What Factors Impact a Greenhouse?

Background

The earth's atmospheric "greenhouse effect" is much more complex than the simple greenhouse experiment described in Activity 12. While the earth's temperature is dependent upon the greenhouse-like action of the atmosphere, the amount of heating and cooling are strongly influenced by several factors.

The type of surface that sunlight first encounters is the most important factor. Forests, grasslands, ocean surfaces, ice caps, deserts, and cities all absorb, reflect, and radiate radiation differently. Sunlight falling on a white glacier surface strongly reflects back into space, resulting in minimal heating of the surface and lower atmosphere. Sunlight falling on a dark desert soil is strongly absorbed, on the other hand, and contributes to significant heating of the surface and lower atmosphere. Cloud cover also affects greenhouse warming by both reducing the amount of solar radiation reaching the earth's surface and by reducing the amount of radiation energy emitted into space.

Scientists use the term albedo to define the percentage of solar energy reflected back by a surface. Understanding local, regional, and global albedo effects is critical to predicting global climate change. The following are some of the factors that influence the earth's albedo.

• Clouds: On a hot, sunny day, we usually welcome a big fluffy cumulus cloud passing overhead because we feel cooler immediately. That's because the top of the cloud reflects sunlight back into space before it ever reaches earth. Depending on their altitude and optical properties, clouds either cool or warm the earth. Large, thick, relatively low-altitude clouds, such as cumulus and cumulonimbus, reflect incoming solar radiation and thereby reduce warming of the surface. The whitewash on plant greenhouses has the same effect on a smaller scale. High-altitude, thinner clouds, such as cirrus clouds, absorb longwave radiation reflected from the earth's surface, causing increased warming.
  
  

  Cirrus	Cumulus	Nimbus

  
  
• Surface albedo: Just as some clouds reflect solar energy into space, so do light-colored land surfaces. This surface albedo effect strongly influences the absorption of sunlight. Snow and ice cover are highly reflective, as are light-colored deserts. Large expanses of reflective surfaces can significantly reduce solar warming. Dark-colored land surfaces, in contrast, are strongly absorptive and contribute to warming. If global temperatures increase, snow and ice cover may shrink. The exposed darker surfaces underneath may absorb more solar radiation, causing further warming. The magnitude of the effect is currently a matter of serious scientific study and debate.
  
  
• Oceans: From space, oceans look much different than adjacent land areas - they often appear darker, suggesting that they should be absorbing far more sunlight. But unlike dry land, water absorbs energy in a dynamic fashion. Some of the solar energy contacting the surface may be carried away by currents, some may go into producing water vapor, and some may penetrate the surface and be mixed meters deep into the water column. These factors combine to make the influence of the ocean surface an extremely complex and difficult phenomenon to predict.
  
  Water also has the capacity to store heat and transport large amounts of heat energy. In addition, oceans are an important sink (storage site) for atmospheric , and their ability to absorb is strongly related to ocean temperature.
  
  Because of their enormous size and depth, oceans are extremely important in determining global climate and the future rate of global temperature change.
  
  
• Forested areas: Like the oceans, the interaction of forests and sunlight is complex. The amount of solar radiation absorbed by forest vegetation depends upon the type and color of vegetation, the time of year, and how well watered and healthy the plants are. In general, plants provide a dark surface, so you might expect high solar absorption. A significant fraction of the solar radiation is captured by the plants and used to make food through photosynthesis (and thus it doesn't re-radiate as heat); some of the energy is dissipated as water evaporates from plant leaves; and some is absorbed and distributed deep within the forest canopy. These complexities make a simple definition of forest influences impossible.
  
  To a lesser extent, the same complexities apply to any relatively continuous-cover ecosystem (for example, grasslands and farmlands).

Source : http://www.ucar.edu/learn/1_3_2_13t.htm