Vegetation effects on climate Rt – r(a) = lE + C + G Vegetation can affect components of surfaceenergy balance 1. Rt is total solar radiation reaching Earth 2. r is reflected radiation, a function of albedo (a) 3. lE is latent heat transfer, driven by evapotranspiration 4. C is convective heat transfer (sometimes called sensible heat flux) 5. G is storage
Vegetation can alter albedo • Leaf color • Land-use change: Grazing, exposes soil, increases albedo, reducing net radiation, decreasing latent heat flux (less evapotrans) Over large enough scales, such changes can alter regional precipitation Similar phenomenon for deforestation • Tree migration into tundra • Tundra is snow-covered in winter, very high albedo • With warming, trees could advance, decreasing winter albedo dramatically • Potentially, creates a positive feedback to warming
II. Changes in climate A. Seasonal (see I.B.) B. Yearly (interannual) C. Millenial scales D. Human impacts - Is global warming for real? - How do we know that it isn’t just a natural fluctuation in temperature? - What are some of the forces that lead to natural climate variability?
II.B. Interannual Variation – El Niño Southern Oscillation - The Pacific Ocean strongly influences the global climate system because it is the largest ocean basin - Normal ocean current and wind direction in central Pacific is easterly 2.9
ENSO events result from weakening of tropical Pacific atmospheric and oceanic circulation Climatic connections carry these climate effects throughout the globe (e.g., El Niño creates warm winters in AK and lots of rain in Calif) 2.19
Changes in orbit cause long-term variations in solar input to Earth Shape of orbit (100,000 yrs) Wobble of tilt (23,000 yrs) Angle of tilt (41,000 yrs) 2.14
Eccentricity: The Earth's orbit around the sun is an ellipse. The shape of the elliptical orbit, which is measured by its eccentricity, varies through time. The eccentricity affects the difference in the amounts of radiation the Earth's surface receives at aphelion and at perihelion. When the orbit is highly elliptical, one hemisphere will have hot summers and cold winters; the other hemisphere will have warm summers and cool winters. When the orbit is nearly circular (now), both hemispheres will have similar seasonal contrasts in temperature.
Rotation axis executes a slow precession with a period of 23,000 years (see following figure) Pole Stars are Transient Wobble inthe tilt
Precession: Present and past orbital locations of the Earth during the N Hemisphere winter
Milankovitch cycles • The interactive effects of Earth’s orbital variation on timing and distribution of total solar input. • Strong effect on glacial/interglacial cycles http://en.wikipedia.org/wiki/Image:Vostok_420ky_4curves_insolation.jpg
D. Human effects • Global warming
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
The Earth’s Energy Balance A budgeting exercise
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
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
Basic physics • Electromagnetic (EM) radiation if generally propagated as a wave • But EM can often behave more like a particle - photon
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
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
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
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
Which passes through a larger unit area? Which has the higher flux?
Think about how that affects energy flux with latitude on earth.
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)
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.
Temperature • Measure of internal heat energy • Rate of motion of molecules in a substance • Faster movement = higher temperature