy is backscattered by the surface, and some is re-radiated from the heated surface, along with radiation from the latent heat of the planet. The outgoing radiation from the top of the atmosphere is about 35% from backscattering from clouds, dust, air, and surface interactions, while 65% is from long wavelength energy radiated from the surface, most of which has been absorbed by clouds and reradiated on the way up. This is a very complex system of energy transport, and of course it is actually a three-dimensional system.
One consequence of the absorption, reflection, scattering processes is that the thermal structure of the atmosphere is complex. Above 100 km altitude, the Thermosphere is quite hot, in the thin atmosphere where there is much reflection and absorption of incident radiation the temperatures can exceed 30 degrees C. There is a relatively cold region at altitudes of 80-90 km, in the Mesopause, and then from 80 to 55 km there is an increase of temperature in the Mesophere. From 45-55 km there is a local maximum temperature near 0 Centigrade in the Stratopause, below which the temperature decreases in the Stratosphere down to about -60 Centigrade at 25 km high. The minimum in the Tropopause is from 15-25 km, with temperature then increasing through the Troposphere to a surface level of about 15 degrees, with a gradient of 7 degrees/km.
So, even if the Earth had uniform radiation from all directions incident on the atmosphere, there would be rather complex thermal structure and associated chemistry. But, in fact, the heating of the atmosphere is very non-uniform, since there is a much greater input of solar radiation at equatorial latitudes (recognizing the tilt of the Earth and the orbital changes that control the geometry relative to the sun. The peak distribution of solar radiation is fairly flat between +/- 30 degrees N/S of the equator. This is in part due to high clouds presence at the equator. Thus, the surface heating has a latitudinal zonation, with hottest sea-surface temperatures in the +/- 30 degree latitude range, a corresponding maximum in evaporation and precipitation In this region, and a consequent latitudinal separation of weather patterns.
The key result of the non-uniform heating of the atmosphere and surface is that differences in pressure in the atmosphere result (high pressure regions are colder and denser, while low pressure regions are hot and expanded). Lateral gradients in pressure cause the gas to flow laterally. This convective motion is not simple in geometry because of the tilt of the Earth relative to the plane of the ecliptic, the effects of rotation, and the non-uniform distribution of continental masses which modify the convective pattern.
In general, there is a system of convection cells at the low latitudes of +/- 30 degrees that are called Hadley cells. These arise due to upwelling hot, water vapor rich equatorial airs, that rise, cool, move either north or south and then precipitate in concentrations near +/- 30 degrees. In the northern hemisphere, northward movement of the air masses assumes a rightward deflection relative to the solid Earth reference frame, leading to clockwise geometry of the Hadley cells, while the southern hemisphere experiences leftward or counterclockwise systems. The Hadley cells are thus three-dimensional cells of rising, rotating and descending currents, all driven by differential heating of the atmosphere. There are strong westerly winds associated with the northern and southern hemisphere systems near the +/- 30 degree latitudes. At higher latitudes the atmospheric circulation is dominated by strong Jet Streams in both hemispheres, with undulatory patterns of strong winds blowing from the west. The jet stream shape is wavelike and is called a Rossby wave. The atmospheric patterns near the poles are complex and variable depending upon the season (position of the tilted axis with respect to the sun).
Within the large-scale patterns of Hadley Cells and Jet Streams, there are many complexities of the regional dynamics. In large part this is the result of interaction with the surface roughness of the planet. Topography effects on atmospheric circulation are called orographic effects. An example is the windward side precipitation induced by mountain ranges as air rises, cools, and water condenses out. Dry areas and even deserts tend to be downwind of mountains. The daily heating of the land causes fluctuating sea and land breezes. Finally, there are complex turbulence and eddies in the dynamic system that give rise to storms, hurricanes, and tornadoes.
Let us leave dynamics for the moment and consider the role of selective absorption by trace gases in the atmosphere. This brings our attention to the controversial area of Ozone depletion. Ozone is a relatively unstable molecule involving three oxygen atoms. Ozone spontaneously generates in the stratosphere, but it is a minor component overall. There is a lot of ozone generated in pollution, at the base of the troposphere, but much of this has a diurnal existence, breaking apart with the cooling off during the night. The lower atmosphere ozone is an important Greenhouse gas, but let us focus on the stratospheric ozone. More than 20 years ago a debate initiated over the fragility of ozone and human disruption of ozone. Already, the importance as a filter of UV light was known. The U.S. adopted then, and continues today a ban on SuperSonic Transports (airplanes that fly faster than the speed of sound and at high altitudes to reduce frictional heating). The reason was that it was demonstrated that the shock wave from sonic booms could disrupt ozone molecules. While this has not kept the French from flying SST's, it is now recognized that a more insidious human activity was rapidly destroying the stratospheric ozone.
This involved the production of chlorofluorocarbons (CFC's), of which the most important is ClO (Chlorine monoxide). CFC's are key components of many refrigerant systems that developed in the U.S. and Europe in the 1950s to 1960s. ClO is a very effective destroyer of ozone, because it reacts with ozone to produce a free chlorine atom with 2 O2 atoms. The chlorine atom is highly reactive and quickly bonds with an oxygen atom to remake a ClO molecule which then can break down another ozone molecule. In this way, chlorine monoxide can gobble up all of the nearby ozone. In the mid 1970s the U.S. banned the production and atmospheric release of CFC's, but there are still 3 billion pounds being produced worldwide today.
The existence of CFC's, some of which are exposed to the atmosphere in the chemical production or in the decay of old refrigerant systems, is blamed for a systematic decrease in ozone detected in the stratosphere over Antarctica commencing in about 1974. From 1974 to 1985 there was a factor of two reduction in ozone over Antarctica that was brought to international attention in 1985. In 1988 mid-latitude depletion of 4-5% of stratospheric ozone over the U.S., Australia and Argentina was detected. Every year since 1985 there have been new record low ozone levels over Antarctica. In November, 1991 there were 3% depletions in ozone in the summer months at temperate latitudes in the northern hemisphere. In 1992 a massive cloud of ClO was detected over Europe, with 30-40% loss of ozone by April of that year. The March, 1992 eruption of Mt. Pinatubo, which ejected gases into the stratosphere appears to have compounded the chlorine content of the stratosphere.
So, where do we stand? It looks likely that ozone depletion will continue, because there is no known natural or artificial way to cleanse the stratosphere of ClO. Some scientists expect the entire ozone layer to be eradicated within a few decades. Already, there are increased levels of skin cancer in Australia and Southern Chile. While ozone is a very minor component of the atmosphere, it is the main UV absorber. It appears that humanity has unknowingly initiated a massive test experiment: can humans survive on a planet in which the UV radiation reaching the surface becomes much more intense than it ever was during the evolution of our species? Get out your sunblock!
![[return icon]](../dinoheadt.jpg)
Return to the Earth Sciences 80A Lectures Page