1 PALEOCEANOGRAPHY 12.740 SPRING 2000 Lecture 14 CLIMATOLOGY OF GLACIATION I. Physical mechanisms governing climate change. A. "Solar Constant": So = ~1370� watts/m2 (amount of sunlight intercepted by a circle with the radius of the earth at the same distance as the earth from the sun). This energy is distributed daily over a sphere of area 4 x that of the circle . Note: 1 cal/cm2/min = 698 watts/m2 Re planar circle 1. So 1/4 So is average amount of solar radiation received per unit surface area of the earth. However, this radiation is not uniformly received over the surfa ce of the earth (seasonal change and latitude). This amounts to 340 watts/m 2 at the top of the atmosphere. 2. How constant is the solar constant? Recent observations by satellites sugge st that it can change measurably (~0.1%) on time scales of a solar cycle. Mo nthly, averages of UV radiation may vary 40% or more (depending on solar ac tivity). From observations of sunspots (Maunder Minimum) and 14C and 10Be, we can surmise that decadal-century scale variability of at least this order o f magnitude has occurred for at least the past 10,000 years. Unfortunately, there is no evidence whether the sun is constant on glacial time scales. The success of the orbital correlation, however, suggests that solar variation ma y not be the important driving parameter. B. Seasonally, at present, the solar beam at the earth is about 31/2% stronger in Ja nuary and 31/2% weaker in July, due to the ellipticity of the earth's orbit. But the earth moves faster in its orbit nearer to sun, so northern winter is 71/2 days s horter than northern summer. 1. The net result of this ("all other things equal") is to make northern winters wa rmer, and summers cooler, than their southern hemisphere equivalents. The effect of unequal land-sea distribution and reflecting variations outweigh this effect, however. The northern hemisphere has much more land, and the conti nental interiors get much colder during winter than oceanic areas.2 C. This energy is converted into thermal motion and re-radiated as infrared radiatio n 1. Black body: outgoing infrared radiation IIR = kT4 2. Real objects: I = e k T4 \ infrared emissivity - a dimensionlessconstant, ~0.95 for earth's surface. Acircle = pr2 ; Asphere = 4pr2 D. Blackbody earth: I = e k T4 = 1/4 So I = outgoing e = infrared k = Stephan- T = absolute Solar emissivity Boltzman temperature IR radiation constant So = solar "constant" I and So are expressed in units of energy/time/area (e.g. watts/m2) e ~ 0.95 (dimensionless; depends on substance) k = 5.68 x 10-8 watts °K-4 T is expressed in absolute °K 1. For So = 1361 watts m-2, the equilibrium T would be 281.8°K (8.5°C, 47°F) C. Of the 340 watts/m2 coming in, 140-170 are absorbed at the surface of the ear th (~60 after passing through clouds and ~20 normally from the scattered shor t wave radiation from a blue sky). ~120 is reflected back into space as visible ra diation. 7 watts/m2 absorbed by stratosphere (O3); 7 watts/m2 by lower atm ( mainly O2); 24 watts/m2 by H2O vapor. 1. Albedo (reflectivity) a; ~0.30 for the earth as a whole; average albedo of clou ds ~0.55; winter snow ~0.80; sea ice~ 0.4-0.6. 2. So total energy absorbed at earth's surface is 1/4 So (1 - ae) B. Effect of Albedo on blackbody earth temperature: 1. Above calculation (II.D.2) applies to a black earth; i.e. all visible solar energy is converted to heat at the earth's (black) surface. Actually, a significant amo unt of light is reflected back into space (mainly by clouds, ice, and the contine nts). So the equation must be modified to: I = e k T4 = 1/4 So (1 - ae) where ae is the surface albedo3 a. Since the albedo potentially could be quite different (more or less ice; diffe rent vegetation patterns; more or less clouds), albedo changes must be co nsidered in evaluating potential changes in climate. Albedo feedback can b e positive (e.g. cooler T --> more ice --> less heating) or negative (higher T --> more clouds (?) --> less heating) 2. Today, ae ~ 0.30, so the equilibrium temperature would be 257.8°K (-15 °C, 4°F). So in the absence of an atmosphere (or in the presence of a radiatio nally-inert atmosphere) (and assuming reflectivity didn't change - no freezing allowed, same clouds), the earth would be much cooler. a. If the solar constant changed by 1%, this T would change by 0.7°K. If albed o changed by 1%, T would change by 0.9°K. In other words, the blackbody cl imate sensitivity is about 1°K per 20 watts/m2/ C. The Greenhouse Effect 1. The average earth surface T is ~15°C (288°K), so the earth surface re-radiat es ~380-410 watts/m2. This is more than total is coming in at the top of the atmosphere! How can this be? Not due to earth's heat flow: 2 x 10-6 cal/c m2/sec => 0.1 watts/m2. Instead, it is because only 40�watts/m2 of this e mitted IR escapes directly to space (straight thru atmosphere). 30-40 watts /m2 is relected back towards the earth by IR absorbers: H2O, CO2, O3; H2O is the most important in toto (hence a substantial T-atm. water vapor positive f eedback: a 1°K rise in SST leads to a 6% rise in atmospheric water vapor) Ho wever, note that because of convection, the equatorial region is a sink for at mospheric moisture, not a source! (90 w/m2 evaporation compared to 250 w /m2 release of latent heat through precipitation). 3. Atmosphere blankets earth:4 a. Because of time lag between absorption and emission, and rotation and vibr ation of gas-phase molecules, IR absorbers re-emit in random direction rel ative to the incident radiation. 2. This effect is mainly due to H2O (H2O IR bands of atmosphere are largely satu rated) But other constituents (e.g. CO2, N2O, CH4, CFC's) are significant. At present, these increase the equilibrium T to ~15°C (+30° more than T expect ed from albedo-earth without greenhouse effect). 3. CO2 has a greater relative effect in cooler or drier areas. There also is a pot ential positive feedback with water vapor:5 (more CO2 -> higher T -> more atm. water vapor -> yet higher T). 5. Convection helps move sensible and latent heat up into the atmosphere. If so mehow convection could be suppressed entirely (as it can in a model), the ear th would be warmer than it is now: ; i.e., convection tends to cancel out some of the greenhouse warming. Doing the simple radiative calculation where the e arth's atmospheric composition is exactly as it is