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  Why isn't Earth as hot as an oven?
 

SunSunlight is the source of energy for the Earth's oceans, atmosphere, land, and biosphere. This energy serves to heat the Earth to temperatures far above the minus 454 degrees Fahrenheit (3 degrees Kelvin) of deep space. Averaged over an entire year and the entire Earth, the Sun deposits 342 Watts of energy into every square meter of the Earth. This is a very large amount of heat—1.7 x 1017 watts of power that the Sun sends to the Earth/atmosphere system. For comparison, a large electric power plant would produce 100 million watts of power, or 108 watts. It would take 1.7 billion such power plants to equal the energy coming to the Earth from the Sun—roughly one for every three people on the Earth! Where does all the Sun's heat go? Why doesn't the Earth just keep getting hotter?

Most of the Sun's heat is deposited into the tropics of the Earth. This is because the Earth's rotational axis is almost perpendicular to the plane of Earth's orbit around the Sun. The polar latitudes receive on average much less solar heating than the equator. If the tilt of the Earth's axis were exactly perpendicular to the orbit plane around the Sun, then there would be no seasons! Climate in January would be the same as climate in April or July, all over the Earth. But the Earth's rotational axis tilts 23.5 degrees away from perpendicular. Consequently, during one part of the orbit around the Sun, the North Pole will be tilted 23.5 degrees toward the Sun and will be in Sunlight 24 hours a day. Six months later at the opposite side of the orbit around the Sun, the North Pole will be in total darkness 24 hours a day. Why is this important? The amount of solar heating of the polar latitudes varies greatly through the year. In the summer, polar latitudes receive almost as much solar energy as the tropics, while in the winter they receive no solar heat at all. Meanwhile, the tropics receive by comparison roughly constant solar heating throughout the year (hence the small seasonal cycles there). As a result, in the winter hemisphere, the difference in solar heating between the equator and the pole is very large—a situation perfect for driving a strong "heat engine," or circulation of the atmosphere. This energy difference drives large mid-latitude storm systems as heat moves from the surplus in the equator to the deficit in the polar regions. In contrast, the summer has very similar heating at the equator and poles, such that the heat engine slows down, and mid-latitude storms lose their source of energy. Summer storms tend to be very small scale and local.

Solar heating of the Earth and its atmosphere drives the large-scale atmospheric circulation patterns, and even the seasons. The difference in solar heating between day and night also drives the strong diurnal (or daily) cycle of surface temperature over land. But with all this solar heating going on, we still haven't answered our earlier question: Why doesn't the Earth just keep getting hotter?

The answer might be loosely called the yang and yin, or the "duality of radiation fluxes." At the same time the solar energy that we can see with our eyes is heating the planet, there is radiation being emitted at much longer wavelengths that our eyes do not see—called "thermal infrared radiation" (basically heat). The amount of heat emitted from a solid surface is proportional to the fourth power of the temperature of the surface. So as the temperature of the Earth rises, it rapidly emits and loses to space an increasing amount of heat. If the Earth were a ball of rock with no atmosphere, a simple calculation that equates the solar energy absorbed by the Earth to the heat emitted by the Earth would predict the global average Earth temperature to be 0 degrees Fahrenheit, or 255 Kelvin-very cold, and not the Earth as we know it (this scenario assumes that an average rock reflects 30 percent of all light that hits it).

Atmospheric Greenhouse Effect
Figure 1: The atmospheric greenhouse effect. Shortwave solar radiation passes through the clear atmosphere relatively unimpeded, but longwave infrared radiation emitted by the warm surface of the Earth is absorbed partially and then re-emitted by a number of trace gases such as water vapor and carbon dioxide.

Atmospheric gases such as water vapor and carbon dioxide absorb the heat emitted from the surface, capturing it in the atmosphere (Figure 1). Because atmospheric temperature decreases with altitude, the heat emission of the atmosphere is at a much lower temperature than the surface. So the Earth and atmosphere keep heating up until the heat emitted roughly balances with the amount of sunlight absorbed. This trapping of heat by carbon dioxide and water vapor is typically called the "greenhouse effect," and these gases are referred to as "greenhouse gases." It is the increase in these gases with time (led by carbon dioxide release from burning oil, gas, and coal) that leads to the potential for future climate change. In fact, most theoretical models predict that as temperatures in the atmosphere increase, the amount of water vapor will increase, thereby acting as a "positive feedback" loop to further increase atmospheric temperatures.

next: Clouds: A hot topic or are we made in the shade?

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Why isn't Earth as hot as an oven?
Introduction
Clouds: A hot topic or are we made in the shade?
Surface Absorption and Reflection
Atmospheric Aerosols: Fossil Fuels and Biomass Burning
From Measurements to Climate Models

Related Data Sets:
Surface Temperature
Outgoing Longwave Radiation

   
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