Earth's Energy Balance and Timescales of Planetary Evolution

1. Timescales • Geological time (big changes globally) • Glacial-interglacial cycles (really recent time) – associated with shifting land masses and effects on ocean circulation • Human time (anthropocene) – relationship to geological forces • Rates of change • Timescales of movement of material and energy • Evolution versus extirpation versus extinction • Human-induced rates of change versus rates of natural self-regulation

2. The Earth’s Energy Balance A budgeting exercise

3. Over Time • Origin of earth (4.6 bybp) • Dating of extra-terrestrial material • Hadean & Archaen (4.6 – 2.5 bybp) • unidirectional change in organization of material/planetary evolution; driven by energy from radioactive decay • Proterozoic & Phanerozoic (2.5 bybp to present) • Solar energy more important/recycling of materials

4. Major points • How do the various components of earth system move energy and matter on the earth’s surface? • Over long time scales the planet must be in steady state (input = output) • In the present system, energy balance at the earth’s surface is driven by solar radiation

6. Basic physics • Electromagnetic (EM) radiation if generally propagated as a wave • But EM can often behave more like a particle - photon

7. Waves defined by their speed (c), wavelength l, and frequency n Frequency and wavelength are inversely related c = ln (cm/cycle or cm) n = c/l Wave number (n) = 1/l (cycles/cm or cm-1) Fig. 3-2

8. Electromagnetic spectrum High energy/high frequency/low l Low energy/low frequency/high l E = hn = hc/l At times, EM radiation behaves more like a particle - photon Fig. 3-3

9. EM • High energy photons (low l/high frequency) – e.g., UV – can break molecular bonds and initiate chemical reactions • Low energy photons (high l/low frequency) interact with molecules affecting their rotation of vibration

10. Flux • How energy (or any material) passes through a unit surface area per unit time • Can think of energy as a particle • Units: some mass per unit area per unit time • mg/m2/hr (= mg m-2 h-1) • mmol quanta/ m2/hr

11. Which passes through a larger unit area? Which has the higher flux?

12. Think about how that affects energy flux with latitude on earth.

13. Back to energy • Energy is expressed as Joules (J) – measures heat, electricity and mechanical work • 1 J = 0.239 calories • 1 J = 2.7778 ×10−7 kilowatt hour • Power (the rate at which work is done or energy is moved) is expressed in Watts (W) • 1 W = 1 J/second • W/m2 then is a unit of energy flux • =J/m2/s • Energy flux is important for global climate • Polar regions cooler due to lower energy flux (mass per unit area per unit time)

14. Flux also depends on distance of an object or observer from the object emitting the radiant energy • Flux of solar energy decreases with distance from the sun. • Relationship is an inverse-square law • Double distance from the sun and intensity of radiation (or energy flux) decreases by a factor of ¼.

15. Temperature • Measure of internal heat energy • Rate of motion of molecules in a substance • Faster movement = higher temperature

16. Blackbody radiation • A body that emits radiation over all possible frequencies (remember inversely related to wavelength) • Wavelength distribution depends on the temperature of the blackbody • The Planck function describes the wavelength distribution relative to radiation intensity

17. Fig. 3-7 Wein’s Law The flux of radiation emitted by a blackbody reaches its peak value at lmax which depends inversely On the body’s absolute temperature The energy flux emitted by a blackbody is related to the fourth power of the body’s absolute temperature Stefan-Boltzmann Law F= sT4

18. UV Vis IR (heat) Energy flux and blackbody radiation temp. are related. Because it is hotter, sun emits more energy per unit area at all wavelengths. Changes in energy flux determine the spectrum of the EM radiation emitted. Shifts along the l axis. Blackbody emission curve for sun and earth – note higher energy shorter wavelength (higher frequency) emissions for the sun.

19. Energy output of the sun • Predicts a maximum of radiation emission in the visible region (0.4 – 0.7 mm) • Large component of high energy UV (remember this is damaging) • Assume Earth is a blackbody radiator • The Earth System responds to both the amount of solar radiation and its EM spectrum

20. Earth’s energy balance • If temperature is constant, the planet has to be in radiative balance

21. Energy input = energy output Fig. 3-19 Earth’s globally averaged atmospheric energy budget. Some incoming solar radiation is lost before reaching the earth’s surface (30%). Some is returned to space as IR (70%)

22. O3 Plus CO2 and H2O Some incoming solar radiation is reflected by clouds Some is absorbed in the atmosphere (ozone at low l; CO2 and H2O at high l)

23. O3 H2O, CO2 UV Vis IR (heat) Fig. 3-8 Blackbody emission curves for the Sun and Earth.

24. O3 Plus CO2 and H2O Only 50% if incoming solar radiation gets through the atmosphere to the Earth’s surface

25. Blackbody radiation of the sun minus that “lost” in the atmosphere. Solar spectrum clipped at high and low ends Energy loss shifts lmaxto lower wavelengths when energy is re-radiated by the Earth. Reradiated as IR (heat). Assumes Earth is a blackbody radiator. Shift in lmax controlled by Wein’s law Net effect of atmosphere on incoming and outgoing radiation

26. Outgoing IR also absorbed by greenhouse gases. CO2 and H2O

27. Greenhouse gas absorption • Major role in warming atmosphere • Traps heat • Leads to re-distribution of radiation before it is re-radiated to space • Without greenhouse gases, planet would be much colder (~ -20oC)

28. Feedback loops • Feedback between lithosphere, hydrosphere, atmosphere and biosphere interact to maintain relatively constant, favorable temperatures over most of geologic history

29. Greenhouse effect • Planetary energy balance in the absence of a greenhouse • Simple model • Planet with no atmosphere and albedo that of Earth • Energy emitted by Earth= energy absorbed by Earth • From distance of sun, Earth is a circle (projected area) • Energy absorbed = energy input – reflected light • Albedo is the average color of the planet • Low albedo = dark color = absorbs heat (warms up) • High albedo = light color = high reflectance (cools down) • Assuming blackbody radiation, Stefan-Boltzman law predicts the temperature of the atmosphereless planet

30. Where S is solar flux A is earth’s albedo We know how to calculate area of a circle Apply Stefan-Boltzman Law and simplify Box Fig. 3-1

31. Low albedo = low reflectivity(light is absorbed) High albedo = high reflectivity Fig. 2-6

32. Box Fig. 3-1

33. Box Fig. 3-1

34. Treat atmosphere as black body T4e = (S/4)*(1-A) Energy balance • T4s = 2T4e Fig. 3-2 The greenhouse effect of a one-layer atmosphere. Provides 33oC of surface warming

36. Fig. 3-19 Earth’s globally averaged atmospheric energy budget.