Understanding Global Climate: Patterns of Radiation Balance

1. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 08:Patterns of Radiation Feb-07-05 (1 of 10) Further Reading: Chapter 04 of the text book Outline - geographic patterns of energy balance - net radiation - meridional transport

2. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University 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 Although the system strives for thermal equilibrium, changes in the system allow for different climates -> “climate change” Myneni Lecture 08:Patterns of Radiation Feb-07-05 (2 of 10) Introduction

3. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 08:Patterns of Radiation Feb-07-05 (3 of 10) Insolation • Today, we will look at the geographic patterns in the radiation balance • Insolation • Albedo • Longwave Radiation • Net Radiation • Insolation: • depends on latitude & time of the year

4. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 08:Patterns of Radiation Feb-07-05 (4 of 10) 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 over equatorial lands can be high due to persistent cloud cover

5. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 08:Patterns of Radiation Feb-07-05 (5 of 10) Absorbed Shortwave Radiation More solar radiation is absorbed by the oceans than adjacent lands about the equator due to clouds on land Less solar radiation is absorbed at the poles than at the equator (in general)

6. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 08:Patterns of Radiation Feb-07-05 (6 of 10) 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 incoming solar radiation • High in tropics • Decreases towards poles

7. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 08:Patterns of Radiation Feb-07-05 (7 of 10) Net Radiation • 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

8. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 08:Patterns of Radiation Feb-07-05 (8 of 10) 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)

9. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 08:Patterns of Radiation Feb-07-05 (9 of 10) Meridional Transport via The Atmosphere • Transports warm tropical air to high latitudes; e.g., tropical air masses

10. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 08:Patterns of Radiation Feb-07-05 (10 of 10) Meridional Transport via The Oceans • Transports warm tropical water to high latitudes; e.g. Gulf Stream