|
|
| Modification of earth's climate | During the past 20 years or so, scientists and policy-makers have been concerned about the effect of human activity on the earth's climate. An industrial economy consumes large quantities of energy, and to the extent that this energy is obtained by burning fossil fuels (oil, coal, and natural gas), carbon dioxide (CO2) is released to the atmosphere. Increased levels of CO2 will increase the earth's temperature, leading to a variety of climatic changes, and consequent effects on living things. | ||||||||||||||||||||||||||||||||||||||||
| Mean temperature | The issue here is a mean, or average, temperature. Clearly there are temperature variations over the day/night cycle, over the cycle of the seasons, and at different latitudes on the the globe. But if we average over these three variables, we find that the earth has a mean temperature of about 14 degrees Celsius (57o Fahrenheit). | ||||||||||||||||||||||||||||||||||||||||
| Equilibrium | To get at the effect of the gases in atmosphere, we first have to understand the concept of thermal equilibrium, which applies to the earth, and to all natural systems. Normally the earth is kept at a constant temperature, because the energy it absorbs from the sun is just balanced by energy that it emits to space. What is this energy that earth emits? | ||||||||||||||||||||||||||||||||||||||||
| Thermal radiation | Every surface that is above the temperature of absolute zero (-273o C, -459o F) emits electromagnetic energy in all directions. If the temperature is high (thousands of degrees) the energy is mostly in the form of light. This is the case for the sun, the stars, as well as the filament of an electric light bulb. At lower temperatures (say, a few hundred degrees Celsius) the energy is mostly infrared radiation. You can feel infrared as heat if you hold your hand near the burner of an electric stove or a toaster oven. It's also true that at lower temperatures the total amount of energy emitted (in a given time) is less than at higher temperatures. Surfaces at room temperature also emit infrared, but this radiation is too weak for you to feel it. | ||||||||||||||||||||||||||||||||||||||||
| Approaching equilibrium | It's not just chance that the infrared emitted to space by the earth exactly balances the energy received from the sun. If we imagine that at some time in the past the earth emitted less energy than it received (and it's believed that this was the case early in the history of the solar system), the effect would be for the earth to get hotter over time - just because there was a net gain in energy. As this happened, the earth's infrared radiation would also increase. But the sun's radiation remains the same. As long as the energy emitted is less than the energy received, the earth's surface temperature keeps going up. But once it reaches the point where the energy emitted just balances the energy received, the temperature stops changing. That's the equilibrium point, and that's how the earth stays. The temperature at which the earth hits this equilibrium depends primarily on how much solar radiation it receives. | ||||||||||||||||||||||||||||||||||||||||
| Albedo/Atmosphere | Two factors modify the discussion above: (1) Not all the incoming solar radiation is absorbed by the earth. A certain fraction (about 30%) is reflected out to space, just as light is reflected from a mirror, or a white wall. This fraction is called the albedo. (2) The earth has an atmosphere. | ||||||||||||||||||||||||||||||||||||||||
| Predicted temperature | If we neglect the effect of the atmosphere, the equilibrium condition above gives us a prediction for the earth's surface temperature: The temperature would be a very chilly -19o C (-2o F). This is obviously not the right answer (although it should be about right for the moon, which is the same distance from the sun and has no atmosphere). The earth is temperate just because it has an atmosphere. | ||||||||||||||||||||||||||||||||||||||||
| Constituents of the atmosphere | The gases in the atmosphere, and their percentage of the whole atmosphere, are as follows:
| ||||||||||||||||||||||||||||||||||||||||
| Life's effect on the atmosphere | It is worth noting that, compared to atmospheres elsewhere in the solar system, the list above has a bizarre cast of characters. Mars and Venus, for example, have atmospheres primarily of CO2. Oxygen is rarely found, except for the earth. There is every reason to believe that N2 and O2 are the major constituents of the earth's atmosphere as a consequence of the existence of life on earth. The metabolism of plants puts nitrogen into the soil and the atmosphere, and oxygen into the atmosphere. | ||||||||||||||||||||||||||||||||||||||||
| Greenhouse gases | But even at very low levels, the molecules of CO2, H2O, methane, and nitrous oxide play an important role in the history of the earth. These molecules continually absorb and re-emit the infrared radiation coming up from the surface. (Molecules do not absorb radiation in the visible range, which makes up most of the sun's energy.) Each molecule remains in a kind of equilibrium, in the sense that it emits a quantum of infrared energy on the average about as many times as it absorbs one. (Infrared, light like, can be thought of as a stream of particles, sometimes called photons, sometimes called quanta.) But the energy absorbed is all radiation on the way up (from the earth's surface); the energy emitted is in all directions. Half of the emitted energy goes up - toward space - and half goes down - back toward the surface. The net effect is that these greenhouse gases trap infrared energy, keeping the atmosphere and the surface hotter than they would be if not for the atmosphere. This is why the earth's temperature is not -19o C, as it would be with no atmosphere, but a comfortable and life-supporting +14o C (57o F). Thus, there is no question about the fact that the greenhouse effect exists, and has created warmth at the earth's surface. | ||||||||||||||||||||||||||||||||||||||||
| Earth's atmosphere in the past | It is also well established that the level of carbon dioxide, the most important greenhouse gas, is higher today than in the past. Geologists determine the makeup of air from distant centuries by analyzing air bubbles locked in Antarctic ice sheets laid down at various times in the past. It is found that for many centuries before the industrial revolution, the percentage of CO2 was 0.028%; it then increased rapidly to the current 0.035% during the last two centuries. | ||||||||||||||||||||||||||||||||||||||||
| Mean temperature in the past | Whether there has been a concurrent increase in average global temperature is a more difficult question. To find the earth's mean temperature we have to average over places and times, and there is a limit to how accurately this can be done. But the best estimates are that a warming of between 0.3o C and 0.6o C has occurred over the past century. The year 2001 was one of the warmest during that period. | ||||||||||||||||||||||||||||||||||||||||
| Temperature/CO2 correlation | There is also good evidence that the mean temperature of the earth correlates well with CO2 levels. Estimates of CO2 going back 160,000 years show variations between 0.028% and 0.018%, and the temperature (which varies up and down by about 10o C) tracks very closely with the CO2 level. | ||||||||||||||||||||||||||||||||||||||||
| Computer modelling | Finally, what can we expect in the future? Climate scientists don't attempt to predict CO2 levels. Rather, they take assumed levels, and try to calculate what the effect will be on temperature. Computer modelling is difficult, because calculations must take account of variations of air temperature, air velocity, and air content at different latitudes and altitudes over the globe, and well as effects of seawater, landmasses, and clouds. One recent compilation finds that if CO2 goes up to 0.056% (twice the pre-industrial level), the increase in mean temperature will be between 1o C and 5o C - giving you an idea of the uncertainty in these estimates. | ||||||||||||||||||||||||||||||||||||||||
| Secondary effects | Nevertheless, temperature increases of this kind can have serious effects on plant, animal, and human life. For one thing, temperature changes can have secondary climatic effects: Higher temperatures would evaporate more water and increase humidity in some locations. Higher temperature melts some of the polar ice caps, and this in turn increases sea and other water levels around the globe. Smaller ice caps reflect less of the incoming solar radiation (reduce the earth's albedo), which further acts to make the earth hotter. Plants and animals, particularly those at the bottom of the food chain, like microscopic plankton in the sea, are sensitive to temperature. Continental areas that are now ideal for growing grains, like the U.S. midwest, may become deserts. Flooding is possible in some densely populated coastal areas, and in low lying countries such as Holland. | ||||||||||||||||||||||||||||||||||||||||
| The Arctic | The poles are more sensitive to atmospheric warming than mid-latitudes. In fact a reduction in the Arctic ice cap has been observed: The area has shrunk by 15% over the past 20 years, and the average thickness of the ice has been reduced from 10 feet to about 6 feet. Loss of Arctic wildlife during this period has recently been documented, including numbers of plankton, fish, birds, and mammals (including caribou and polar bears). | ||||||||||||||||||||||||||||||||||||||||
| Responses | One response to the risk of global warming is to use energy sources other than fossil fuels, namely nuclear reactors and/or solar collectors. Another response is to call for more conservation, and, in effect, for a less energy-intensive economy, which may or may not mean a lowered standard of living. Thus society faces the question of whether to take fairly serious steps to avoid an outcome that is not absolutely certain to occur, but that could have drastic consequences if it does occur. | ||||||||||||||||||||||||||||||||||||||||
| Developing countries | These issues become much more intense when one considers the position of the less developed nations. They comprise a much larger population than the industrialized world, and will seek a higher standard of living via increased energy consumption. In fact, as a result largely of conservation, CO2 production in the industrialized countries has been approximately constant over the past 25 years, but it has increased in the developing world and in the countries of the former Soviet Union and eastern Europe. | ||||||||||||||||||||||||||||||||||||||||
| The Kyoto Protocol | In 1997 a United Nations-sponsored conference was held in Kyoto, Japan, aimed at world-wide reduction in the release of greenhouse gases. The Kyoto Protocol has at this point been signed by 84 nations, including the United States. It calls for a reduction in emissions to 5% (averaged over all these nations) below 1990 levels, the reduction to take place between 2008 and 2012. The industrialized countries commit to higher percentage reductions; the developing countries, lower. The U.S. Senate has not ratified the agreement, and support for it is not strong. The Protocol did not turn out to be a major issue in the 2000 and 2004 presidential elections, although the two political parties hold sharply different views. President Bush is not a supporter of the Protocol. |
KEY CONCEPTS
|