Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 08:Patterns of Radiation Feb-02-07 (1 of 11) Further Reading: Chapter 04 of the text book Outline - geographic patterns of energy balance - net radiation - meridional (latitudinal) transport

Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 08:Patterns of Radiation Feb-02-07 (2 of 11) Introduction Previously, we discussed the energy balance on a global scale and found that there are complex interactions between subsystems but that overall there is no net gain or loss of energy within any subsystem Has important implications: Global energy balance is the primary control on global climate The natural Greenhouse effect (i.e. absorption and re-emission of longwave radiation by the atmosphere) is the key to temperature control of the entire system The concept of thermal equilibrium suggest that the system establishes an energy balance by adjusting the temperature of the system If incoming energy is greater than outgoing energy, then the temperature increases, increasing the outgoing radiation This is the key to understanding the global warming debate: if humans add CO2, the Greenhouse effect increases, more re-radiated energy reaches the surface, hence incoming radiation increases, thus the temperature increases, increasing outgoing radiation until the system is balanced again

Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 08:Patterns of Radiation Feb-02-07 (3 of 11) Insolation Today, we shall look at the geographic patterns in the radiation balance Insolation Albedo and Reflected (Absorbed) Shortwave Radiation Longwave Radiation Net Radiation Insolation: depends on latitude & time of the year TOA Insolation for January note the darkness in Northern Hemisp.

Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 08:Patterns of Radiation Feb-02-07 (4 of 11) Albedo Albedo is the fraction of incident shortwave radiation that is reflected by a surface It determines the amount of solar radiation absorbed by a surface Albedo is high near the poles (snow & ice), low over water bodies, medium over land Albedo of barren lands is higher than vegetated lands

Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 08:Patterns of Radiation Feb-02-07 (5 of 11) Reflected Shortwave Radiation On Winter Solstice, the polar North receives no energy from the Sun. In contrast, the amount of incoming solar energy the Earth receives on June 21, Summer Solstice, is 30 percent higher at the North Pole than at the Equator. In the top image, taken on Winter Solstice 2004, the far North is dark blue, indicating that no sunlight is being reflected back into space. The most sunlight is being reflected out of the Southern Hemisphere, where December 22 marked the longest day of the year. The lower image shows Summer Solstice 2005. Darkness dominates the South, while the North is now the location receiving and reflecting the most light. In both images, bright white clouds stand out because they are reflecting so much light back into space.

Less solar radiation is absorbed at the poles than at the equator Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 08:Patterns of Radiation Feb-02-07 (6 of 11) Absorbed Shortwave Radiation More solar radiation is absorbed by the oceans than adjacent lands Less solar radiation is absorbed at the poles than at the equator

Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 08:Patterns of Radiation Feb-02-07 (7 of 11) Outgoing Longwave Radiation Outgoing longwave radiation is very similar to absorbed shortwave radiation This is because longwave radiation is determined by the underlying temperature which is determined by the amount of absorbed solar radiation High in tropics Decreases towards poles

Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 08:Patterns of Radiation Feb-02-07 (8 of 11) Outgoing Longwave Radiation Note the North-South shift in max OLR emission Note the change in OLR emission in Antarctica

Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 08:Patterns of Radiation Feb-02-07 (9 of 11) Net Radiation = (ABS_SW - OLR) Remember, globally net radiation was zero, i.e. incoming balanced outgoing radiation This is not true for given latitudes At the tropics, the incoming is greater than the outgoing due to high insolation and low albedo At the poles, outgoing is greater than incoming due to low insolation and high albedo Why does the temperature at the equator not rise and the temperature at the poles not decrease? Because energy is physically transported from the equator to the poles by the atmosphere and the ocean -> it is this difference in energy between the poles and equator which drives almost all dynamics in the system

Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 08:Patterns of Radiation Feb-02-07 (10 of 11) Meridional Transport Driven by gradient in TOA net radiation Net Radiation at TOA Low latitudes: Positive High latitudes: Negative Outgoing energy partly derived from tropics Transport accomplished via two mechanisms (Atmosphere & Oceans)

Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 08:Patterns of Radiation Feb-02-07 (11 of 11) Meridional Transport via The Oceans Transports warm tropical water to high latitudes; e.g. Gulf Stream