MET 112 Global Climate Change The Earth’s Energy Balance Dr. Craig Clements San José State University MET 112 Global Climate Change

Review: Why is CO2 So Important? Carbon Dioxide is a greenhouse gas. Greenhouse gases are those gases that cause the greenhouse effect. The greenhouse effect makes a planet’s surface temperature warmer than it would otherwise be. The stronger the greenhouse effect, the warmer the surface (other factors being equal). Consider the blanket analogy So, we’ve introduced an obviously important term. The greenhouse effect. If you’ve ever seen a greenhouse, it’s a glass house used to grow plants usually in the winter when it’s very cold. Inside a greenhouse, the sun’s energy can come through the glass, but the energy trying to leave the greenhouse cannot escape. While this is the basic idea, we’ll see that it’s actually a bit more complicated than this, but for now this is ok. So, there are certain type of gases that we call greenhouse gases. These greenhouse gases are special because sunlight passes through them, but they trap heat trying to leave the planet, similar to a greenhouse.

Earth’s Energy Balance Energy entering top of atmosphere Energy entering the Earth’s surface = Energy leaving top of atmosphere = Energy leaving Earth’s surface So today, after these very broad introductory comments, we will spend a short time looking at the nature of energy in the E-A system, then we will look at some very basic radiation concepts (here I don’t want to get bogged down in complex detail, I just want to give you an overview of basic radiation concepts). However it is important that you have a basic understanding, because radiation is the primary energy source for driving atmospheric and ocean circulation and for all life on earth. Finally today, we’ll examine the mean annual global energy budget, concentrating especially on those significant climate leverage points. Conservation of Energy

There are three modes of energy transmission in the atmosphere. Conduction: the transfer of energy in a substance by means of molecular excitation without any net external motion. Convection: the transfer of energy by mass motions within a fluid or gas, resulting in actual transport of energy. Radiation: the propagation of electromagnetic waves through space. We will talk about radiation in a bit more detail in a minute. Conduction does occur in the atmosphere, but is very inefficient because of the distance between air molecules. Conduction is much more important in solid substances, such as associated with heat transfer in soil Convection occurs both in the atmosphere and ocean. For example turbulence in the atmosphere is a very efficient means of vertical transfer of heat. The term ADVECTION (applied to atmosphere or ocean) refers to horizontal “convection”. One convenient way at looking at flows and transformations of energy in the E-A system is by taking a systems approach

Conduction

Convection

Electromagnetic radiation Radiation is the transfer of energy by rapid oscillations of electromagnetic fields. The most important general characteristic is its wavelength (), ______________________________. Radiation travels through space at the speed of light (3 x 108 m s-1). Defined as the crest-to-crest distance The energy from the sun is transmitted by e-m radiation, which is emitted by all objects that have temperatures above absolute zero

Radiation What emits radiation? All objects with a temperature greater than 0°K emit some type of radiation (energy) Examples: Radiation laws: Warmer objects emit more intensely than cold objects. (Stefan-Boltzmann Law) Warmer objects emit a higher proportion of their energy at short wavelengths than cold objects. (Wien’s Law)

Stefan-Boltzmann Law: λ = w / T Wien’s Law: λ = maximum wavelength (μm) w = constant = 0.2897 (μm K) T= temperature of the object (K) Stefan-Boltzmann Law: E = σT4 E = radiation emitted (W m-2) σ = Stefan-Boltzmann constant= 5.67 x 10-8 (W m-2 K-4) T= temperature of the object (K)

Solar Radiation (Sunlight) Sunlight is primarily made up of the following: Visible Light (44%) Infrared Radiation (48%) Ultraviolet Radiation (7%) Unit: 1 m = 0.000001 m

Terrestrial or Longwave Radiation Planets mainly emit infrared radiation Radiation emitted by planets occurs mainly at wavelengths _____ than those contained in solar radiation Terrestrial Radiation (“Longwave”) Solar Radiation (“Shortwave”) longer

Solar vs. Terrestrial Radiation The sun is much hotter than planets; therefore, sunlight consists of shorter wavelengths than planetary radiation; Thus …

Review questions Sun Earth infrared Considering the previous discussion Which object would emit more (intensity) radiation: Earth or Sun? If you were examining the radiation emitted by both the Sun and Earth, which would have a longer wavelength? What wavelength radiation are you emitting right now? Sun Earth infrared

Energy from the Sun Obviously, the Sun provides the Earth with it’s energy. The question is, how much of the Sun’s energy does the Earth get? Sun’s energy is either Scattered (reflected away) or Absorbed Scattering happens by bouncing off Particles in the atmosphere Earth’s surface Absorption happens when certain gases absorb the energy The reality is the only certain gases absorb certain wavelengths.

Absorption of radiation Absorption of shortwave radiation by atmospheric gas molecules is fairly weak; most absorption of shortwave radiation occurs at the Earth’s surface. Most gases do not interact strongly with longwave radiation, however Greenhouse gas molecules absorb certain wavelengths of longwave radiation.

Absorption of Radiation in the Earth’s Atmosphere

Incoming solar radiation Each ‘beam’ of incoming sunlight can be either: Reflected back to space: Clouds Atmosphere Surface Or absorbed; either by atmosphere (e.g. clouds or ozone) or Earth’s surface. Albedo So today, after these very broad introductory comments, we will spend a short time looking at the nature of energy in the E-A system, then we will look at some very basic radiation concepts (here I don’t want to get bogged down in complex detail, I just want to give you an overview of basic radiation concepts). However it is important that you have a basic understanding, because radiation is the primary energy source for driving atmospheric and ocean circulation and for all life on earth. Finally today, we’ll examine the mean annual global energy budget, concentrating especially on those significant climate leverage points.

Longwave or terrestrial Recap ______________ radiation comes from the sun and is composed of both ultraviolet and visible radiation __________________ radiation comes from the Earth and is composed of infrared radiation Recall that everything (above a temperature of 0K) emits some type of radiation (energy) with a particular wavelength. Shortwave or solar Longwave or terrestrial

Review - sensors that measure radiation Pyranometer A _________________ measures solar radiation. A__________________ measures infrared radiation (terrestrial) that comes from the Earth. Pyrgeometer

Some atmospheric radiation escapes to space Some surface radiation escapes to space Greenhouse gases emit longwave upward and downward Most outgoing longwave is absorbed in atmosphere (by greenhouse gases) Some atmospheric radiation is absorbed at the surface Longwave radiation is emitted from surface.

Result: warmer surface temperature Greenhouse Effect Sequence of steps: Solar radiation absorbed by earth’s surface. Earth gives off infrared radiation. Greenhouse gases absorb some of the Earth’s infrared radiation. Greenhouse gases (water and CO2) give off infrared radiation in all directions. Earth absorbs downward directed infrared radiation Result: warmer surface temperature So today, after these very broad introductory comments, we will spend a short time looking at the nature of energy in the E-A system, then we will look at some very basic radiation concepts (here I don’t want to get bogged down in complex detail, I just want to give you an overview of basic radiation concepts). However it is important that you have a basic understanding, because radiation is the primary energy source for driving atmospheric and ocean circulation and for all life on earth. Finally today, we’ll examine the mean annual global energy budget, concentrating especially on those significant climate leverage points.

Energy Balance Assume that the Earth’s surface is in thermodynamic equilibrium: Thermodynamic Equilibrium: The flow of energy away the surface equals the flow of energy toward the surface Surface Average surface temperature = 15°C

Sudden Removal of all Greenhouse Gases Removal of greenhouse gases would decrease downward flow of energy; now energy away from surface is greater than energy toward surface.

Sudden Removal of all Greenhouse Gases Removal of greenhouse gases would decrease downward flow of energy; now energy away from surface is greater than energy toward surface. Thus, average surface temperature starts to decrease.

Sudden Removal of all Greenhouse Gases As surface cools, emission of radiation decreases until balance is restored. At this point, cooling stops

Result: A Very Cold Planet! As surface cools, emission of radiation decreases until balance is restored. At this point, cooling stops and equilibrium is restored. Average surface temperature = -18°C

Earth’s Greenhouse Effect Without the greenhouse effect, the surface temperature of Earth would be Way Cold (-18°C) Greenhouse gases play an important role in shaping climate. More GHGs – warmer climate Less GHGs – cooler climate

Classwork 1-1 Surface Average surface temperature = 15°C Start with the following diagram and assume the earth’s surface temperature is 15C and that the atmosphere has greenhouse gases. Imagine that the concentrations of greenhouse gases were to increase by 50%. 1. Draw two more diagrams illustrating (with arrows) how the energy balance would change with the increase in greenhouse gases and explain why. 2. How would the average surface temperature change? Surface Average surface temperature = 15°C