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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
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 1026 WATTS.
HOWEVER, THE SUN IS 1.5 X 108 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.
1368 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 -19o C.
WITH OUR ATMOSPHERE, THE EFFECTIVE RADIATING
TEMPERATURE IS 15oC.
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 30o 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:
NITROGEN
OXYGEN
ARGON
CO2
METHANE
N2O
OZONE
78%
21%
0.93%
390 ppm
1.8 ppm
314 ppb
10-100 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 30o C (55o F)
COLDER.
DURING THE LAST ICE AGE, THE GMST AVERAGED 6 – 9o 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 CO2 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 CO2 TO ABSORB ENERGY AS 1. THEN, WE RATE
THE OTHER GREENHOUSE GASES RELATIVE TO IT.
SOME HALOCARBONS HAVE VALUES OF AT LEAST 1000.
METHANE HAS A VALUE OF 21.
CO2 IS AN IMPORTNT GREENHOUSE GASE BECAUSE WE
PRODUCE SO MUCH OF IT.
ALSO, CO2, METHANE, AND N2O HAVE NATURAL SINKS.
HOWEVER, CO2 HAS A VERY LONG LIFETIME IN THE
ATMOSPHERE – HUNDREDS OF YEARS.
METHANE AND N2O 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 18TH 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, N2O – 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.