Earth s Heat Budget Solar Energy and the global Heat Budget Transfer of heat drives weather and climate Ocean circulation Should we talk about this? What causes the seasons? Before you answer, think. What are seasons, what is their frequency? 1
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 Transmit Energy to Oceans: Three ways of transmitting Energy/Heat to/through oceans: Radiation: direct transfer of E via light waves from source. Conduction: Vibrating molecules collide Convection: flow advects/carries heat 2
Transmission of E to oceans: Sunlight adds Energy to oceans! Light hits Oceans: Scattering by particles Absorption by molecules (added heat) Attenuation... Attenuation of sunlight: Decrease in energy with depth Longer wavelengths (reds) absorbed shallow Photic zone < 100 m on average 3
Instantaneous Solar Radiation (ISR) ISR = amount of solar radiation per area of Earth surface Where is it greatest? Is there a trend? The Atmosphere s effect on ISR Atmosphere diminishes ISR b/c it absorbs & scatters light The atmospheres effect increases with latitude b/c the apparent atmospheric thickness increases with latitude 4
Daylight hours & ISR Longer day = More solar radiation Equator vs. High latitude Changes during seasons What about poles 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? Distance too Sun 5
Instantaneous Solar Radiation: results of 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 indicate Surplus (+0.85 W/m 2 ) 6
Budget details: heat loss Heat is lost by Reflection off the surface and atmosphere And reradiated from surface and atmosphere More heat is lost from the atmosphere to space than from the surface. Albedo effect of reflective surfaces (ice). Ice ages and the albedo positive feedback Heat budget: Absorbed heat Surface gains more heat from ISR than the Atmosphere! 7
Notice imbalance (internal) Atmosphere looses more heat than it gains from ISR. Therefore, surface MUST transfer heat to atmosphere to maintain balance! JUMP UP & DOWN! - how is atmosphere heated? How is heat transferred between Atmosphere and Surface? Latent heat of vaporization accounts for ~70% of heat transfer! Evaporation of water from surface absorbs heat from surface (heat into the water, not the surface) Precipitation liberates that heat ~30% from conduction and re-radiation 8
Latitudinal imbalance in heat budget Heat gained at <30º latitude (Surplus radiation) Heat lost at >30º latitude (deficit heat) Why? - ISR changes with latitude. So there is a thermal or heat Gradient So what? What drives atmospheric and surface circulation? Earth surface temperature Notice Temp. 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 9
Average Annual Temperature Change Land Temperature change > ocean Temperature change. Why? Very little change in temperature at the equator - Why? Greatest temperature change at temperate latitudes - Why? Small temperature changes in Polar regions - Why? Latent heat of fusion and sea ice formation. 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 10
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. 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 11
Sea Ice Forms during winter and insulates ocean maintaining Temperature Latent heat of fusion liberated too atmosphere & ocean Max thickness 2 m - why? Latent heat can t conduct out if ice > 2m thick and it melts Forms a layer of dense (salt-rich) cold water that sinks (circulates) Most of sea ice melts in summer Latent heat of fusion absorbed from ocean and atmosphere (cools oceans) Remaining sea ice reflects 80% or ISR - high albedo effect Average 1973-1976 Loss of Sea Ice Average 1999-2002 2002 annual loss (400,000 mi 2 ) relative to 99-00 average. Cause: Changes in Atmospheric Circulation (Climate Change?) Result: increased climate warming (heat transfer to atm. from oceans in winter). Dramatic change in ecosystem both on and below the ice. 12
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. Implications for the effects of global warming Snowball Earth = Sea Ice and Deficit in Heat Budget ~750-550 Ma Dim Sun (6% less ISR) Rhodinia Supercontinent = poor ocean circulation Sea Ice grows - Albedo effect reflects ISR. (positive feedback) Sum effect = super global cooling Glaciers cover continents Sea ice covers oceans Massive volcanism during rifting of Rhodinia and resumed global ocean circulation reverse the cooling. 13
Ice bergs are not sea ice Sea level increasing 1.8 mm/y and ~50% of that is from melting glaciers. Recent studies show that rates of glacial melting are increasing relative to rates from the 1990s. 14