1998 Year of the Ocean Impacts of Global Climate Change 



INTRODUCTION 



Developing an understanding of both weather and climate means developing an 

 understanding of the Earth's heat budget, i.e., how the energy from incoming solar radiation is 

 redistributed around the globe. The major source of heat to the surface of the earth is the Sun, 

 principally through incoming visible radiation. Heat is generated in the Earth's interior, for 

 example through the decay of radioisotopes; however, this contribution to the heat balance of the 

 surface of the Earth, where most life exists, is small compared to the solar flux. Clouds, ice caps 

 and snow make the Earth a relatively bright planet such that about 30 percent of the incoming 

 solar radiation received at the top of the atmosphere is reflected back into space. A very small 

 fraction is absorbed directly by gases and aerosols as it passes through the atmosphere. Most of it 

 is absorbed by the Earth's surface. This radiation is redistributed by the ocean and the 

 atmosphere and the excess is radiated back into the atmosphere and space as longer wavelength, 

 infrared radiation. 



If our atmosphere consisted of just nitrogen and oxygen, the global average surface 

 temperature of the planet would be about 33°C (60°F ) colder than it is now and the Earth would 

 be a frozen wasteland. This is not the case because of the presence of clouds and small quantities 

 of other gases, primarily water vapor and carbon dioxide, that absorb much of the infrared 

 radiation emitted by the Earth's surface and reemit their own heat, as radiation, at much lower 

 temperatures. This "traps" the Earth's radiation and is the mechanism for planetary warming, , 

 "the greenhouse effect" (Figure 1). 



Aside from the gases in the atmosphere, clouds also play a major role in climate. By 

 reflecting solar radiation away from Earth, some clouds act to cool the planet while other types 

 of clouds warm the Earth by trapping heat near the surface. For years, it was not known whether 

 clouds warmed or cooled the planet. Recent satellite measurements have proven that clouds exert 

 an overall powerful cooling effect on the Earth. In some areas, however, such as the tropics, 

 heavy clouds may markedly warm the regional climate. 



Clouds and greenhouse gases fit into a global radiation budget, a budget that must 

 balance itself Most of the incoming solar radiation is received in tropical regions while very 

 little is received in polar regions especially during winter months. Over time, energy absorbed 

 near the equator spreads to the colder regions of the globe, carried by winds in the atmosphere 

 and by currents in the ocean. The small amount of energy retained in the atmosphere is 

 redistributed, basically, by winds. The time it takes for the atmosphere to mix around the globe is 

 approximately one month. In its simplest form, an understanding of weather is an attempt to 

 understand winds. Compared to the atmosphere, the ocean is much denser and has a much greater 

 ability to store heat. The ocean also moves much more slowly than the atmosphere and 

 distributes heat at a much slower rate. Because the ocean covers nearly two-thirds of the surface 

 of the earth, the combined effect of the ocean's heat capacity and its coverage of the Earth's 

 surface means that much of the heat received from the sun that is retained within the biosphere is 

 stored in the ocean. 



G-6 



