4. Third rock from the Sun - restless Earth
The Earth's changing atmosphere
Our Sun-layered atmosphere
Our thin atmosphere is pulled close to the Earth by its gravity, and suspended above the ground by molecular motion. The atmosphere near the ground is compacted to its greatest density and pressure by the weight of the overlying air. At greater heights there is less air pushing down from above, so the compression is less and the density and pressure of the air falls off into the near vacuum of space.
Not only does the atmospheric pressure decrease as we go upward, the temperature of the air also changes. It decreases steadily with increasing height in the lowest region of our atmosphere, called the troposphere from the Greek tropo for "turning". The temperature falls at higher altitudes because the air expands in the lower pressure and becomes cooler. But the temperature is not a simple fall-off with height. It falls and rises in two full cycles as we move off into space. The temperature increases are produced by the Sun's invisible radiation.
When absorbed in our air, the invisible short-wavelength radiation from the Sun transfers its energy to the atoms and molecules there, causing the temperature to rise. There is, for example, a gradual increase in temperature just above the troposphere, within the next atmospheric layer named the stratosphere. This layer is located between 10 thousand and 50 thousand meters above the Earth's surface. Its name is coined from the words "stratum" and "sphere". The Sun’s invisible ultraviolet radiation is largely absorbed in the stratosphere, where it warms the gas and helps make ozone.
The vanishing ozone
The mesosphere, from the Greek meso for "intermediate", lies just above the stratosphere. The temperature declines rapidly with increasing height in the mesosphere, reaching the lowest levels in the entire atmosphere. The main reason for the decreasing temperatures is the falling ozone concentration and decreased absorption of solar ultraviolet.
The temperature then begins to rise again with altitude in the ionosphere, a permanent, spherical shell of ions and electrons, reaching temperatures that are hotter than the ground. The ionosphere is created and heated by absorbing the extreme ultraviolet and X-ray portions of the Sun’s energy. This radiation tears electrons off the atoms and molecules in the upper atmosphere, thereby creating ions and free electrons that are not attached to atoms.
Solar ultraviolet radiation supplies ozone to the stratosphere, at a rate that depends on the varying ultraviolet output of the Sun, and we have recently been punching holes in the ozone layer with chemicals used in our everyday lives. The threat of dangerous ultraviolet rays passing through the damaged ozone layer led to international agreement to ban the use of the destructive chemicals. However, because of their long lifetime and slow diffusion into the stratosphere, the ozone layer is not expected to regain full strength until well into the latter half of the 21st century. Further details are given in Ozone Depletion at the Human Impact part of this web site.
Heating by the greenhouse effect
Visible sunlight passes through our transparent atmosphere to warm the Earth’s land and oceans, and some of this heat is reradiated in infrared form. The longer infrared rays are less energetic than visible ones and do not slice through the atmosphere as easily as visible light. So our atmosphere absorbs some of the infrared heat radiation, and some of the trapped heat is reradiated downward to warm the planet’s surface and the air immediately above it. The warming by heat-trapping gases in the air is now commonly known as the “greenhouse effect”.
Right now, the warming influence is literally a matter of life and death. It keeps the average surface temperature of the planet at 288 degrees kelvin (15 degrees Celsius or 59 degrees Fahrenheit). Without this greenhouse effect, the average surface temperature would be 255 degrees kelvin (-18 degrees Celsius or 0 degrees Fahrenheit); a temperature so low that all water on Earth would freeze, the oceans would turn into ice and life, as we know it, would not exist.
The gases that absorb infrared heat radiation are minor ingredients of our atmosphere. The main ones are water vapor and carbon dioxide, with water vapor the most powerful heat trapping gas of the two.
Humans are pumping increasing amounts of carbon dioxide in the air
For hundreds of years, humans have been filling the sky with carbon dioxide. The invisible waste gas is dumped into the air by burning fossil fuels – coal, oil and natural gas. When these materials are burned, their carbon atoms, denoted C, enter the air and combine with oxygen atoms, O, or oxygen molecules, O2, to make carbon dioxide, abbreviated CO2.
Just a few decades ago, no one knew if any of the carbon dioxide stayed in the atmosphere or if it was all being absorbed in the oceans. Then in 1958 Charles D. Keeling (1928-) began measurements of its abundance in the clean air at the Mauna Loa Observatory in Hawaii. It is located at a remote high-altitude site in the midst of a barren lava field, far from cars and people that produce carbon dioxide and from nearby plants that might absorb it.
The sensitive measurements shown that the amount of carbon dioxide in the atmosphere increases and decreases in an annual cycle. Every spring plants bloom, sucking CO2 out of the air, and every fall CO2 is released back into the air as plants either decay or lose their leaves. The measurements had recorded the breathing of the plants all over the Northern Hemisphere.
But more importantly, Keeling’s measurements showed that humans are also changing the composition of the atmosphere. Superimposed on the annual fluctuations, there was a systematic increase over the entire period of observation, continuing nonstop since 1958. Year by year the total measured concentration of carbon dioxide grew, as inexorably as the expansion of the world’s population and human industry.
Since 1958, atmospheric concentrations of CO2 have increased from 315 parts per million (106), abbreviated 315 ppm, to 365 ppm at the turn of the century. Studies of ice deposits in Antarctica indicate that the amount of CO2 has been increasing at an exponential rate ever since the beginning of the industrial revolution in the mid-18th century. Concentrations of the gas averaged 280 ppm just before the industrial era. In the succeeding two and a half centuries, a mere blink in the eye of cosmic time, the atmospheric concentration of carbon dioxide has increased 31 percent.
The atmosphere now contains almost 800 billion tons of carbon dioxide. Humans continue to release about 7 billion tons of it each year. In other words, each person on Earth is, on average, dumping about a ton of carbon dioxide into the air every year, and there is no end in sight. (The world population in January 2002 was 6.202 billion, increasing at the rate of about 6 million people every month.)
Once added to the air, carbon dioxide spreads throughout the entire atmosphere. And it remains in the air for a long time, taking decades and even centuries to disappear. So future generations will have to contend with our present activities.
There are now many signs of a recent rise in global temperature, attributed at least in part to heat-trapping gases deposited in the atmosphere by human activity. Uncertain computer models forecast a wide range of possible consequences of global warming for a century from now, including catastrophic ones. Global warming predictions, natural and human-induced warming, likely consequences of global warming, the heated debates over global warming, and individual and political action to limit the emission of heat-trapping gases are given in Global Warming at the Human Impact part of this web site.
Using past records to separate the warming effects of the Sun and humans
The Sun is the driving force for all climate and weather on the Earth, including terrestrial winds and the seasons. Yet, our Sun is a variable star, and its changing luminosity can affect the Earth’s atmosphere and climate.
To most of us, the Sun looks like a perfect, white-hot globe, smooth and without a blemish. However, detailed scrutiny indicates that our star is not perfectly smooth, just as the texture of a beautiful face increases when viewed close up. Magnetism protrudes to darken the skin of the Sun in Earth-sized spots detected on the visual disk.
The magnetized atmosphere in, around and above groups of sunspot is called a solar active region. The sunspots come in pairs of opposite magnetic polarity, or direction, and they are connected by magnetic loops that rise above the visible solar disk. These loops of magnetism contain the hot, million-degree atmosphere of the Sun, which emits intense radiation at X-ray wavelengths. So, active regions shine brightly in X-rays, illuminating the thin magnetic loops that stitch the solar atmosphere together.
The total number of spots on the Sun varies periodically, from a maximum to a minimum and back to a maximum in about 11 years. The number of active regions, with their bipolar spots and the magnetic loops that join them, varies in step with the 11-year sunspot cycle, peaking at the sunspot maximum. The sunspot cycle is therefore also known as the solar cycle of magnetic activity.
Because active regions emit intense ultraviolet and X-ray radiation, the Sun is brightest at these wavelengths during the peak of the magnetic activity cycle and dimmest at cycle minimum. At the minimum of the 11-year cycle, the active regions are largely absent and the strength of the ultraviolet and X-ray emission of the Sun is greatly reduced. The ultraviolet emission doubles from activity minimum to activity maximum, while the Sun's X-ray emission increases by a factor of 100.
Our lives depend on the Sun's continued presence and steady output. It illuminates our days, warms our world, and makes life possible. Yet, as reliable as the Sun seems, it is an inconstant companion, with a luminosity that varies in tandem with the Sun's 11-year magnetic activity cycle.
The total amount of the Sun’s life-sustaining energy is called the “solar constant”, perhaps because no variations could be detected in it for a very long time. Until the early 1980s, it was not known if the Sun's visible light was anything but rock-steady because no variations could be reliably detected from the ground. The required measurement precision could not be attained here on Earth because of the changing amount of sunlight absorbed and scattered by our atmosphere.
Stable detectors placed aboard satellites above the Earth’s atmosphere have been precisely monitoring the Sun’s total irradiance of the Earth since 1978, providing conclusive evidence for small variations in the solar constant. It is almost always changing, in amounts of up to a few tenths of a percent and on time scales from 1 second to 20 years, and probably longer. This inconstant behavior can be traced to changing magnetic fields in the solar atmosphere.
Historical records indicate that the changing Sun has indeed been drastically altering the climate for thousands of years. The Little Ice Age (1400-1800), for example, overlaps the Spörer Minimum (1420-1500) and Maunder Minimum (1645-1715) in solar activity, when sunspots virtually disappeared from the face of the Sun. During the Little Ice Age, alpine glaciers expanded, the river Thames, England and the canals of Venice, Italy, regularly froze over, and painters depicted unusually harsh winters in Europe.
Humans seem to have taken over the climate in more recent times. The Earth is now hotter than it has been any time during the previous 1,000 years, and heat-trapping gases produced by our industrial societies are probably responsible.
Ice and fire
During the past one million years the Earth has undergone a series of warm and cold periods. During the cold periods, called ice ages, huge ice sheets build up on the continents and in the polar seas. The growing layer of continental ice flows towards the equator, scouring and covering large areas of land. Then the climate warms and the ice retreats.
Cores extracted from the glacial ice in Greenland and Antarctica have provided a natural archive of the Earth’s past climate over the past 420 thousand years. They strongly support the idea that changes in the Earth’s orbit and spin axis cause variations in the intensity and distribution of sunlight arriving at Earth, which in turn initiate natural climate changes and trigger the ebb and flow of glacial ice.
Air trapped in the polar ice cores indicates that the Antarctica air-temperature changes are associated with varying concentrations of atmospheric carbon dioxide and methane. The temperatures go up whenever the levels of carbon dioxide and methane do, and they decrease together as well. Scientists cannot however, yet agree whether the increase in greenhouse gases preceded or followed the rising temperatures.
On the other hand, the current level of greenhouse gases, recently deposited in our atmosphere by humans, far surpasses any natural fluctuation of these substances recorded during past ice ages. So we really don't know for certain what is going to happen to the environment in the next few thousand years.
But then, in about 7 billion years from now, the Sun will balloon into a red giant star with a dramatic increase in size and a powerful rise in luminosity. The giant Sun will be 2,300 times brighter than it is now, resulting in a substantial rise in temperatures throughout the solar system. It will become hot enough to melt the Earth’s surface. The frozen moons of the outer planets might then spring to life as the Sun melts their ice into seas of liquid water. The only imaginable escape would then be interplanetary migration to distant moons or planets with a warm, pleasant climate.
(page 4 of 5)
Copyright 2010, Professor Kenneth R. Lang, Tufts University