ATM S 111, Global Warming: Understanding the Forecast DARGAN M. W. FRIERSON DEPARTMENT OF ATMOSPHERIC SCIENCES DAY 1: OCTOBER 1, 2015
Outline How exactly the Sun heats the Earth How strong? Important concept of albedo : reflectivity How the greenhouse effect works How the Earth cools And how greenhouse gases lead to less cooling
From Before We Asked What factors influence climate at a given place? Sunshine (and latitude) Topography/mountains Proximity to oceans and large lakes Ocean currents Presence of trees/vegetation Etc. But what are the main factors that control the global climate? We ll study this next
The Sun Driver of everything in the climate system!
How Does Energy Arrive From the Sun? Energy from the Sun is electromagnetic radiation or just radiation for short Goes through space at the speed of light Radiation is absorbed or reflected once it gets to Earth Radiation with shorter wavelengths is more energetic And radiation is classified in terms of its wavelength ß This has long wavelength and low energy ß This has short wavelength and high energy
Types of Radiation Types of electromagnetic radiation, from most to least powerful (or shortest wavelength to longest wavelength) Gamma rays X-rays Ultraviolet (UV) radiation Visible light Infrared radiation Microwaves Radio waves
Sun s Radiation The Sun emits: Visible light (duh) Also near infrared radiation (infrared with very short wavelength) A small (but dangerous) amount of ultraviolet radiation This is what makes us sunburn! These three bands together we call shortwave radiation
How Strong is the Sun? By the time it gets to the top of Earth s atmosphere, the Sun shines at a strength of 1366 Watts per square meter Watt (abbreviated as W): unit of power or energy per unit time 1366 W/m 2 is roughly what s experienced in the tropics when the sun is directly overhead
Average Solar Radiation The average incoming solar radiation is not 1366 W/ m 2 though It s only 342 W/m 2 (exactly ¼ of this). Why? Half the planet is dark at all times High latitudes get less direct radiation, which spreads out more Here it s nighttime
Reason for Seasons Directness of solar radiation is key for seasons as well Winter is tilted away from the Sun, gets less direct light, and thus is colder Winter solstice December 21-22 Equinox March 20-21 or Sept 22-23 Summer solstice June 20-21 Current sunlight South pole sky soon after equinox (October maybe? Sun goes around low in the sky)
Winter in the Northern Hemisphere When the Earth is tilted away from the Sun The Sun is lower in the sky Days are shorter
When Solar Radiation Hits the Atmosphere Average incoming solar radiation = 342 W/m 2 Only 20% gets absorbed in the atmosphere This includes absorption of dangerous UV by the ozone layer 50% is absorbed at the surface Meaning much of the sunlight makes it directly through the atmosphere! 30% is reflected back to space What does the reflecting?
Key Concept for Climate: Albedo Albedo: fraction of incident light that s reflected away Albedo ranges from 0 to 1: 0 = no reflection 1 = all reflection Things that are white tend to reflect more (high albedo) Darker things absorb more radiation (low albedo)
Albedo Values for Earth Clouds, ice, and snow have high albedo Cloud albedo varies from 0.2 to 0.7 Thicker clouds have higher albedo (reflect more) Snow has albedo ranging from 0.4 to 0.9 (depending on how old the snow is) and ice is approximately 0.4 Ocean is very dark (< 0.1), as are forests (0.15) Desert has albedo of 0.3
Relative Contributions to Earth Albedo Remember we said 30% of incoming solar radiation is reflected away? 20% is from clouds 5% is by the surface 5% is by the atmosphere (things like dust from deserts and air pollution are key players here)
Total Solar Input Total absorbed solar radiation is 70% of the incoming solar radiation Because 30% is reflected away 70% of 341 W/m 2 = 240 W/m 2
Summary So Far The Sun heats the Earth Some is reflected back, a bit is absorbed in the atmosphere But other than that, the atmosphere is rather transparent to solar radiation (half is absorbed right at the surface!) Clouds and snow/ice are primary contributors to the albedo of Earth Next, how energy escapes from Earth and the greenhouse effect
Longwave Radiation The Sun is the energy input to the climate system How does the Earth lose energy? Turns out it s also by radiation! But it s not visible light like from the Sun, it s infrared radiation AKA longwave radiation Infrared satellite image à
Longwave Radiation Everything actually emits radiation Depends partly on the substance but mostly on temperature Neck = hotter Hair = colder Infrared thermometer
Longwave Radiation Higher temperature means more radiation Eyes and inner ears are warmest: they radiate the most Nose is the coldest: it radiates less Thermal night vision technology detects longwave radiation
From my cat Longwave radiation
Temperature & Radiation Higher temperature = more radiation and more energetic radiation (shorter wavelengths) Explains the Sun s radiation too Sun is really hot à It emits much more radiation It emits shortwave radiation instead of longwave radiation like the Earth
Energy Into and Out of the Earth Heating/cooling of Earth The Earth is heated by the Sun (shortwave radiation) The Earth loses energy by longwave radiation (out to space)
Energy Balance If the energy into a system is greater than the energy out, the temperature will increase A temperature increase then results in an increase of energy out Hotter things radiate more This will happen until: E in = E out Energy in Energy out When energy in equals energy out, we call this energy balance
Energy Balance on Earth If the solar radiation into Earth is greater than the outgoing longwave radiation, the temperature will increase A temperature increase then results in an increase of the longwave radiation out (hotter things radiate more) This will happen until: E in = E out Shortwave in Longwave out Global warming upsets the energy balance of the planet
Energy Balance with No Atmosphere If there was no atmosphere, for energy balance to occur, the mean temperature of Earth would be 0 o F (-18 o C) -18 o C (0 o F) Missing piece: the greenhouse effect All longwave radiation doesn t escape directly to space
The Greenhouse Effect Greenhouse gases block longwave radiation from escaping directly to space These gases re-radiate both upward and downward The extra radiation causes additional warming of the surface Extra downward radiation due to greenhouse gases 15 o C (59 o F)
The Greenhouse Effect Greenhouse gases cause the outgoing radiation to happen at higher levels (no longer from the surface) Air gets much colder as you go upward So the radiation to space is much less (colder à less emission) 15 o C (59 o F)
The Greenhouse Effect Greenhouse effect is intuitive if you pay attention to the weather! Cloudy nights cool less quickly In the desert, temperatures plunge at night! No clouds & little water vapor in the desert: little greenhouse effect
Summary The Earth is heated by the Sun This is shortwave radiation Albedo: key factor that determines how much radiation is absorbed vs reflected Earth loses energy due to longwave radiation The greenhouse effect causes less heat loss due to longwave radiation If the Earth is pushed out of energy balance, it warms or cools in response Warming can occur from increased solar radiation or increased greenhouse effect