lGlobalwarming refers to an unequivocal and continuing rise in the average temperature of Earth's climate system. Since 1971, 90% of the warming has occurred in the oceans. Despite the oceans' dominant role in energy storage, the term "global warming" is also used to refer to increases in average temperature of the air and sea at Earth's surface. Since the early 20th century, the global air and sea surface temperature has increased about 0.8 °C (1.4 °F), with about two-thirds of the increase occurring since 1980. Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850. Scientific understanding of the cause of global warming has been increasing. In its fourth assessment (AR4 2007) of the relevantscientific literature, the International Panel on Climate Change (IPCC) reported that scientists were more than 90% certain that most of global warming was being caused by increasing concentrations of greenhouse gases produced by human activities. In 2010 that finding was recognized by the national science academies of all major industrialized nations. Affirming these findings in 2013, the IPCC stated that the largest driver of global warming is carbon dioxide (CO2) emissions from fossil fuel combustion, cement production, and land use changes such as deforestation. Its 2013 report states Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes. This evidence for human influence has grown since AR4. It is extremely likely (95-100%) that human influence has been the dominant cause of the observed warming since the mid-20th century
Initial causes of temperature changes
This graph, known as the Keeling Curve, shows the increase of atmosphericcarbon dioxide (CO2) concentrations from 1958–2013. Monthly CO2measurements display seasonal oscillations in an upward trend; each year's maximum occurs during theNorthern Hemisphere's late spring, and declines during its growing season as plants remove some atmospheric CO2. The climate system can respond to changes in external forcings. External forcings can "push" the climate in the direction of warming or cooling.Examples of external forcings include changes in atmospheric composition (e.g., increased concentrations of greenhouse gases), solar luminosity,volcanic eruptions, and variations in Earth's orbit around the Sun. Orbital cycles vary slowly over tens of thousands of years and at present are in an overall cooling trend which would be expected to lead towards an ice age, but the 20th century instrumental temperature record shows a sudden rise in global temperatures. Greenhouse gases
The greenhouse effect is the process by which absorption and emission of infrared radiation by gases in a planet's atmosphere warm its lower atmosphere and surface. It was proposed by Joseph Fourier in 1824, discovered in 1860 by John Tyndall, was first investigated quantitatively by Svante Arrhenius in 1896,[ and was developed in the 1930s through 1960s by Guy Stewart Callendar.
On earth, naturally occurring amounts of greenhouse gases have a mean warming effect of about 33 °C (59 °F). Without the Earth's atmosphere, the temperature across almost the entire surface of the Earth would be below freezing. The major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect; carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; andozone (O3), which causes 3–7%. Clouds also affect the radiation balance through cloud forcingssimilar to greenhouse gases. Human activity since the Industrial Revolution has increased the amount of greenhouse gases in the atmosphere, leading to increased radiative forcing from CO2, methane, tropospheric ozone, CFCs and nitrous oxide. According to work published in 2007,...
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