Global warming is the increase in global temperatures caused by the emission of greenhouse gases that trap the sun's heat in the Earth's atmosphere. The major greenhouse gases that contribute to global warming include carbon dioxide, methane, nitrous oxides, chlorofluorocarbons and halocarbons. Carbon dioxide emissions are primarily caused by the use of fossil fuels (oil and coal) for energy.
Global warming, as used in the popular context, is the phenomenon that attributes an increase in the average annual surface temperature of Earth to increased atmospheric concentrations of carbon dioxide and other gases. Global warming describes only one of several components involved in climate change and specifically refers to a warming of Earth's surface outside of the range of normal fluctuations that have occurred throughout Earth's history.
Climate describes the long-term meteorological conditions or average weather for a region. Throughout Earth's history there have been dramatic and cyclic changes in climatic weather patterns corresponding to cycles of glacial advance and retreat that occur on the scale of 100,000 years. Within these larger cycles are shorter duration warming and cooling trends that last from 20,000 to 40,000 years. Scientists estimate that approximately 10,000 years have elapsed since the end of the last ice age, and examination of physical and biological processes establishes that since the end of the last ice age there have been fluctuating periods of global warming and cooling.
Measurements made of weather and climate trends during the last decades of the twentieth century raised concern that global temperatures are rising not in response to natural cyclic fluctuations, but rather in response to increasing concentrations of atmospheric gases that are critical to the natural and life-enabling greenhouse effect (infrared re-radiation, mostly from water vapour and clouds, that warms the earth's surface).
Observations collected over the last century indicate that the average land surface temperature increased by 0.7°C. The effects of temperature increase, however, cannot be fully isolated and many meteorological models suggest that such increases temperatures also result in increased precipitation and rising sea levels.
Measurements and estimates of global precipitation indicate that precipitation over the world's landmasses has increased by approximately 1% during the twentieth century. Further, as predicted by many global warming models, the increases in precipitation were not uniform. High latitude regions tended to experience greater increases in precipitation while precipitation declined in tropical areas.
Measurements and estimates of sea level show increases of 6–8 in (15–20 cm) during the twentieth century. Geologists and meteorologists estimate that approximately 25% of the sea level rise resulted from the melting of mountain glaciers. The remainder of the rise can be accounted for by the expansion of ocean water in response to higher atmospheric temperatures.
Estimates of atmospheric greenhouse gases prior to the nineteenth century (extrapolated from measurements involving ice cores) indicate that of the last few million years the concentration of greenhouse gases remained relatively unchanged prior to the European and American industrial revolutions. During the last two centuries, however, increased emissions from internal combustion engines and the use of certain chemicals have measurably increased concentrations of greenhouse gases that might result in an abnormal amount of global warming.
Although most greenhouse gases occur naturally, the evolution of an industrial civilization has significantly increased levels of these naturally occurring gases. In addition, new gases have been put into the atmosphere that increase the greenhouse effect. Important greenhouse gases in the modern Earth atmosphere include carbon dioxide, methane, nitrous oxides, ozone, halogens (bromine, chlorine, and fluorine), halocarbons, and other trace gases.