Earth s Heat Budget Solar energy and the global heat budget Transfer of heat drives weather and climate Ocean circulation A. Rotation of the Earth B. Distance from the Sun C. Variations of Earth s orbit around the Sun D. Tilt of Earth s rotation axis relative to it s orbital plane E. Variations is Sun s heat What causes the seasons? Seasons Angle of sun in the sky varies over the year, why? This effects: Southern vs. Northern summer Daylight hours in summer & at poles And thus heating of Earth s surface 1
Three ways of Transmitting Energy: Radiation: direct transfer of energy via absorption of electromagnetic waves. Conduction: Two bodies in contact will transfer heat from hot to cold. Vibrating molecules of warm body collide with neighbors and cause them to vibrate Advection: flow of a fluid containing heat carries the heat as the fluid circulates. Transmission of E to oceans: Sunlight adds Energy to oceans by absorption of light radiation Light hits Oceans: Light s energy is Absorbed by water molecules (added heat) Adsorption occurs when the frequency of the light interacts with the electrons of the water molecule. Light is an electromagnetic wave and its frequency causes electrons of water molecule to oscillate in phase with the light. This oscillation absorbs the energy from the light wave and vibrates the molecule Attenuation... Attenuation of sunlight: Decrease in energy with depth due to absorption Each wavelength of light has a different frequency Long wavelengths have short frequencies Longer wavelengths (reds) absorbed shallower Shorter wavelengths penetrate deeper before being completely absorbed Photic zone < 100 m on average Below the photic zone, all visible light is absorbed 2
The Sun does not heat the surface equally at all latitudes ISR vs. latitude and atmosphere effect Instantaneous Solar Radiation (ISR) ISR = amount of solar radiation per area of Earth surface Where is it greatest? Is there a trend? 3
The Atmosphere s effect on ISR Atmosphere diminishes ISR b/c it absorbs & scatters light The sky is blue b/c the atm preferentially scatters blue wavelengths - so you see them. Looking near the horizon away from the Sun and the sky is less blue (mixing in longer wavelengths) Looking near the Sun we see yellows and reds of un-scattered light direct from the Sun The atmosphere s effect increases with latitude b/c the apparent atmospheric thickness increases with latitude At sunset, the light must travel a greater distance through the atmosphere and all the blues are scattered out of view - thus reds and yellows fill the sky for colorful sunsets. Daylight hours & ISR Longer day = More solar radiation Equator vs. High latitude Changes during seasons What about poles What causes the seasons? Solar Radiation: Daylight hours & poles ISR greatest during Summer (Duh!) ISR at poles helps to melt sea ice ISR is greater at South pole than at North pole! Why? 4
Instantaneous Solar Radiation: combining all factors Warming in tropics Maintain cool polar regions THERMAL GRADIENT: change in surface heating with latitude Earth s Heat Budget Is Earth s heat budget balanced (i.e. in = out) Has there ever been an imbalance? Global warming Ice ages Today! Current model results indicates a surplus of heat ~ 0.85 W/m 2 Budget details: heat loss Heat is lost by Reflection off the surface (ice, and water) and reflection off the atmosphere and clouds (albedo effect) And re-radiated from surface and atmosphere More heat is lost from the atmosphere to space than from the surface. 5
Heat budget: Absorbed heat Surface gains more heat from ISR than the Atmosphere! Notice the internal imbalance Atmosphere looses more heat than it gains from ISR. Therefore, surface MUST transfer heat to the atmosphere to maintain balance! JUMP UP & DOWN! - how is atmosphere heated? How is heat transferred between Surface and Atmosphere? Latent heat of vaporization accounts for ~70% of this heat transfer! Evaporation of water from surface absorbs heat from the sun and surface (heat flows into the water) Precipitation/condensation liberates that heat to the atmosphere This is advection of heat The remaining ~30% from conduction and re-radiation More heat is transferred to the atmosphere by LHV than is absorbed by ISR 6
Heat is gained by the surface at <30º latitude (Surplus heat from Sun) Heat is lost by the surface at >30º latitude deficit heat more heat is lost to atmosphere than gained from the Sun) Why? - ISR changes with latitude. So there is a thermal heat Gradient So what? This drives atmospheric and surface circulation. That s what! Latitudinal imbalance in radiated heat Earth surface temperature Notice temperature distribution (isotherms): Irregular pattern in N Hemisphere, Sub-parallel isotherms in Southern Ocean Why? Recall Location of continents effect communication of oceans Southern ocean communication smooths temperature distribution Average Annual Temperature Change Land Temperature change > ocean Temperature change. Why? Very little change in temperature at the equator - Why? Greatest ocean surface temperature change at temperate latitudes (~40º) - Why? Small ocean surface temperature changes in polar regions - Why? Thermostatic effect of latent heat of fusion of water (sea ice formation), that s why. 7
Compare seasonal changes in surface T 60º 60ºN - wide T variation 60ºS - minor T variation What is different between N. and S. Hemispheres? Continental mass Heat capacity Circulation Note also the warm ocean currents 0º 60º Northern Summer Northern Winter NOW heat budget & circulation! Heat in tropics Evaporation transfers heat to atmosphere and precipitation liberates heat into the atmosphere. Flow of heat is down the gradient! From equator to poles. In the oceans this transfer is along the surface currents Ocean surface currents result from atmospheric circulation that is a direct result of heat flow in the atmosphere from equator to higher latitudes Recall Heat transported by Gulf Stream Gulf Stream outflow is 10ºC warmer than return flow. Flow is 55 x 10 6 m 3 per second Calculate 550 trillion calories per second! 8
SST and Climate We know that oceans effect land temps (recall lake effect ). Research indicates this is a strong effect. 2004 SSTs 3rd warmest in 125 yr 2004 Global T avg 4th warmest in 125 yrs Sea Ice Forms during fall and insulates oceans maintaining Temperature Latent heat of fusion liberated to atmosphere & ocean Max thickness of sea ice is ~2 m - why? Latent heat can t conduct out if ice > 2m thick, causing ice to melt Forms a layer of dense (salt-rich) cold water that sinks (circulates) Most of sea ice melts in spring-summer Latent heat of fusion absorbed from ocean and atmosphere (cools oceans) Remaining sea ice reflects 80% of ISR delivered to polar regions - high albedo effect Antarctic Sea Ice & African Climate During times of abundant sea ice, sub-saharan Africa receives substantial rain (due to thermal gradient and resulting flow). During times of limited sea ice coverage, sub-saharan Africa is in drought. Similarly, variations in Arctic sea ice coverage may affect the climate of the northern hemisphere by altering the thermal gradient and thus changing the dynamics of heat flow. Francis and Vavrus, GRL 2012 Note: These results are not confirmed, and as yet not consensus but rather the topic of active research. 9
Snowball Earth = Sea Ice and Deficit in Heat Budget ~750-550 Ma The Earth was frozen solid = snowball Earth. How did this happen? The Sun was dimmer then (6% less ISR) Rhodinia Supercontinent resulted in poor ocean circulation and heat transfer. Thus, Sea Ice Extent increased resulting in increased Albedo effect (positive feedback - cooling causing further cooling) Sum effect = super global cooling at high latitudes resulting in Glaciers growing into low latitudes and covering continents, increasing Albedo effect further and cooling oceans at low latitudes Result: Sea ice extends to low latitude nearly covering oceans How did Earth ever recover? Massive volcanism during rifting of Rhodinia and resumed global ocean circulation reversed the cooling. Volcanic gases are greenhouse gases Separation of continents allowed oceans to circulate around them, warming the surface and melting the glaciers Thus, Albedo decreases and a warming trend sets in. Movie inserted on next page Review Questions What causes the seasons? Contrast the three ways heat energy can be transferred. Why is the photic zone 100m deep? How and why does ISR vary (1) with latitude and (2) throughout the year. Consider the Heat Budget: How is heat lost? How does the surface gain heat? Compare the solar energy added to the atmosphere versus the heat lost by the atmosphere. How is the atmosphere heated? Is the heat budget in balance? What is the root cause for circulation of Earth s oceans and Atmosphere? 10
Review Questions Why does surface temperature of the continents show wide ranges in temperature compared to the oceans? Why don t polar seas experience wide ranges in temperature? Explain why the variations in seasonal sea surface temperature are different in the Northern Hemisphere compared to the Southern. How does extent of seasonal sea ice impact Earth s climate? 11