Ole Humlum; Oslo University

Transcription

1 Ole Humlum; Oslo University

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4 Earth

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8 Milankovitch Cycles Axial tilt is the inclination of the Earth's axis in relation to its plane of orbit around the Sun. Oscillations in the degree of Earth's axial tilt occur on a periodicity of 41,000 years from 21.5 to 24.5 degrees. Today the Earth's axial tilt is about 23.5 degrees, which largely accounts for our seasons. Because of the periodic variations of this angle the severity of the Earth's seasons changes. With less axial tilt the Sun's solar radiation is more evenly distributed between winter and summer. However, less tilt also increases the difference in radiation receipts between the equatorial and polar regions.

9 Milankovitch Cycles Eccentricity describes the shape of the Earth's orbit around the Sun. The constantly fluctuating orbital shape ranges between more and less elliptical on a cycle of about 100,000 years. These oscillations are of prime importance to glaciation in that it alters the distance from the Earth to the Sun, thus changing the distance the Sun's radiation must travel to reach Earth, subsequently reducing or increasing the amount of radiation received at the Earth's surface in different seasons.

10 Milankovitch Cycles Precession is the Earth's slow wobble as it spins around its axis. The precession of Earth wobbles from pointing at Polaris (North Star) to pointing at the star Vega. Precession, has a periodicity of 23,000 years.

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12 Daily average insolation (W/m2) at the top of the atmosphere.

13 Annual mean and solstice insolation at the top of the atmosphere.

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25 Processes cooling the terrain surface

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28 Heat loss from surface to atmosphere: 6 W/m 2 Heat loss from surface to atmosphere: 200 W/m 2 Surface cooling by evaporation

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30 Energy budget for atmosphere and surface.

31 Surface cooling by convection

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33 Greenhouse effect Processes cooling the terrain surface

34 The greenhouse effect: Since the Earth s atmosphere is relatively transparent to incoming solar shortwave radiation, and relatively opaque to the infrared radiation, due to the presence of substances which absorb in the infrared (the so called greenhouse gasses: mainly water vapor and condensed water in the form of clouds, but also carbon dioxide, methane, nitrous oxide, and ozone), the surface temperature must be higher than it would in the absence of these greenhouse gasses, in order for surface cooling to balance incoming solar radiation. The most important greenhouse gas in the atmosphere is water vapor. Changes in relative humidity on the order of 1.3-4% are equivalent to the effect of doubling CO 2.

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38 The greenhouse effect: Since the Earth s atmosphere is relatively transparent to incoming solar shortwave radiation, and relatively opaque to the infrared radiation, due to the presence of substances which absorb in the infrared (the so called greenhouse gasses: mainly water vapor and condensed water in the form of clouds, but also carbon dioxide, methane, nitrous oxide, and ozone), the surface temperature must be higher than it would in the absence of these greenhouse gasses, in order for surface cooling to balance incoming solar radiation. The most important greenhouse gas in the atmosphere is water vapor. Changes in relative humidity on the order of 1.3-4% are equivalent to the effect of doubling CO 2. The radiational effect of both water vapor and CO 2 is nonlinear (effect decreasing with increasing concentration), and the effect of small changes in dry regions (e.g. Arctic) will matter much more to the radiative balance than changes in moist regions.

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40 The greenhouse effect: Since the Earth s atmosphere is relatively transparent to incoming solar shortwave radiation, and relatively opaque to the infrared radiation, due to the presence of substances which absorb in the infrared (the so called greenhouse gasses: mainly water vapor and condensed water in the form of clouds, but also carbon dioxide, methane, nitrous oxide, and ozone), the surface temperature must be higher than it would in the absence of these greenhouse gasses, in order for surface cooling to balance incoming solar radiation. The most important greenhouse gas in the atmosphere is water vapor. Changes in relative humidity on the order of 1.3-4% are equivalent to the effect of doubling CO 2. The radiational effect of both water vapor and CO 2 is nonlinear (effect decreasing with increasing concentration), and the effect of small changes in dry regions (e.g. Arctic) will matter much more to the radiative balance than changes in moist regions. The direct effect of doubling CO 2 on the Earth s temperature is rather small: about o C. Larger model predictions depend on positive feedbacks, primarily from water vapor and upper atmosphere temperature, acting in such a manner as to greatly magnify the effect of CO 2.

41 Because of the greenhouse gasses the emission temperature of Earth is not the temperature at the surface, but corresponds to some upper level temperature at approximately one optical depth into the atmosphere. This level varies with location, but is typically near 500 mb (or 5-6 km). Greenhouse effect Processes cooling the terrain surface

42 Jupiter; what we see is not the solid surface but some upper level in the atmosphere.

43 NW Europe 19. March 2007; infrared channel.

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45 Radiative balance at 5-6 km altitude Increasing the atmospheric concentration of greenhouse gasses makes the atmosphere less transparent (more opaque) for infrared radiation, and the critical altitude for radiation to space therefore increases. 1: Now we have an imbalance because of lower temperatures at higher altitude. Loss Gain 2: Now we again have balance by warming the atmosphere, especially at the critical altitude. Critical altitude for radiative heat loss to space The temperature effect of increasing greenhouse gasses in the atmosphere will first be seen at the critical altitude.

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