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Atmospheric Composition The composition of Earth’s atmosphere both controls the radiative environment and is significantly regulated by biological activity. On geological timescales, the recycling of carbon via plate tectonics is critical for Earth’s climate (c.f. Mars & Venus). Table 1.1 lists, in order of abundance, the constituents of Earth’s atmosphere. 3.9 x 10-4 3.8 x 10-4 1 Atmospheric Mass: The Hydrostatic Balance Atmospheric pressure is the weight exerted by the overhead atmosphere on a unit area of surface. Consider the mercury barometer: "vacuum" Patm H The weight of the mercury column, H, must equal the weight of the atmospheric column, or: Patm = ρHg g H = (at sea level) 13.6 g cm-3 × 9.8 m s-2 × 76 cm = 1.013 × 105 kg m-1 s-2 in SI units. The SI unit is Pascal (Pa); 1 Pa = 1 kg m-1 s-2. Other units for atmospheric pressure in widespread use: the atmosphere, 1 atm = 1.013 x 105 Pa, the bar (b) (1b = 1 x 105 Pa), the millibar (mb) (1mb = 100 Pa) and for Chemists, the torr = 1 mm Hg = 134 Pa. To be good internationalists, Pa are it! Often you will now see pressure in hectopascals, hPa, which are equivalent to mb. 2 Consider the following forces acting on a slab of atmosphere: Surface area, A Z+dz Z At equilibrium (or hydrostatic balance!) the weight (acting downward) must be balanced by the pressure gradient force: ρ g A dz = A × (P(z) - P(z+dz)) rearranging: dP/dz = -ρ g From the ideal gas law, ρ = P Ma/RT Substituting yields: dP/P = - (Ma g / RT) dz For isothermal atmospheres (and this is true on Earth to ~20%), we can integrate: Pz = Po exp(- [(Ma g)/RT]z) = Po exp(- z / H) where H, the scale height, is about 7 1/2 km. Mass of Earth’s atmosphere Mass = 4 π R2 P(surface)/ g ≈ 5 × 1018 kg. (1 kg / cm2). This is the same weight as a column of water 10 meters deep. Air is 78% N2, 21 % O2, and 1% Ar (+ some water and other things). The mean molecular weight, Ma, is: (0.78 × 28) + (0.21 × 32) + (0.01 × 40) = 29 g mole-1. For a column weight of 1 kg cm-2 we have ~35 moles cm-2 or 2 × 1025 molecules cm-2 in the column. 3 3.9 x 10-4 3.8 x 10-4 The Sun The sun is a relatively small star whose projected lifetime on the main sequence is ~ 11 billion years. Theory and observations of stars similar to the sun suggest that the luminosity has increased 25-30% over the last 4.5 billion years. Despite the increasing energy delivered to Earth, many paleo climate records suggest that mean surface temperatures have not changed much and in general, perhaps, cooled. This has been called by some the faint-sun paradox. The current solar luminosity, Lo, is presently 3.9 x 1026 W. This energy is emitted by the sun's photosphere whose radius is ~7 x 108 m. The flux density at the photosphere is then: Flux Density = flux/area = Lo/(4 π r2) = 6 x 107 W m-2 Compare San Onofre Nuclear Station 1 or 2 or 3 = 1 x 109 W (actually not 1 as it is now decommissioned) 4 The Solar "Constant" Since space is effectively a vacuum, the amount of energy passing outward through any sphere centered on the sun will be equal to the solar luminosity, Lo. If the radiation is isotropic: Flux = Lo = Sd 4 π d2 Sd, the Solar Constant (at distance d) = Lo/(4 π d2) – see figure to follow At the mean distance of Earth from Sun (1AU = 1.5 x 1011 m): So = 1368 W m-2. For a rotating sphere at 1 A.U., average radiance: So / 4 = 342 W m-2 [c.f. average new refrigerator (or average person) ~ 100 W.] In addition to 25-30% change in Lo over 4.5 billion years, So changes on various timescales: 1. Annual due to orbital eccentricity (~3% change in d between perihelion - presently Jan. 3rd - and aphelion) 2. "11-year" (solar cycle) (0.25% change in Lo) Numerous attempts to link decadal solar variation with climate have been (largely) unsuccessful. Figure 5. Ruddiman 2001 Emission Temperature of Planets 5 Figure 6. Earth’s Orbit Climate Puzzles: The Solar "Constant" Figure 7. Ruddiman 2001. 6 Figure 7. Ruddiman 2001. The Solar "Constant" The current climate epoch: The Holocene (though some now say we have entered a new epoch, the anthropocene) The climate during the last 10,000 years has been notably quiescent Ice core record from Greenland – Emergence of civilization (see Diamond "Guns, Germs, and Steel") Figure to follow Some variability, however Climate record of 20th century – Figure to follow Nevertheless, recent (and relatively small) local climate shifts have had significant influence. Dust Bowl – 1930s Recent (last several decades): abnormal strength and consistency of Indian monsoon 1816 - "year without a summer" (Tambora eruption) 7 • Figure 2. Paleoclimate record for last 150,000 yrs. Ruddiman, 2001. The current climate epoch - the Holocene Schneider and Held, Discriminants of 20th Century Changes in Earth Surface Temperature, J. Climate, 14, 249, 2001 8 Some “Climate Puzzles”: • Faint Sun • Warm Ages - Cretaceous (100 Ma) Eocene (50 Ma) smaller pole-equator gradient(?) • Glaciation - No ice first 2.5 Ga; generally ice free until 0.1 Ga. • Transition from 40 Kyr to ~100 Kyr glaciation ~0.7-1 million years ago. • Present - warm extratropics / ice covered poles 9