Atmospheric Temperature

Solar energy is not the only determinant of atmospheric temperature. As noted above, Earth's surface, after absorbing solar radiation in the visible region, emits infrared radiation back to space. Several atmospheric gases absorb this heat radiation and re-radiate it in all directions, Lines are drawn on the plot connecting points of equal temperature (like contour lines on a map), given in degrees C. Figure 2 is for December though February, which is winter in the northern hemisphere and summer in the southern. As one might expect, the warmest temperature is found at the surface near the equator, and drops as one travels toward either pole and/or as one increases in altitude. Surprisingly, however, the coldest spot in the lower atmosphere is at the tropopause over the equator, which is colder than even over polar regions. Illustration by Hans & Cassidy. Courtesy of Gale Group. The temperature plot (Figure 3) for June through August (Southern hemisphere winter, Northern summer) shows that the equatorial temperature does not change much with the seasons. The middle and high latitudes have experienced much more change, as the temperature contours have shifted northward. The tropopause over the equator is still extremely cold, surpassed only by the stratosphere over the Antarctic. Illustration by Hans & Cassidy. Courtesy of Gale Group.
including back toward the surface. These so-called greenhouse gases thus trap infrared radiation within the atmosphere, raising its temperature. Important greenhouse gases include water vapor (H₂O), carbon dioxide (CO₂), and methane (CH₄). It is estimated that the Earth's surface temperature would average about 32°C (90°F) cooler in the absence of greenhouse gases. Since this temperature is well below the freezing point of water, it is apparent that the planet would be much less hospitable to life in the absence of the greenhouse effect.

While greenhouse gases are essential to supporting life on the planet, more is not necessarily better. Since the beginning of the industrial revolution in the mid- nineteenth century, humans have released increasing amounts of carbon dioxide to the atmosphere through the burning of fossil fuels. The level of carbon dioxide measured in the remote atmosphere has shown a continuous increase since record keeping began in 1958. If this increase translates into a like rise in atmospheric temperature, the results would be dire indeed: melting polar ice caps and swelling seas, resulting in coastal cities being covered by the ocean; radical shifts in climate, dooming plants and animals that could not adapt quickly enough; and unpredictable changes in wind and weather patterns, posing significant challenges for agriculture. The problem in forecasting the changes that increasing greenhouse gases may bring is that the Earth's climate is a very complicated, interconnected system. The interplay of the atmosphere, the oceans, the continents and the ice caps is not completely understood. While it is known that some of the emitted carbon dioxide is absorbed by the oceans and eventually deposited as carbonate rock (such as limestone), we do not know if this is a steady process or if it can keep pace with our constant releases. Computer models designed to mimic the Earth's climate must make many approximations. Nonetheless, calculations by these less-than-perfect models suggest that a doubling of carbon dioxide levels would mean an increase in the average Northern hemisphere surface temperatures of 39–43°F (4–6°C). While this may not sound like much, note that during the last ice age, when large ice sheets covered much of the northern hemisphere, the Earth's average temperature was only 41°F (5°C) below current levels.