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Global cooling is a theory first posed in the 1970s by Russian scientists, which states that the worldwide yearly temperature is steadily declining. The idea of global cooling created modern ice age scares in the 1970s and 1980s but was largely ignored by popular culture after global warming became a public concern. Global cooling may be caused by human pollution, orbital forcing, or global warming.
Similar to global warming, global cooling may be caused in part by pollution factors. Aerosol particles, such as smog, get trapped in the atmosphere and block some of the sun's rays, thus lowering the temperature on earth. Scientists believe this effect is generally minimal, however, and is not the main contributor to global cooling, although it is still a concern.
The largest contributor to global cooling is probably orbital forcing. Orbital forcing is a completely natural phenomenon that occurs because the earth wobbles slightly on its axis as it spins. This wobbling marginally changes the orientation of the Earth and the shape of its orbit over the course of time, which in turn changes the amount of sunlight that reaches Earth. Orbital forcing is theorized to have caused the previous ice ages.
Additionally, global warming may also cause global cooling. As greenhouse gases are trapped in the atmosphere, raising the temperature, and the ice caps melt, large amounts of cold water will rush into the ocean, thus lowering the temperature and causing a shift in climate. This concept is only a theory, however, and later testing has determined that it is unlikely to have drastic consequences over a short amount of time.
Initial research in the 1970s and into the 80s lead to some panic about the possibility of extreme temperature drops and a modern day ice age. Although initial reports in 1972 claimed the next ice age was approximately 20,000 years away, the suggestion that human interference may have sped up the process caused worry that the world population could push the impending ice age up far enough for it to be an immediate concern. These reports were exaggerations, however, and now with more accurate data on climate patterns and changes, many scientists believe that even with human interference, the next ice age is still tens of thousands of years away.
Ultimately, global cooling is still a heatedly debated subject, though it does not receive as much publicity as global warming. It is, however, often used in attempts to discount the validity of global warming. Despite debates and scientific disagreements, one thing remains the same. World temperatures do fluctuate. Through recorded history, and likely before, there have been periods where temperatures steadily rose and periods where they steadily fell.