DARGAN M. W. FRIERSON DEPARTMENT OF ATMOSPHERIC SCIENCES DAY 1: MARCH 30, 2010 ATM S 111, Global Warming: Understanding the Forecast

Reading Assignments From last time:  Make sure you’ve read Rough Guide p “Climate Change: A Primer”  The Big Picture  The Outlook  What Can We Do About It Next reading assignment:  Rough Guide p “The Greenhouse Effect”  If it’s about hotels in Melbourne you might have bought the wrong Rough Guide

Outline of This Lecture 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 What are the main greenhouse gases?  And which are changed by human activity?

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!

We Are Small! Fun facts (i.e., won’t be on the HW/tests): Sun has approximately: 100 times the radius of the Earth 1 million times the volume of Earth 300,000 times the mass of Earth The Sun is 10 light minutes away from Earth (100 Sun diameters away)

Fun Facts: Other Heat Sources? The Sun is by far the main heat source for the atmosphere/ocean system What are other ways the atmosphere and ocean can be heated?  Heat from volcanoes or the solid Earth: 0.025% of the Sun’s heating  Direct heating of the atmosphere/ocean from burning of fossil fuels or nuclear power: 0.007% of the Sun  This is not the greenhouse effect  Tides: 0.002% of the Sun  Caused by the moon, lead to motions which eventually turn to heat from friction So the Sun contributes over 99.97% of the energy input!

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 are 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

Types of Radiation Types of electromagnetic radiation, from most powerful 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… Here it’s nighttime High latitudes get less direct radiation, which spreads out more Reason for seasons: Winter is tilted away from the Sun, gets less direct light, and thus is colder

Next: When Solar Radiation Hits the Atmosphere So, the average incoming solar radiation is 342 W/m 2 What happens when this encounters the atmosphere? Only about 20% gets absorbed within the atmosphere  This includes absorption of dangerous UV by the ozone layer 50% is absorbed right at the surface  Meaning much of the sunlight makes it directly through the atmosphere! 30% is reflected away, back into 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 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

“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 

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

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

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: When energy in equals energy out, we call this “energy balance” Energy inEnergy out

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 inLongwave out

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) Missing piece: the greenhouse effect  All longwave radiation doesn’t escape directly to space -18 o C (0 o F)

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  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

What are the Major Greenhouse Gases? Our atmosphere is mostly nitrogen (N 2, 78%), oxygen (O 2, 21%), and argon (Ar, 0.9%)  But these are not greenhouse gases Molecules with 1 atom or 2 of the same atoms aren’t greenhouse gases though  Just like the atmosphere is almost transparent to solar radiation, the primary gases in our atmosphere are transparent to longwave radiation If our atmosphere was only nitrogen, oxygen, and argon, this picture with no greenhouse effect would be accurate!

Greenhouse Gases Polyatomic molecules are greenhouse gases  Water vapor (H 2 O)  Carbon dioxide (CO 2 )  Methane (CH 4 )  Nitrous oxide (N 2 O)  Ozone (O 3 )  Chlorofluorocarbons (the ozone depleting chemicals which have been banned) The fact that they can rotate and vibrate means they can absorb the right frequencies of longwave

Water Vapor Gas form of water  AKA humidity The number one greenhouse gas!  Powerful because there’s a lot of it Not controlled by humans!  It’s a feedback not a forcing (topic of the next lecture)  Observed to be increasing with global warming

Carbon Dioxide CO 2  It’s what we breathe out, what plants breathe in The primary contributor to the anthropogenic (human-caused) greenhouse effect  63% of the anthropogenic greenhouse effect so far Increases primarily due to fossil fuel burning (80%) and biomass burning (e.g., forest fires; 20%)  Preindustrial value: 280 ppm  Current value: 386 ppm

Carbon Dioxide CO 2 will also be the main problem in the future It’s extremely long-lived in the atmosphere  50% of what we emit quickly gets taken up by the ocean or land  We’ll discuss this more later  Most of the rest sticks around for over 100 years  Some of what we emit will still be in the atmosphere over 1000 years from now!

Methane CH 4  Natural gas like in stoves/heating systems Much more potent on a per molecule basis than CO 2  Only 1.7 ppm though – much smaller concentration than CO 2 Natural sources from marshes (swamp gas) and other wetlands Increases anthropogenically due to farm animals (cow burps), landfills, natural gas leakage, rice farming

Methane The lifetime of CH 4 is significantly shorter than carbon dioxide  Breaks down in the atmosphere in chemical reactions  Lifetime of methane is only 8 years Methane concentrations have been leveling off in recent years, possibly due to drought in wetlands at high latitudes

Global Warming Potential CO 2 lifetime > 100 years Methane lifetime = 8 years  But methane is a much stronger greenhouse gas How to put these on similar terms? Global warming potential (or GWP)  Global warming potential is how much greenhouse effect emissions of a given gas causes over a fixed amount of time (usually 100 years)  Measured relative to CO 2 (so CO 2 = 1)  Methane’s global warming potential is 25  Much more potent than CO 2 even though it doesn’t stay as long

Nitrous Oxide N 2 O  Laughing gas Also more potent on a per molecule basis than CO 2  Global warming potential: 310 Comes from agriculture, chemical industry, deforestation Small concentrations of only 0.3 ppm

Ozone Ozone or O 3 occurs in two places in the atmosphere  In the ozone layer very high up  This is “good ozone” which protects us from ultraviolet radiation & skin cancer  Remember ozone depletion is not global warming!  Near the surface where it is caused by air pollution: “bad ozone” Bad ozone is a greenhouse gas, and is more potent on a per molecule basis than CO 2  But very very short-lived  Fun fact: Global warming potential for ozone is not usually calculated – rather it’s wrapped into the GWPs of the other gases that lead to its chemical creation

CFCs CFCs or chlorofluorocarbons are the ozone depleting chemicals  Have been almost entirely phased out CFCs are strong greenhouse gases  Their reduction likely saved significant global warming in addition to the ozone layer! Some replacements for CFCs (called HFCs) are strong greenhouse gases though Global warming potentials of up to 11,000!

The Natural Greenhouse Effect Contributions to the natural greenhouse effect:  H 2 O (water vapor): 60%  CO 2 (carbon dioxide): 26%  All others: 14% These numbers are computed with a very accurate radiation model  First running with all substances, then removing each individual gas

The Unnatural Greenhouse Effect Increasing levels of CO 2 and other greenhouse gases leads to a stronger greenhouse effect  With more greenhouse gases, it becomes harder for outgoing radiation to escape to space It’s like this picture from before, but more. More radiation trapped before it gets out to space. Longwave radiation is emitted from a higher (and colder) level on average.

The Unnatural Greenhouse Effect Contributors to the “anthropogenic” greenhouse effect  Numbers for the whole world up to this point:  Carbon dioxide: 63%  Methane: 18%  CFCs, HFCs: 12%  Nitrous oxide: 6%

The Anthropogenic Greenhouse Effect Contributors to the “anthropogenic” greenhouse effect  Numbers for the US based on current (2008) emissions CO 2 is the big problem in the US currently. Note how much lower the HFCs are than on the previous slide. This is b/c we basically don’t emit CFCs any more. From US EPA 2010 report (draft)

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 Greenhouse gases:  Number one is water vapor  Number two is CO 2  Global warming potential: key concept

Survey Question about Types of Radiation Which of the following are types of electromagnetic radiation?  A. X-rays  B. Sonar  C. Radio waves  D. A + B  E. A + C  F. B + C  G. All