9. Our Changing Atmosphere

The varying Sun and its affect on the Earth's atmosphere

The Sun is a magnetic variable star

Our lives depend on the Sunís continued presence and steady output. It illuminates our days, warms our world, and makes life on Earth possible.The total amount of the Sunís life-sustaining energy is called the ďsolar constantĒ, perhaps because no variations could be detected in it for a very long time. Yet, as reliable as the Sun appears, it is an inconstant companion. Its luminous output varies in tandem with the Sunís 11-year magnetic activity cycle.

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Stable detectors placed aboard satellites above the Earthís atmosphere have been precisely monitoring the Sunís total irradiance of the Earth since 1978, providing conclusive evidence for small variations in the solar constant (Fig. 9.1). 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. This inconstant behavior can be traced to changing magnetic fields in the solar atmosphere.

The Earthís varying Sun-layered atmosphere

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Not only does the atmospheric pressure decrease as we go upward, the temperature of the air also changes, but it is not a simple fall-off. It falls and rises in two full cycles as we move off into space (Fig. 9.2).

The temperature decreases steadily with increasing height in the lowest region of our atmosphere, called the troposphere from the Greek tropo for turning. Visible sunlight passes harmlessly through this region to warm the ground below. The temperature above the ground tends to fall at higher altitudes where the air expands in the lower pressure and becomes cooler. The temperature increases at greater heights within the next atmospheric layer, named the stratosphere. The Sunís invisible ultraviolet radiation is largely absorbed in the stratosphere, where it warms the gas and helps make ozone.

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The threat of dangerous and even lethal ultraviolet rays caused world-wide concern when it was discovered that everyday, man-made chemicals are punching a hole in the ozone layer (Fig. 9.3). The chemicals, called chlorofluorocarbons or CFCs for short, were therefore completely banned by international agreement in 1990. Still, the ozone layer is not expected to regain full strength until well into the latter half of the twenty-first century.

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 electrons and ions, 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 Sunís 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.

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Solar X-rays and extreme ultraviolet radiation both produce and significantly alter the Earthís ionosphere. Their greater intensity near the maximum of the 11-year magnetic activity cycle produces increased ionization, greater heat, and expansion of our upper atmosphere. At a given height, the temperature, the density of free electrons, and the density of neutral, unionized atoms all rise and fall in synchronism with solar activity over its 11-year cycle (Fig. 9.4). This Sun-induced change in the content and structure of the ionosphere affects its ability to mirror radio waves.

The Sun's role in warming and cooling the Earth

The Sunís radiation and global warming

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The land temperatures have been correlated with the length of the solar cycle. The yearly mean air temperature over land in the Northern Hemisphere has moved higher or lower, by about 0.2 degrees Centigrade, in close synchronism with the solar-cycle length during the past 130 years (Fig. 9.5). Short cycles are characteristic of greater solar activity that apparently warm our planet, while longer cycles signify decreased activity on the Sun and cooler times at the Earthís surface. These temperature variations might be attributed to solar-driven changes in cloud cover, caused by the Sunís 11-year modulation of the amount of cosmic rays reaching Earth.

At times of enhanced activity on the Sun, the solar wind is pumped up with intense magnetic fields that extend far out into interplanetary space, blocking more cosmic rays that would otherwise arrive at Earth. The resulting decrease in cosmic rays means that fewer energetic charged particles penetrate to the lower atmosphere where they may help produce clouds, particularly at higher latitudes where the shielding by Earthís magnetic field is less. The reduction in clouds, that reflect sunlight, would explain why the Earthís surface temperature gets hotter when the Sun is more active.

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Many of the temperature changes on Earth during the first half of the 20th century could be directly related to brightening and dimming of the Sun. Solar variability provides a reasonable match to the detailed ups and downs of the temperature record during this period (Fig. 9.6.

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To fully understand the temperature measurements, scientists have examined historical records of the variable brightness of the Sun and other stars. Their reconstruction of the varying solar irradiance of Earth (Fig. 9.7) show that the Sunís changing brightness dominated our climate for two centuries, from 1600 to 1800. Cooling by hazy emission from volcanoes next played an important role, but the Sun noticeably warmed the climate for another century, from 1870 to 1970. After that, heat-trapping gases apparently took control of our climate.

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The Earth is now hotter than it has been any time during the previous 1,000 years (Fig. 9.8). Global warming by the greenhouse effect is probably responsible for this recent, unprecedented rise in temperature. Minor ingredients of the atmosphere, such as carbon dioxide and water vapor, absorb the groundís infrared radiation, holding it close to the planetís surface and elevating the temperature there. Methane and nitrous oxide also act as greenhouse gases, but they are less abundant than carbon dioxide and water vapor.

The continued accelerated burning of fossil fuels will someday cause great damage to the environment, so both the developing and industrial nations should now do more to stop it. The Sunís activity can nevertheless substantially enhance or moderate this warming, and there isnít very much we can do about the Sunís changing temperament except monitor it. Moreover, there may be relief on its way when the next ice age begins.

Cooling the Earth down

Spacecraft observations of the varying solar brightness over the past two decades indicate that it has varied by about 0.1 percent. The observed brightness and magnetic variations of stars, with masses and ages close to those of the Sun, indicate that more substantial variations of the Sunís luminosity are possible. They may be associated with dramatic changes in the Earthís climate on time scales of hundreds, thousands, and hundreds of thousands of years.

Profound Sun-driven transformations in climate are suggested by past solar activity recorded in sunspot observations, tree rings and ice cores. An example is the period from 1645 to 1715, now known as the Maunder Minimum, when sunspot activity dropped to unusually low levels and the world experienced one of the coldest periods of the Little Ice Age in Europe. Comparisons with the brightness variations of Sun-like stars indicate that the Sun was approximately 0.25 percent dimmer at the time of the Maunder Minimum then currently, and was therefore capable of explaining the estimated drop of about 0.5 degrees Celsius in global mean temperature.

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The radiocarbon records confirm that the Maunder Minimum corresponded to a dramatic reduction in solar activity, and show that such are a fairly common aspect of the Sunís behavior. During the past two thousand years, the Sun has spent nearly a third of the time in a relatively inactive state (Fig. 9.9). Extended periods of solar inactivity must therefore be considered to be a permanent feature of the Sun, and can be expected to occur again in the future.

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The changing Sun has been drastically altering the climate for thousands of years. The Little Ice Age (1400-1800), for example, overlaps the SpŲrer Minimum (1400-1530) and Maunder Minimum (1645-1715) in solar activity. During this long period of unusual cold weather, alpine glaciers expanded, the Thames River and the canals of Venice regularly froze over, and painters depicted unusually harsh winters in Europe (Fig. 9.10).

Further back in time, during the past one million years, our climate has been dominated by the recurrent ice ages, each lasting about 100 thousand years. At the height of each long ice age, the great polar ice sheets advance down to lower latitudes. These glaciations are punctuated every 100 thousand years or so by a relatively short interval of unusual warmth, called an interglacial, lasting 10 or 20 thousand years, when the glaciers retreat. We now live in such a warm interglacial interval, called the Holocene period, in which human civilization has flowered. Still, the die is cast for the next glaciation, and the ice will come again.

The rhythmic alteration of glacial and interglacial intervals is related to periodic alterations in the amount and distribution of sunlight received by Earth over tens of thousands of years. When less sunlight is received in far northern latitudes, the winter temperatures are milder there, but so too are summer temperatures. So, less polar ice then melts in the summer, and over time the winter snows are compressed into ice to make the glaciers grow.

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Three astronomical cycles combine to alter the angles and distance at which sunlight strikes the far northern latitudes of Earth, triggering the ice ages. This explanation was fully developed by Milutin Milankovitch from 1920 to 1941, so the astronomical cycles are now sometimes called the Milankovitch cycles. They involve periodic wobbles in the Earthís rotation and changes in the tilt of its axis and the shape of its orbit, occurring over tens of thousands of years (Fig.9.11).

Cores extracted from the glacial ice in Greenland and Antarctica provide the longest natural archive of the Earthís past climate. They strongly support the idea that changes in the Earthís 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.

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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 (Fig. 9.12). Scientists cannot however, yet agree whether the increase in greenhouse gases preceded or followed the rising temperatures.

Still, we should not discount recent global warming. The concentrations of carbon dioxide and methane have now risen to unprecedented levels in our air, vastly exceeding those at any time during the past 420 thousand years. The warming produced by their greenhouse effect might counteract the cold of the next ice age.