The greenhouse effect occurs as light is absorbed by surface and oceans of the earth, transformed into heat, and re-radiated as infrared radiation. Certain portions of the Earth's atmosphere, the greenhouse gases, absorb the heat, and once again re-radiate it in all directions. The continual process of absorbing and radiating heat serves to retain the heat in the atmosphere, reducing the amount of heat sent back into space. Under normal circumstances, a natural greenhouse effect helps moderate temperatures, and keeps the planet warm enough to sustain life. The rapid increase in greenhouse gases during the 20th century has created an enhanced greenhouse effect, contributing to global warming.
Most mainstream scientists support the notion that increasing levels of greenhouse gases are due to human activity. The burning of fossil fuels and deforestation are two activities that increase concentrations of carbon dioxide in the atmosphere. According to measurements taken at the Mauna Loa observatory in Hawaii, the concentration of carbon dioxide in the atmosphere has risen from 313 ppm to 389 ppm in the past 50 years, with most of the rise attributable to fossil fuels. Rising temperatures can create synergistic processes that lead to even more warming, increasing water vapor in the atmosphere, or releasing methane from the arctic.
Data from human records, tree rings, corals, and other sources show that the global temperatures rose by .74 degree Fahrenheit during the 20th century, with the increase accelerating in the second half of the century. Climate models indicate that temperatures are likely to rise one more degree during the 21st century. Temperature changes vary widely over the planet, with larger changes occurring over land than over ocean. Some scientists suggest that climate change may result in cooling in some areas, as ocean and air currents change, and increased ocean evaporation results in cases of heavy localized snowfall.
There are many reasons to be concerned about the impacts of global warming. Rising temperatures are likely to result in widespread ecological change. Many animal and plant species are likely to become extinct as ecosystems adjust to climate change. While adaptable species will survive, and other migrate, the end result will be lost biodiversity. Global warming also has potential to melt ice caps, raise sea levels and displace human populations due to coastal flooding and droughts. The planet has already been experiencing heightened occurrence and severity of heat waves and extreme weather events, which promise to become worse as the climate becomes more destabilized.