ATM S 111, Global Warming: Understanding the Forecast

1. ATM S 111, Global Warming: Understanding the Forecast Dargan M. W. Frierson Department of Atmospheric Sciences Day 1: April 1, 2014

2. 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

3. 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

4. The Sun Driver of everything in the climate system!

5. 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 • This has long wavelength and low energy • This has short wavelength and high energy

6. 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

7. 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”

8. 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/m2 is roughly what’s experienced in the tropics when the sun is directly overhead

9. Average Solar Radiation • The average incoming solar radiation is not 1366 W/m2 though • It’s only 342 W/m2 (exactly ¼ of this). Why? Half the planet is dark at all times… Here it’s nighttime High latitudes get less direct radiation, which spreads out more

10. 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)

11. When Solar Radiation Hits the Atmosphere • Average incoming solar radiation = 342 W/m2 • 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?

12. 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)

13. Albedo Values for Earth • Clouds, ice, and snow have high albedo • Cloudalbedo 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

14. 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)

15. Total Solar Input • Total absorbed solar radiation is 70% of the incoming solar radiation • Because 30% is reflected away • 70% of 341 W/m2 = 240 W/m2

16. 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 pretty much transparent when it comes 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

17. “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 

18. “Longwave Radiation” • Everything actually emits radiation • Depends partly on the substance but mostly on temperature Neck = hotter Hair = colder Infrared thermometer

19. Longwave Radiation • Higher temperature means more radiation Eyes and inner ears are warmest: they radiate the most Nose is the coldest: it radiates less A WARM CAT…. IZ A HAPPY CAT Thermal night vision technology detects longwave radiation

20. Longwave radiation From my cat

21. Temperature & Radiation • Higher temperature = more radiation and more energetic radiation (shorter wavelengths) • Explains the Sun’s radiation too • Sun is really hot  • It emits much more radiation • It emits shortwave radiation instead of longwave radiation like the Earth

22. 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)

23. “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: • When energy in equals energy out, we call this “energy balance” Energy in Energy out

24. 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: • Global warming upsets the energy balance of the planet Shortwave in Longwave out

25. Energy Balance with No Atmosphere • If there was no atmosphere, for energy balance to occur, the mean temperature of Earth would be 0o F (-18o C) • Missing piece: the greenhouse effect • All longwave radiation doesn’t escape directly to space -18o C (0o F)

26. The Greenhouse Effect • Greenhouse gases block longwaveradiation 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 15o C (59o F)

27. 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  less emission) 15o C (59o F)

28. 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

29. 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