While Earth's temperature is dependent upon the composition and actions of the atmosphere, the amount of energy retained by Earth is strongly dependant on the albedo of Earth's surfaces.
Just as some clouds reflect sunlight into space, so do light-colored land surfaces. Scientists use the term albedo to define the percentage of sunlight reflected back by a surface. This surface albedo effect strongly influences the absorption of sunlight. Forests, grasslands, ocean surfaces, ice caps, deserts, and cities all absorb, reflect, and radiate solar energy differently. Sunlight falling on a white glacier surface strongly reflects back into space, resulting in minimal heating of the surface and the air immediately above. Sunlight falling on dark soil or rock is strongly absorbed and contributes to significant heating of the Earth's surface and the air above.
Understanding local, regional, and global albedo effects is critical to predicting global climate change. Light-colored ice and snow are very weakly absorptive, reflecting 80-90% of incoming solar energy. Dark-colored land surfaces, are strongly absorptive and contribute to warming, reflecting only 10-20% of the incoming sunlight. If global temperatures increase, snow and ice cover may shrink. The exposed darker surfaces underneath may absorb more sunlight, causing further warming. The magnitude of the effect is currently a matter of serious scientific study and debate.
About Urban Heat Islands
The air in urban areas can be 2 to 5°C (3.6 to 9°F) warmer than nearby rural areas. This is known as the urban heat island effect. It’s most noticeable when there is little wind. An urban heat island can increase the temperature and length of a heatwave. City heat can influence the weather - changing wind patterns, clouds, and precipitation.
What makes cities warmer? There are many factors that can influence the urban heat island effect. Changes to the land surface that are made in urban areas have a large impact on whether a heat island forms. For example, many cities have fewer trees than surrounding rural areas. Trees shade the ground, preventing the Sun’s radiation from being absorbed. Without trees, the ground surface heats up. Dark rooftops and pavement absorb more sunlight too. Automobiles, which make heat from their engines and exhaust, also contribute to the heat island effect. Fewer plants in urban settings mean that less evapotranspiration, a process that cools the air, occurs.
Some people have wondered whether the growth of cities has caused global warming because of their urban heat islands. There is very strong evidence that this is not the cause of climate change. According to the Intergovernmental Panel on Climate Change, global warming is very unlikely to be affected significantly by growing urban areas.
Today, many cities are making an effort to combat the urban heat island effect. White or reflective materials are being used for roofing and roads. Trees are being planted along city streets. Also, in many areas, green roofs - living plants on rooftops – are being installed.
Developed by Lisa Gardiner for the UCAR Center for Science Education based on educational resources from NESTA and resources developed by teacher Emilio Macalalad as a part of the RETI Professional Development Program, 2012.