The greenhouse effect is a natural process that plays a major part in shaping the earth’s climate. It produces the relatively warm and hospitable environment near the earth’s surface where humans and other life-forms have been able to develop and prosper. It is one of a large number of physical, chemical and biological processes that combine and interact to determine the earth’s climate. Climate, whether of the earth as a whole or of a single country or location, is often described as the synthesis of weather recorded over a long period of time. It is defined in terms of long-term averages and other statistics of weather conditions, including the frequencies of extreme events. Climate is far from static. Just as weather patterns change from day to day, the climate changes too, over a range of time frames from years, decades and centuries to millennia, and on the longer time-scales corresponding to the geological history of the earth. These naturally occurring changes, driven by factors both internal and external to the climate system, are intrinsic to climate itself. But not all changes in climate are due to natural processes. Humans have also exerted an influence. Through building cities and altering patterns of land use, people have changed climate at the local scale. Through a range of activities since the industrial era of the mid-19th century, such as accelerated use of fossil fuels and broadscale deforestation and land use changes, humans have contributed to an enhancement of the natural greenhouse effect. This enhanced greenhouse effect results from an increase in the atmospheric concentrations of the so-called greenhouse gases, such as carbon dioxide and methane, and is widely believed to be responsible for the observed increase in global mean temperatures through the 20th century.
