GORAL WARMING THE GREENHOUSE EFFECT The greenhouse effect is so named because it resembles the warming of a botanical green house but the comparison is not a perfect one The glass walls of a greenhouse hold heat inside the building by trapping warmed air preventing heat loss via convection In contrast greenhouse gases absorb radiation emitted from Earth's surface and direct it back to Earth preventing radiant heat loss The greenhouse effect is a natural phe nomenon that helps maintain temperatures suitable for life As sunlight enters Earth's atmosphere some of it is reflected back into space Most of it passes through the atmo sphere however and heats Earth s surface which in turn emits some of this energy as infrared radiation In the atmosphere some infrared radiation escapes into space but most is absorbed by greenhouse gases and directed back toward Earth where it further warms Earth s surface and lower atmosphere Without the warming due to the greenhouse effect Earth's surface temperature would be much colder than it is with an average global temperature aroundo °F 18 °C As a model of comparison the planet Venus has a high concentration of carbon 12
GORAL WARMING EARTH's CHANGING CLIMATE EARTH's ATMOSPHERE Earth like other planets of the solar system is enclosed in a thin shell of gas called an atmo sphere The atmosphere clings tightly to Earth by the attraction of gravity The air composing the atmosphere moves freely sometimes vio lently Earth's atmosphere consists mainly of It is an in areas undermine Rainbow in the Scottish countryside Gary Yeowell Stone Getty Images 13
GLOBAL WARMING nitrogen oxygen argon water vapor carbon dioxide and small amounts of other gases and solid and liquid particles The atmosphere serves to moderate the extremes of heat and cold on Earth During the day as the heat of the Sun penetrates the air and warms Earth the atmosphere traps this heat so that it escapes more slowly into space making the night warmer than it would be without this effect The atmosphere also protects Earth's inhabitants to some extent from meteor particles cosmic rays radiation from the sun and stars atmospheric dust and other hazards The atmosphere is in constant motion due to Earth's rotation and changes in tem perature and pressure The sometimes violent changes that take place in the atmosphere are experienced on Earth as weather wind ocean currents lightning and rainbows dioxide in its atmosphere The carbon diox ide does not allow the heat from the planet to escape into space thus causing an extreme greenhouse effect resulting in surface tem peratures as high as 840 °F 450 °C Mars's atmosphere on the other hand contains low concentrations of carbon dioxide and so does not keep nearly as much of the planet s ther mal radiation from escaping In other words Mars has a weak greenhouse effect making 14
GLOBAL WARMING EARTH's CHANGING CLIMATE its surface much colder than it would be if it had a strong one The average surface tem perature of Mars is about 82 °F 63°C GREENHOUSE GASEs The most important greenhouse gas is water vapor which occurs naturally in Earth's atmo sphere Most other greenhouse gases also occur naturally but some are produced by human activity as well The main greenhouse gases with human sources are carbon dioxide methane nitrous oxide and halocarbons Carbon dioxide is produced naturally by animals through respiration The primary human produced source of carbon dioxide is the burning of fossil fuels coal oil and natural gas These fuels are widely used in electricity generation transportation and industry Another source of carbon dioxide from human activity is the clearing of forests for agriculture and other purposes Atmospheric methane concentrations are much lower than those of carbon dioxide but they are more potent By weight meth ane is 25 times more powerful at trapping heat than carbon dioxide over a 100 year time period Methane is produced naturally by for example the decay of vegetation in 15
