The process of global warming begins with the release of greenhouse gases, such as methane, carbon dioxide, nitrous oxide, water vapour and fluorinated gases. Outgoing infrared radiation, or longwave radiation, is absorbed from the Earth’s surface by these gases as well as aerosols, hence the lower layers of the atmosphere become warmer and less energy is emitted by the Earth’s surface. This is known as the greenhouse effect; without it, the Earth would be a very cold place, with a mean surface temperature about 33°C lower than it is now. But approximately since the beginning of the Industrial Age, the concentrations of greenhouse gases have reached unprecedented levels. The amount of carbon dioxide in the troposphere, or the lowest layer of the atmosphere, has risen from 280 ppm to about 400 ppm. methane levels have exceeded 1800 ppb, an increase from approximately 700 ppb in pre-Industrial times.
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