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In response, the Earth's surface emits longwave radiation that is mostly absorbed by greenhouse gases. The absorption of longwave radiation prevents it from reaching space, reducing the rate at which the Earth can cool off. Without the greenhouse effect, the Earth's average surface temperature would be as cold as −18 °C (−0.4 °F).
This acts to reduce solar energy reaching the surface and lets infrared energy escape, cooling Titan's surface. Titan has both a greenhouse and an anti-greenhouse effect which compete with one another. The greenhouse effect warms Titan by 21 K while the anti-greenhouse effect cools Titan by 9 K, so the net warming is 12 K (= 21 K - 9 K). [3] [4]
[13] [14] Similarly, Earth has an effective temperature of 255 K (−18 °C; −1 °F), [14] but a surface temperature of about 288 K (15 °C; 59 °F) [15] due to the greenhouse effect in our lower atmosphere. [5] [4] The surface temperatures of such planets are more accurately estimated by modeling thermal radiation transport through the ...
Anthropogenic changes to the natural greenhouse effect are sometimes referred to as the enhanced greenhouse effect. [19]: 2223 This table shows the most important contributions to the overall greenhouse effect, without which the average temperature of Earth's surface would be about −18 °C (0 °F), [2] instead of around 15 °C (59 °F). [3]
The runaway greenhouse effect is often formulated in terms of how the surface temperature of a planet changes with differing amounts of received starlight. [13] If the planet is assumed to be in radiative equilibrium, then the runaway greenhouse state is calculated as the equilibrium state at which water cannot exist in liquid form. [3]
A new statement, adopted by the society in 2003, revised in 2007, and revised and expanded in 2013, [14] affirms that rising levels of greenhouse gases have caused and will continue to cause the global surface temperature to be warmer: Human activities are changing Earth's climate.
The Arctic has warmed the most, and temperatures on land have generally increased more than sea surface temperatures. Earth's average surface air temperature has increased almost 1.5 °C (about 2.5 °F) since the Industrial Revolution. Natural forces cause some variability, but the 20-year average shows the progressive influence of human activity.
Billions of years ago, when the Sun was 25 to 30% dimmer, Venus's surface temperature could have been much cooler, and its climate could have resembled current Earth's, complete with a hydrological cycle—before it experienced a runaway greenhouse effect. [44]