Earth and sky, woods and fields, lakes and rivers, the mountain and the sea, are excellent schoolmasters, and teach some of us more than we can ever learn from books. John Lubbock John Lubbock

WHAT DETERMINES THE EARTH’S GLOBAL MEAN SURFACE TEMPERATURE (GMST)? AND, HOW DOES THE COMPOSITION OF THE ATMOSPHERE COME INTO THAT EQUATION?

THE ONLY ENERGY THAT EARTH RECEIVES, OTHER THAN A SMALL AMOUNT OF INTERNAL ENERGY GENERATED BY INTERNAL FISSION REACTIONS, IS ELECTROMAGNETIC RADIATION FROM THE SUN.

ELECTROMAGNETIC RADIATION IS THE ONLY FORM OF ENERGY THAT CAN TRAVEL THROUGH THE VACUUM OF SPACE. A STABLE TEMPERATURE IS ACHIEVED WHEN THERE IS A BALANCE BETWEEN THE RATE OF ENERGY RECEIVED FROM THE SUN AND THE RATE AT WHICH IT IS RETURNED TO SPACE. IF THERE IS NO BALANCE, THE EARTH WILL EITHER COOL DOWN OR HEAT UP UNTIL A BALANCE IS REACHED.

THE SUN IS EMITTING ENERGY AT THE RATE OF 3.85 X WATTS. HOWEVER, THE SUN IS 1.5 X 10 8 KM FROM EARTH. SO, THE EARTH ONLY RECEIVES A SMALL FRACTION OF THE ENERGY THE SUN EMITS. IF WE IMAGINE A DISK WITH A DIAMETER EQUAL TO THE EARTH’S DIAMETER AT RIGHT ANGLES TO THE SUN’S RAYS JUST OUTSIDE THE EARTH’S ATMOSPHERE, WE CAN CALCULATE THE ENERGY INPUT PER UNIT AREA WATTS M -2.

HOWEVER, THE EARTH IS A ROTATING SPHERE, AND NOT A FLAT DISK, SO IF WE ALLOW FOR THIS, THE RADIATION AVERAGE (SOLAR CONSTANT) IS 342 WATTS M -2. FOR OUR DISCUSSION PURPOSES, WE WILL REFER TO THIS AS 100 ENERGY UNITS. 342 WATTS M -2 = 100 ENERGY UNITS ALBEDO = PROPORTION OF INCIDENT SOLAR RADIATION THAT IS REFLECTED BACK INTO SPACE CLOUDS AND ICE HAVE A HIGH ALBEDO = 90% OCEANS HAVE A LOW ALBEDO = 5% LAND SURFACES VARY FROM FORESTS AT 10-20% TO GRASSLANDS AND DESERTS AT 35% AVERAGE EARTH ALBEDO = 31%

THIS MEANS THAT 31% OF THE INCOMING SOLAR RADIATION IS REFLECTED BACK INTO SPACE. THE REMAINDER (69%) IS ABSORBED BY THE ATMOSPHERE AND EARTH’S SURFACE. 100% 31% 69% ABSORBED BY SURFACE WITH NO ATMOSPHERE, THIS RADIATION WOULD BE ABSORBED BY THE SURFACE AND REEMITTED AS LONGER WAVELENGTH RADIATION.

THE EFFECTIVE RADIATING TEMPERATURE AT EQUILIBRIUM WITH NO ATMOSPHERE WOULD BE -19 o C. WITH OUR ATMOSPHERE, THE EFFECTIVE RADIATING TEMPERATURE IS 15 o C. THE ATMOSPHERE DEFINITELY PLAYS A PART IN HOW MUCH HEAT ENERGY THE EARTH KEEPS OR LOSES. THIS IS CALLED THE GREENHOUSE EFFECT. SO, WITH THE GREENHOUSE EFFECT, EARTH IS OVER 30 o C WARMER THAN IT WOULD BE WITHOUT IT. SOME OF THE GASES IN THE ATMOSPHERE CAN ABSORB INFRARED RADIATION. THEY ARE CALLED GREENHOUSE GASES.

THE PRIMARY GREENHOUSE GASES ARE: WATER VAPOR CARBON DIOXIDE METHANE NITROUS OXIDE OZONE THESE MOLECULES CAN ABSORB OR REEMIT RADIATION. SOME OF THIS RADIATION IS EMITTED BACK TO EARTH AND ABSORBED BY THE SURFACE. THE CONTRIBUTION A GAS MAKES TO THE GREENHOUSE EFFECT DEPENDS ON ITS CONCENTRATION AND ITS ABILITY TO ABSORB RADIATION.

ATMOSPHERIC CONCENTRATIIONS ARE: NITROGEN78% OXYGEN21% ARGON0.93% CO ppm METHANE1.8 ppm N 2 O314 ppb OZONE ppb WATER VAPOR VARIES WITH TIME AND PLACE, BUT AVERAGED OVER BOTH, THE WATER VAPOR REPRESENTS 0.5% OF THE ATMOSPHERIC GASES. WATER VAPOR IS THE SINGLE MOST IMPORTANT NATURAL GREENHOUSE GAS. IT CONTRIBUTES ABOUT 60% TO THE SURFACE WARMING. THE CARBON DIOXIDE CONTRIBUTES ABOUT 25%.

THE NATURAL GREENHOUSE EFFECT IS A GOOD THING. IMAGINE EARTH IF THE GMST AVERAGED OVER 30 o C (55 o F) COLDER. DURING THE LAST ICE AGE, THE GMST AVERAGED 6 – 9 o C COLDER. IT IS THE ADDITIONAL WARMING OR THE ENHANCED GREENHOUSE EFFECT THAT IS A CONCERN.

IN THE GREENHOUSE EFFECT THE EARTH’S SURFACE ABSORBS VISIBLE AND UV RADIATION AND REEMITS IT AS INFRARED RADIATION. THE GREENHOUSE GASES CAN THEN ABSORB THIS IR RADIATION. AS YOU GO UP IN ALTITUDE, THE AIR GETS THINNER (FEWER MOLECULES PER UNIT VOLUME). 80% OF THE TOTAL MASS IS BELOW 10 KM. 99.9% IS BELOW 50 KM. AS THESE MOLECULES ARE MORE ABUNDANT AT LOWER ALTITUDES, IT MAKES SENSE THAT THE ATMOSPHERE SHOULD BE WARMER AT GROUND LEVEL.

IN THE STRATOSPHERE, IT MAKES SENSE THAT THE AIR SHOULD BE WARMER AT HIGHER ALTITUDES, AS IT IS THE INCOMING UV RADIATION THAT IS BEING ABSORBED BY THE OZONE THAT RESULTS IN WARMING.

THE CONCENTRATION OF CO 2 WAS RELATIVELY CONSTANT FOR HUNDREDS OF YEARS. AT AROUND 1750 AT THE START OF THE INDUSTRIAL REVOLUTION, IT STARTED TO INCREASE. IN A LITTLE OVER 200 YEARS, FROM 1750 TO AROUND 1975, IT INCREASED FROM 280 ppm TO 330 ppm. IN JUST 30 YEARS, IT HAS INCREASED AN ADDITIONAL 50 ppm.

WHEN COMPARING GREENHOUSE GASES, WE RATE THE ABILITY OF CO 2 TO ABSORB ENERGY AS 1. THEN, WE RATE THE OTHER GREENHOUSE GASES RELATIVE TO IT. SOME HALOCARBONS HAVE VALUES OF AT LEAST METHANE HAS A VALUE OF 21. CO 2 IS AN IMPORTNT GREENHOUSE GASE BECAUSE WE PRODUCE SO MUCH OF IT. ALSO, CO 2, METHANE, AND N 2 O HAVE NATURAL SINKS. HOWEVER, CO 2 HAS A VERY LONG LIFETIME IN THE ATMOSPHERE – HUNDREDS OF YEARS. METHANE AND N 2 O HAVE SHORT LIFETIMES IN THE ATMOSPHERE.

PRIOR TO THE INDUSTRIAL REVOLUTIION, THE CONCENTRATIONS OF THESE GASES IN THE ATMOSPHERE WERE RELATIVELY BALANCED. THE SOURCES AND SINKS WERE RELATIVELY BALANCED. POPULATION TELLS PART OF THE STORY. AT THE END OF THE 18 TH CENTURY, THERE WERE LESS THAT 1 BILLION PEOPLE ON EARTH. NOW THERE ARE OVER 7 BILLION. BEST ESTIMATES PUT THE POPULATION AT 9 BILLION BY 2050.

WHERE DO THESE GREENHOUSE GASES COME FROM? CARBON DIOXIDE COMES FROM BURNING FOSSIL FUELS. METHANE – 30% COMES FROM ANAEROBIC BREAKDOWN OF ORGANIC MATTER (SWAMPS AND WETLANDS). RICE PADDIES CONTRIBUTE 11%. LIVESTOCK FLATULENCE CONTRIBUTES 16%. LANDFILLS CONTRIBUTE 17%. NATURAL GAS PRODUCTION AND COAL MINES PRODUCE 19%. NITROUS OXIDE, N 2 O – AGRICULTURAL ACTIVITIES AND BURNING FOSSIL FUELS. OZONE – GROUND LEVEL AMOUNTS ARE PRODUCED BY PHOTOCHEMICAL SMOG.

CONTRIUBTIONS OF GREENHOUSE GASES TO INCREASE IN GMST: CARBON DIOXIDE – 53% METHANE – 17% NITROUS OXIDE – 5% HALOCARBONS – 12% TROPOSPHERIC OZONE – 13% WE TALK A LOT ABOUT CARBON DIOXIDE, BUT IT IS IMPORTANT NOT TO FORGET ABOUT THE OTHER GASES. WE HAVEN’T MENTIONED WATER VAPOR. HUMANS DON’T DIRECTLY IMPACT WATER VAPOR CONTENT, BUT HUMANS DO INDIRECTLY INFLUENCE IT. THE AMOUNT DEPENDS ON TEMPERATURE.