Lecture 7: Natural Climate Change. Instructor: Prof. Johnny Luo.

Transcription

1 Lecture 7: Natural Climate Change Instructor: Prof. Johnny Luo

2 Final Exam: May :15pm at MR O44

3 Outlines (Chapter 11, Edition 1) 1. Variation in solar luminosity 2. Natural aerosols (e.g., volcanoes) 3. Orbital parameter theory of ice ages

4 S 0 4 (1 α) σt 4 e = 0 Solar theory suggests that during early history of the Sun, its luminosity was 30% weaker the Faint Young Sun Problem. Given that for current climate, T e for Earth is 255 K, calculate the T e when S 0 is 30% less (assume albedo stays the same) S then 1 30% = S now 1 4 T then = T now 0.7 = T 4 then 4 T now = = 233 K But geologic evidence suggests that the Earth s surface back then was as warm as or even warmer than it is today. One theory is that the atmospheric greenhouse effect was stronger (much higher CO 2 level).

5 S 0 4 (1 α) σt 4 e = 0 Venus Earth Mars Distance to the Sun Planetary albedo Effectitve temperature 0.7 AU 1 AU 1.5 AU K 255 K 211 K T s : 735 K 288 K 227 K CO 2 : 97% N 2 :78%,O 2 :21% CO 2: 96% 93 bar 1 bar bar Solar constant is inversely proportional to d 2

6 dimming Sunspots are cold spots (dimming the Sun), but they are accompanied by bright regions called facula. The net effect would be a brightening. brightening Solar irridiance ΔS 0 1 W m -2 If we assume the Earth s climate sensitivity is 0.5 K (W m -2 ) -1, calculate the climate response ΔT s ΔS 0 (1 0.3) = W m 2 4 λ dt s dq ~ 0.5 K /(W m 2 ) ΔT s 0.09 K

7 Outlines 1. Variation in solar luminosity 2. Natural aerosols 3. Orbital parameter theory of ice ages

8 Volcanic eruption (a natural source of aerosols): Ashes and rock particles fall out of the atmosphere very quickly, most of the longer-lasting volcanic aerosols (residing in the stratosphere) is sulfur dioxide (SO 2 ), which is then oxidized to sulfur acid (H 2 SO 4 ). H 2 SO 4 condenses to form aerosols. Mt. Pinatubo 1991 (photo taken from space shuttle 9/9/91)

9 El Chichon (1982) and Pinatubo (1991) are two major volcanoes that we have plenty of data, especially from satellites Optical depth = 0.1 Aerosols are like clouds: they reflect sunlight (cooling) and traps IR radiation (warming) Depending on aerosol size, volcanic aerosols can have either warming or cooling effect. The threshold is about 2 µm, above which aerosols will have warming effect. Since smaller aerosols stay longer in the stratosphere, a volcano almost always has a cooling effect.

10 Shortwave cooling dominates longwave warming Mt. Pinatubo 1991 shortwave longwave λ dt s dq Soden et al. 2002

11 Outlines 1. Variation in solar luminosity 2. Natural aerosols 3. Orbital parameter theory of ice ages

12 The motivation for the orbital parameter theory is to explain the ice ages and interglacial period. Since these variations look very cyclic or periodic, it s natural to think they are associated with periodic variation of Earth s orbit.

13 Several important orbital parameters: 1. Obliquity (Φ): measuring the tilting angle of the Earth s rotation axis 2. Eccentricity (e): how elliptical the orbit is (between 0 and 1) 3. Perihelion relative to the equinox (Λ): precession of the equinox

14 Let s think how these parameters will affect the annual mean solar insolation a good measure of the energy input for the Earth 1. Obliquity (Φ): if the Earth s obliquity angle is 40 0 (instead of ), will this planet be hotter or colder as a whole?

15 Nowhere do we see obliquity in this equation! The Earth is a symmetric sphere. Obliquity does NOT affect annual mean insolation because the Earth is symmetric in shape.

16 Let s think how these parameters will affect the annual mean solar insolation a good measure of the energy input for the Earth 1. Obliquity (Φ): if the Earth s obliquity angle is 40 0 (instead of ), will this planet be hotter or colder as a whole? 2. Eccentricity (ε): if the Earth s orbit is very elliptical, will this planet be hotter or colder? 3. Perihelion relative to the equinox (Λ): how will this parameter affect the Earth s mean temperature?

17 Annual average insolation (S): S = (1+e 2 /2)S 0, where S 0 is for e = 0 The range of e is < 0.06 for the Earth. 1. Calculate S for e = 0.06 versus e = If solar constant S 0 is 1361 W m -2 and the climate sensitivity is 0.5 K (W m -2 ) -1, how much change in surface temperature do we expect? (1+e 2 /2) = , so 0.18% change in S 0 (~ 2.5 W m -2 ). ΔS/4 (1-α) = 2.5 1/4 (1-0.3) = 0.44 W m -2 climate forcing. dq λ = = 0.22 K, climate response - not enough for an Ice Age. Conclusion: Change in eccentricity has minimum effect on annual-mean insolation.

18 Milankovich Theory: onset of ice age depends, not on annual mean insolation, but what happens in the polar region during summer & winter. The following conditions are conducive to the onset of an ice age: 1) (Cold Summer) A colder summer means less melting of glacier. 2) (Warm Winter) Winter pole is almost always cold enough to form snow. A slightly warmer winter means more moisture in the air that will produce more snow. Note that colder summer & warmer winter happen under the same orbital condition (e.g., when the obliquity is smaller). Once these conditions are met, ice albedo feedback will help push the Earth into a full-blown ice age. The essence of the Milankovich Theory is that it moved away from the previous school of thoughts that focused on changes on annual mean.

19 Several important orbital parameters: 1. Obliquity (Φ): measuring the tilting angle of the Earth s rotation axis 2. Eccentricity (e): how elliptical the orbit is (between 0 and 1) 3. Perihelion relative to the equinox (Λ): precession of the equinox Let s think about what kind of arrangement for each of these parameters will minimize summertime solar insolation for the pole. 1. Small obliquity; 2. large eccentricity + summer solstice at aphelion

20 Effect of obliquity Hotter summer near pole Colder winter Normalized distribution function for solar insolation S(23.5 0,lat,time) S(24.5 0,lat,time) - S(22.5 0,lat,time) The largest & smallest possible tilting angles Higher latitude summer receives 10% more insolation (~ 40 W m -2 ) if obliquity increases by 2 0. So, smaller obliquity results in colder summer/warmer winter, and vice versa.

21 Ellipse: d = a 0(1 e 2 ) 1+ ecosv At perihelion : (v = 0) At aphelion: (v = ) d = a 0(1 e 2 ) 1+ e d = a 0(1 e 2 ) 1 e = a 0 (1 e) = a 0 (1+ e) Solar insolation (S) is proportional to 1/d 2. For the Earth, the maximum e = 0.055, so S( perihelion) S(aphelion) = ( )2 ( ) Currently, e = 0.015, so S( perihelion) S(aphelion) = ( )2 ( ) So, NH winter receives ~ 6% more solar insolation than SH winter

22 Effect of eccentricity & precession combined At Northern summer, insolation S 0 s 4 (Φ,lat,v)(1+ 2esinΛ) Precessional parameter Precessional parameter results in 15% change in high-latitude summer insolation. Note it s a combined effect (with eccentricity). For northern summer to receive less solar radiation, we need 1) e to be large (orbit being elliptical) 2) longitude of perihelion = or sinλ = -1 (meaning aphelion coincides with the northern summer solstice).