One of the parameters that allows Earth to sustain life is its ability to reflect solar radiation, but to this day there is no existing theory that explains how the planet's "albedo," or reflectivity, is achieved or maintained.
Scripps Institution of Oceanography at UC San Diego scientist V. Ramanathan has pondered planetary albedo for years but argues that the topic now warrants a serious examination from the greater scientific community. As two consequences of pollution-global warming and global dimming-influence Earth's climate, their trends could jeopardize a condition now optimally set to support life.
Percentages of the sunlight that is reflected from Earth's surface and atmosphere vary widely. Fresh snow and some clouds, for instance, can have albedos greater than 50 percent while dark surfaces such as the surface of the ocean reflect less than five percent of the light that strikes them. Clouds overall double the planetary albedo. Earth and the atmosphere as a whole maintain an albedo that hovers around 29 percent.
Ramanathan notes that if Earth's albedo were to increase only three percent, the resulting climate change would throw the planet into an ice age. A three-percent decrease would create a severe heating effect comparable to that caused by a sixfold increase in atmospheric carbon dioxide, far greater than anything projected by today's climate models.
Global warming and global dimming both have the potential to alter Earth's prevailing albedo through complex climate feedbacks. Ramanathan suggests there is evidence that they already have. Despite their common origin in human activities, however, they have done so in ironically contradictory fashion.
In the past century, industrial and agricultural activities have generated an infusion of particulate pollution into the atmosphere. Inputs of soot and other aerosols into the atmosphere have made the planet dimmer by limiting the solar radiation that reaches its surface. The phenomenon also impacts human health by exposing people to smog and on agriculture by limiting the production of rain-bearing clouds.
Human activity has also introduced unprecedented concentrations of carbon dioxide into the atmosphere, causing the planet to trap more heat and retain more water vapor. After correctly predicting in 1980 that global warming would be detected by 2000, Ramanathan observed at the end of the 20-year period that the amount of warming was roughly half what he'd predicted. The stifling of the warming trend is generally attributed to a counteracting cooling effect caused by global dimming, an inference that has since been supported by data collected by Ramanathan and others from a number of field campaigns.
But what will happen when the counterbalance is eliminated? Already Western nations have been successful in reducing particulate pollution and Ramanathan believes emerging nations, most importantly in south Asia, will soon follow suit. As one form of pollution is eliminated, the mask concealing the true impact of global warming will be stripped away. He predicts an acceleration of warming trends to take place in coming decades but what that means for cloud formation, hydrological cycles and other events that affect albedo is unknown.
"We're sort of in uncharted territory when it comes to what happens 30 or 40 years from now," Ramanathan said.
Related to AGU Fall Meeting 2006 "Bjerknes Lecture: Global Dimming and Its Masking Effect on Global Warming"