Friday, 24 December 2021

The greenhouse effect and drivers of climate change

 

The Earth’s climate system is powered by energy from the sun reaching the Earth in the form of sunlight. Some of the incoming solar radiation is reflected back to space, but the rest is absorbed by the atmosphere and at the Earth’s surface, which warms the planet. The Earth cools down by emitting radiation back to space at a rate that depends on the temperature of Earth. Since the Earth is much colder than the sun, it emits infrared radiation in the lower-energy, longwave part of the energy spectrum (infrared radiation, invisible to the human eye), whereas the sun emits mainly high-energy, shortwave radiation (visible and ultraviolet light).

The Earth’s average temperature is determined by the overall balance between the amount of absorbed incoming energy (as light) from the sun and the amount of outgoing energy (as infrared radiation) from Earth to space. Only a portion of the incoming energy from the sun is used to warm the Earth, as some of it is reflected by the Earth’s atmosphere and surface. About two-thirds of the incoming solar energy (about 240 W/ m2) is absorbed and used to warm the planet (Hartman et al., 2013). Some of the outgoing infrared radiation (heat radiation) is absorbed and then re-emitted by clouds and GHGs in the lower atmosphere. This process is known as the greenhouse effect, and it leads to heat being trapped in the lower atmosphere, which warms the Earth’s surface. Naturally occurring GHGs in the atmosphere — mainly water vapour and CO2 from natural sources — produce a natural greenhouse effect that raises the Earth’s mean surface temperature from about −16ºC to about +15ºC (Lacis et al., 2010). This higher temperature creates conditions favourable for life on Earth and also increases the flow of heat from Earth to space (to about 240 W/m2) so that it balances the flow of incoming solar energy.

In a stable climate, global average temperature remains roughly constant because of this balance between incoming and outgoing energy. However, the Earth’s energy balance can be perturbed. Factors that disrupt this balance and cause climate warming or cooling are called climate drivers or climate forcing agents. Climate drivers can be either natural or human-caused. They can disrupt the Earth’s energy balance by 1) changing the amount of incoming solar radiation; 2) changing the Earth’s albedo, that is, how much incoming solar radiation is reflected from the Earth’s surface and atmosphere; and 3) changing the amount of outgoing infrared radiation by changing the composition of the atmosphere.

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