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Global Warming

Global warming is the increase in the average measured temperature of the earth's near-surface air and oceans and it is due to human activity caused most of the warming observed since the start of the industrial era and increase in anthropogenic greenhouse gas concentrations.The term global warming is often used with the term climate change, but the two terms have different meanings. Global warming is the mixed result of anthropogenic (human-caused) emissions of greenhouse gases and changes in the solar irradiance, while climate change refers to any change in the state of the climate that can be identified by changes in the average and/or the variability of its properties (e.g., temperature, precipitation), and that persists for an extended period, typically decades or longer.

Many scientists found that warming of the climate system is unequivocal, and it is evident from observations of increases in global air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level" and, furthermore, they conclude with that the globally averaged net effect of human activities The scientists found that the combined radiative forcing due to increases in Carbon Di Oxide (CO2), Methane and Nitrous Oxide is the largest climate driver and its rate of increase during the industrial age is very unprecedented in more than 10,000 years. Furthermore, the carbon dioxide radiative forcing increased by 20% from the last 10 years, the largest change for any decade in at least the last 200 years. The IPCC(Intergovernmental Panel on Climate Change) also found that anthropogenic contributions to aerosols in the atmosphere produce cooling effects, referred to as global dimming. However the cooling (global dimming) effects due to human-caused aerosols are equivalent to about half of the warming effects due to the combined radiative forcing of human-produced greenhouse gases, causing a net warming.

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's lower atmosphere and surface. From the records it has been found that carbon dioxide concentration has been increased from 310 ppmv to 390 ppmv since 1960. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable. On Earth, the major greenhouse gases are water vapors , which causes about 36–70 percent of the greenhouse effect ; Carbon dioxide(CO2), which causes 9–26 percent; methane(CH4), which causes 4–9 percent; and ozone , which causes 3–7 percent. Human activity since the industrial revolution has increased the concentration of various greenhouse gases, leading to increased in radiative forcing from CO2, methane , tropospheric ozone ,CFCsand nitrous oxide . For molecule, methane is a more effective greenhouse gas than CO2 , but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. Some other naturally occurring gases contribute small fractions of the greenhouse effect; one of these, nitrous oxide(N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively since the beginning of the industrial evolution . From some geological evidence, it is believed that CO2 values this high were last attained 20 million years ago. Fossil fuel burning has produced approximately three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.

Some other hypotheses departing from the consensus view have been suggested to explain most of the temperature increase. One such hypothesis proposes that warming may be the result of variations in solar activity. One predicted effect of an increase in solar activity would be a warming of most of the stratosphere, whereas an increase in greenhouse gases should produce cooling there. These effects forces the change in the climate. When a warming trend results in effects that induce further warming, it produces positive results when the effects induce cooling, the process results negative . The primary positive feedback involves water vapor. The primary negative feedback is temperature-radiation: as the temperature of a body increases, the emitted radiation increases with the fourth power of its absolute temperature . This provides a powerful negative feedback which stabilizes the climate system over time.

One of the most positive feedback effects relates to the evaporation of water. If the atmosphere is warmed, the saturation vapour pressure increases, and the quantity of water vapors in the atmosphere will tend to increase. Since water vapors is a greenhouse gas, the increase in water vapors content makes the atmosphere warm further; this warming causes the atmosphere to hold still more water vapors (a positive effect), and so on until other processes stop the feedback loop. The result is a much larger greenhouse effect than that due to CO2 alone. Although this feedback process causes an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer. This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.