9. Our Changing Atmosphere

    • The atmosphere colors the sky blue and makes sunsets red.

    • Oxygen, carbon dioxide and water are cycled around the globe.

    • The ocean waters were supplied to the young Earth by colliding comets and asteroids or by volcanoes.

    • The seasons are due to changes in the incident sunlight, caused when the relevant hemisphere is tilted toward or away from the Sun.

    • Invisible carbon dioxide and water vapor in our atmosphere help to warm the Earth by trapping the Sunís heat and preventing some of it from being reflected back into space. This process, commonly known as the greenhouse effect, keeps the oceans from freezing and the Earth from becoming a frozen ball of ice.

    • The Earthís upper atmosphere is heated and ionized by the Sunís variable X-ray and ultraviolet radiation, respectively producing the ionosphere and stratosphere.

    • The ionosphere is hotter than the ground.

    • The ozone layer in our stratosphere is both created and modulated by solar ultraviolet radiation, while also protecting us from dangerous ultraviolet sunlight.

    • An ozone hole forms above the South Pole in the local spring. Chlorine compounds known as chlorofluorocarbons, which have been released into the atmosphere by past human activity, are destroying the ozone. Such compounds have now been outlawed by international agreement.

    • The Sun is a variable star, with a brightness that increases, decreases and increases again every 11 years.

    • The total solar irradiance of the Earth, the so-called solar constant, rises and falls in step with the 11-year magnetic activity cycle, but with a total recent change of only about 0.1 percent.

    • When sunspots cross the visible solar disk, they produce, in themselves, a brief dimming of the Sunís radiative output, amounting to a few tenths of one percent for just a few days; a brightness increase caused by faculae and plage exceeds the overall sunspot decrease at times of high solar activity.

    • Over the Sunís 11-year magnetic activity cycle, there are small changes in the amount of visible light emitted by the Sun, but there are huge variations in the Sunís X-ray and ultraviolet output during the 11-year cycle.

    • Solar X-rays and extreme-ultraviolet radiation both produce and significantly alter the Earthís ionosphere. The solar X-rays fluctuate in intensity by two orders of magnitude, or a factor of one hundred, during the Sunís 11-year magnetic activity cycle. Near activity maximum greater amounts of X-rays produce increased ionization, greater heat, and expansion of our upper atmosphere, altering satellite orbits and disrupting communications.

    • During the past one thousand years, there have been century-long periods of very low solar activity.

    • Sun-like stars can vary in brightness by more than the solar constant changes observed so far.

    • The Earth is now hotter than any time during the previous 1,000 years. This global warming is attributed to carbon dioxide and other heat-trapping greenhouse gases, such as methane and nitrous oxide, which have been released into the atmosphere by humans. These gases have been increasing in the atmosphere for more than a century as wastes from industry, cars and trucks are added to the air.

    • By burning coal and oil, humans have increased the amount of carbon dioxide in the Earthís atmosphere by 30 percent since the industrial revolution.

    • Rising air and ocean temperatures, retreating glaciers, rising sea levels, climate changes, and many alterations of physical and biological systems indicate that global warming is increasing and the added heat is here.

    • Incomplete knowledge of the role of clouds and the oceans in global warming make the computations uncertain.

    • If current emissions of carbon dioxide and other greenhouse gases go unchecked over the next 100 years, global warming could produce agricultural disasters in the worldís poorest countries, and rising seas with coastal flooding throughout the world.

    • The Kyoto Protocol limits the production of greenhouse gases, but the United States has not signed it and the developing countries are exempt from the agreement.

    • The major ice ages, which repeat every one hundred thousand years, are caused by astronomical rhythms that alter the angles and distances from which sunlight strikes the Earth.

    • The Milankovich cycles that produce variations in the amount of sunlight incident on Earth include changes in the Earthís rotational wobble, axial tilt, and orbit shape, with respective periodicities of 23,000, 41,000 and 100,000 years.

    • Analysis of the deep ice core taken from Vostok, Antarctica reveal the roughly 100,000-year periodicity of the ice ages over the past 420,000 years, and indicate that transitions from glacial to warm epochs are accompanied by an increase in the atmospheric concentration of the three principal greenhouse gases Ė carbon dioxide, methane and nitrous oxide. A relativity small increase in the intensity of incident solar radiation, associated with a 100,000-year periodic change in the Earthís orbit, may be amplified by an increase in greenhouse gases to produce a warm, interglacial epoch.

Copyright 2010, Professor Kenneth R. Lang, Tufts University