CAPHER GLOBAL WARMING EARTH's CHANGING CLIMATE The rise of air temperatures near Earth's surface over the past century is known as global warming Earth has experienced periods of gradual warming and cooling throughout its existence due to natural causes such as volcanic eruptions and variations in the Sun's output However scientists have attributed the current increase in global temperatures to human causes primarily the release of certain gases into the atmosphere as a result of industrial activity These gases collectively termed greenhouse gases absorb and trap heat emitted from Earth's surface through a phe nomenon known as the greenhouse effect In addition to the rise in near surface air temperatures global warming encom passes other climatic changes caused by this warming such as variations in precipi tation patterns winds and ocean currents For this reason the terms global warm ing and climate change are sometimes used interchangeably IO
GLOBAL WARMING EARTH's CHANGING CLIMATE Sun reflected ight atmosphere absorbed and reradiated Earth Some incoming sunlight is reflected by Earth's atmosphere and sur face but most is absorbed by the surface which is warmed Infrared IR radiation is emitted from Earth's surface Some IR radiation escapes to space but some is absorbed by greenhouse gases and rera diated toward Earth where it further warms the surface and lower atmosphere Encyclopaedia Britannica Inc II
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
