Greenhouse Effect
The greenhouse effect, first discovered by Joseph Fourier in 1824, and first investigated quantitatively by Svante Arrhenius in 1896, is the process by which an atmosphere warms a planet.
Mars, Venus and other celestial bodies with atmospheres (such as Titan) have greenhouse effects, but for simplicity this article mostly refers to the case of Earth.
In common parlance, the term greenhouse effect may be used to refer either to the natural greenhouse effect, which is the greenhouse effect which occurs naturally on Earth, or to the enhanced (anthropogenic) greenhouse effect, which results from gases emitted as a result of human activities (see also global warming). No-one disputes the former, or its magnitude; the latter is accepted by a large majority of scientists, although there is some dispute as to its magnitude (see scientific opinion on climate change and attribution of recent climate change).
The Natural Greenhouse Effect
T h e e a r t h r e c e i v e s a n e n o r m o u s a m o u n t o f s o l a r r a d i a t i o n . J u s t a b o v e t h e a t m o s p h e r e , t h e s o l a r p o w e r f l u x d e n s i t y a v e r a g e s a b o u t 1 3 6 6 w a t t s p e r s q u a r e m e t e r , o r 1 . 7 4 0 × 1 0 1 7 W o v e r t h e e n t i r e E a r t h . T h i s f i g u r e g r e a t l y e x c e e d s t h e p o w e r g e n e r a t e d b y h u m a n a c t i v i t i e s . T h e d i f f e r e n c e b e t w e e n t h e n a t u r a l g r e e n h o u s e e f f e c t a n d g l o b a l w a r m i n g i s t h a t - g l o b a l w a r m i n g i s a n t h r o p o g e n i c w h e r e a s g r e e n h o u s e e f f e c t i s n o t .
T h e s o l a r p o w e r h i t t i n g E a r t h i s b a l a n c e d o v e r t i m e b y a n e q u a l a m o u n t o f p o w e r r a d i a t i n g f r o m t h e E a r t h ( a s t h e a m o u n t o f e n e r g y f r o m t h e S u n t h a t i s s t o r e d i s s m a l l ) . A l m o s t a l l r a d i a t i o n l e a v i n g t h e E a r t h t a k e s t w o f o r m s : r e f l e c t e d s o l a r r a d i a t i o n a n d t h e r m a l b l a c k b o d y r a d i a t i o n .
Reflected solar radiation accounts for 30% of the Earth's total radiation: on average, 6% of the incoming solar radiation is reflected by the atmosphere, 20% is reflected by clouds, and 4% is reflected by the surface.
The remaining 70% of the incoming solar radiation is absorbed: 16% by the atmosphere (including the almost complete absorption of shortwave ultraviolet over most areas by the stratospheric ozone layer); 3% by clouds; and 51% by the land and oceans.
This absorbed energy heats the atmosphere, oceans, and land and powers life on the planet. It should be noted that the surface of the Earth is in constant flux with daily, yearly and age long cycles and trends in temperature and other variables for a variety of causes; thus these percentages apply on average only.
Like the Sun, the Earth is a thermal radiator. Because the Earth's surface is much cooler than the Sun (287 K vs 5780 K), Wien's displacement law dictates that Earth radiates its thermal energy at longer wavelengths than the Sun. While the Sun's radiation peaks at a visible wavelength of 500 nanometers, Earth's radiation peak is in the longwave (far) infrared at about 10 micrometres.
The Earth's atmosphere is largely transparent at visible and near-infrared wavelengths, but not at 10 micrometres (this is, probably, not entirely coincidental: the transparency to "visible" wavelengths makes eyes adapted to seeing these wavelengths useful; and eye that could see in a strongly-absorbed wavelength would not be so useful).
Only about 6% of the Earth's total radiation to space is direct thermal radiation from the surface. The atmosphere absorbs 71% of the surface thermal radiation before it can escape. The atmosphere itself behaves as a radiator in the far infrared, so it re-radiates this energy.
The Earth's atmosphere and clouds therefore account for 91.4% of its longwave infrared radiation and 64% of Earth's total emissions at all wavelengths. The atmosphere and clouds get this energy from the solar energy they directly absorb; thermal radiation from the surface; and from heat brought up by convection and the condensation of water vapor.
Because the atmosphere is such a good absorber of longwave infrared, it effectively forms a one-way blanket over Earth's surface. Visible and near-visible radiation from the Sun easily gets through, but thermal radiation from the surface can't easily get back out. In response, Earth's surface warms up.
The power of the surface radiation increases by the Stefan-Boltzmann law until it (over time) compensates for the atmospheric absorption. Another, simpler, but essentially equivalent way of looking at this is that the surface is heated by two sources: direct solar radiation, and thermal radiation from the atmosphere; it is thus warmer than if heated by solar radiation alone.
The result of the greenhouse effect is that average surface temperatures are considerably higher than they would otherwise be if the Earth's surface temperature were determined solely by the albedo and blackbody properties of the surface.
It is commonplace for simplistic descriptions of the "greenhouse" effect to assert that the same mechanism warms greenhouses, but this is an incorrect oversimplification.
The above description (and many other simplified expositions of the greenhouse effect) may give the impression that radiation is the most important method for transmitting heat through the atmosphere.
In the lower atmosphere, particularly in the tropics, convection and latent heat transport is very important in moving heat vertically upwards from the surface; the greenhouse effect dynamics described above do operate, but become important higher in the atmosphere.
L i m i t i n g f a c t o r s
T h e d e g r e e o f t h e g r e e n h o u s e e f f e c t i s d e p e n d e n t p r i m a r i l y o n t h e c o n c e n t r a t i o n o f g r e e n h o u s e g a s e s i n t h e p l a n e t a r y a t m o s p h e r e . T h e d e e p a n d c a r b o n d i o x i d e - r i c h a t m o s p h e r e o f V e n u s ( c o m b i n e d w i t h a n o r b i t c l o s e r t o t h e s u n t h a n t h a t o f E a r t h ) c a u s e s s u r f a c e t e m p e r a t u r e s h o t e n o u g h t o m e l t l e a d , t h e a t m o s p h e r e o f E a r t h c r e a t e s h a b i t a b l e t e m p e r a t u r e s , a n d t h e t h i n a t m o s p h e r e o f M a r s c a u s e s a m i n i m a l g r e e n h o u s e e f f e c t .
A r u n a w a y g r e e n h o u s e e f f e c t o c c u r r e d o n V e n u s b e c a u s e o f a n i n t e r a c t i o n o f t h e g r e e n h o u s e e f f e c t w i t h o t h e r p r o c e s s e s i n f e e d b a c k c y c l e s . V e n u s i s s u f f i c i e n t l y s t r o n g l y h e a t e d b y t h e S u n t h a t w a t e r v a p o u r c a n r i s e m u c h h i g h e r i n t h e a t m o s p h e r e a n d i s s p l i t i n t o h y d r o g e n a n d o x y g e n b y u l t r a v i o l e t l i g h t . T h e h y d r o g e n c a n t h e n e s c a p e f r o m t h e a t m o s p h e r e a n d t h e o x y g e n c o m b i n e s .
T h e r e f o r e l e s s c a r b o n d i o x i d e i s r e a b s o r b e d b y t h e p l a n e t a r y c r u s t c a u s i n g a n e v e n h i g h e r t e m p e r a t u r e . A s a r e s u l t , t h e g r e e n h o u s e e f f e c t w a s p r o g r e s s i v e l y i n t e n s i f i e d b y p o s i t i v e f e e d b a c k .
O n E a r t h t h e r e i s a s u b s t a n t i a l h y d r o s p h e r e a n d b i o s p h e r e w h i c h r e s p o n d s t o h i g h e r t e m p e r a t u r e s b y r e c y c l i n g a t m o s p h e r i c c a r b o n m o r e q u i c k l y ( i n g e o l o g i c t e r m s ; t h e t i m e s c a l e f o r t h e o c e a n / b i o s p h e r e t o r e m o v e a C O 2 p e r t u r b a t i o n i s o n t h e o r d e r o f s e v e r a l h u n d r e d y e a r s ) .
T h e p r e s e n c e o f l i q u i d w a t e r t h u s l i m i t s t h e i n c r e a s e i n t h e g r e e n h o u s e e f f e c t t h r o u g h n e g a t i v e f e e d b a c k . T h i s s t a t e o f a f f a i r s i s e x p e c t e d t o p e r s i s t f o r a t l e a s t h u n d r e d s o f m i l l i o n s o f y e a r s , b u t , u l t i m a t e l y , t h e w a r m i n g o f a n a g i n g S u n w i l l o v e r w h e l m t h i s r e g u l a t o r y e f f e c t .
T h e a v e r a g e s u r f a c e t e m p e r a t u r e w o u l d b e "-1 8 ƒ C i f t h e a t m o s p h e r e p l a y e d n o r o l e . I n r e a l i t y t h i s t e m p e r a t u r e i s c l o s e r t o 1 5 ƒ C a b o v e z e r o d u e t o t h e c o m b i n a t i o n o f t h e g r e e n h o u s e e f f e c t a n d t h e c o n v e c t i v e f l o w o f h e a t e n e r g y w i t h i n t h e a t m o s p h e r e .
B e c a u s e c o n v e c t i o n ( v e r t i c a l e x c h a n g e s o f u n s t a b l y s t r a t i f i e d a i r , p r e d o m i n a n t l y b y s t o r m c l o u d s ) m o v e s h e a t a b o v e m u c h o f t h e t h e r m a l I R a b s o r b a n c e o f t h e a t m o s p h e r e , t h e g r e e n h o u s e e f f e c t o n t h e s u r f a c e i s s m a l l e r t h a n i t w o u l d b e i n t h e a b s e n c e o f s u c h c o n v e c t i o n .
R e c e n t m e a s u r e m e n t s o f c a r b o n d i o x i d e a m o u n t s f r o m M a u n a L o a o b s e r v a t o r y s h o w t h a t C O 2 h a s i n c r e a s e d f r o m a b o u t 3 1 3 p p m ( p a r t s p e r m i l l i o n ) i n 1 9 6 0 t o a b o u t 3 7 5 p p m i n 2 0 0 5 . T h e c u r r e n t o b s e r v e d a m o u n t o f C O 2 e x c e e d s t h e g e o l o g i c a l r e c o r d o f C O 2 m a x i m a ( ~ 3 0 0 p p m ) f r o m i c e c o r e d a t a ( H a n s e n , J . , C l i m a t i c C h a n g e , 6 8 , 2 6 9 , 2 0 0 5 ) .
C O 2 p r o d u c t i o n r a t e f r o m i n c r e a s e d i n d u s t r i a l a c t i v i t y ( f o s s i l f u e l b u r n i n g ) a n d o t h e r h u m a n a c t i v i t i e s s u c h a s l a n d - u s e c h a n g e s h a s o v e r w h e l m e d t h e n o r m a l f e e d b a c k c o n t r o l m e c h a n i s m s . G l o b a l c l i m a t e m o d e l c a l c u l a t i o n s i n d i c a t e t h a t t h e e l e v a t e d C O 2 l e v e l s a r e l i k e l y t o l e a d t o g l o b a l w a r m i n g .
T h e r e h a s b e e n a n o b s e r v e d g l o b a l a v e r a g e t e m p e r a t u r e i n c r e a s e o f a b o u t 0 . 5 o C s i n c e 1 9 6 0 ( S c i e n c e 3 0 8 , 1 4 3 1 , 2 0 0 5 ) . T h e r e i s s t i l l s o m e p u b l i c c o n t r o v e r s y a b o u t t h e r o l e o f h u m a n a c t i v i t i e s a n d t h a t o f C O 2 a n d o t h e r g r e e n h o u s e g a s i n c r e a s e s f o r g l o b a l w a r m i n g .
The greenhouse gases
Water vapor (H2O) causes about 60% of Earth's naturally-occurring greenhouse effect. Other gases influencing the effect include carbon dioxide (CO2) (about 26%), methane (CH4), nitrous oxide (N2O) and ozone (O3) (about 8%). Collectively, these gases are known as greenhouse gases. The greenhouse effect due to carbon dioxide is specifically known as the Callendar effect.
The wavelengths of light that a gas absorbs can be modelled with quantum mechanics based on molecular properties of the different gas molecules. It so happens that heteronuclear diatomic molecules and tri- (and more) atomic gases absorb at infrared wavelengths but homonuclear diatomic molecules do not absorb infrared light. This is why H2O and CO2 are greenhouse gases but the major atmospheric constituents (N2 and O2) are not.
Between the absorptions of water vapor and those of carbon dioxide, there is an atmospheric window where, prior to the industrial era, no infrared radiation was trapped, lying between 8 and 15 micrometres. Compounds such as perflurocarbons (CF4, C2F6 etc.), chlorofluorocarbons, halons and SF6 absorb very strongly in this window. This means that they are extremely potent greenhouse gases, especially given the absence of natural sinks to remove them. Perfluorocarbons can have a lifetime of 50,000 years.
Ancient Fossil Fuels Caused Jurassic Warming Discovery - May 26, 2005
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