Weather Banter - View Single Post - Stephen Hawking and Carl Sagan on the Greenhouse Effect

Giga2

On 13 Aug, 04:31, "
wrote:
On Aug 11, 8:26*pm, Roger Coppock wrote:

On Aug 11, 8:17*pm, Roger Coppock wrote:

Please see:

http://www.youtube.com/watch?v=mu1PicT0TMU

Below are lecture notes from a college course:

The greenhouse effect of Venus

--- Venus' perpendicular solar flux is ~2600 W/m2

From geometry, we can calculate the average solar flux over the
surface of Venus. It is approximately 661 W/m2.
Venus is very reflective of sunshine. In fact, it has a reflectivity
(or albedo) of 0.8, so the planet absorbs approximately 661 X 0.2 =
132 W/m2.
By assuming that the incoming radiation equals the outgoing radiation
(energy balance), we can convert this into an effective radiating
temperature by invoking the Stefan-Boltzmann law (total energy = _
T4). We find that T=220K.
But Venus’ surface has a temperature of 730K!!!
The explanation for this huge discrepancy is the planet’s greenhouse
effect.

The greenhouse effect of Earth

--- Earth's perpendicular solar flux is ~1350 W/m2

From geometry, we can calculate the average solar flux over the
surface of Earth. It is approximately 343 W/m2.
The earth has a much lower albedo than Venus (0.3), so the planet
absorbs approximately 343 X 0.7 = 240 W/m2.
By assuming that the incoming radiation equals the outgoing radiation,
we can convert this into an effective radiating temperature by
invoking the Stefan-Boltzmann law (total energy = _ T4). We find that
T=255K.
Earth’s surface has a temperature of 288K
While much smaller than Venus’ greenhouse effect, earth’s is crucial
for the planet’s habitability. Without the greenhouse effect, the
temperature today in Los Angeles would be about 0 degrees Fahrenheit.

* Venus' surface atmospheric pressure is about 90 times that of
earth. *Venus's *surface gravity is about 10% less than that of earth
so Venus's atmosphere, just about all CO2,, is about 100 times as
dense as earth's.

CO2 makes up not quite 0.04% of earth's atmosphere, so Venus has about
100/0.0004 = 250,000 times as much CO2 as earth.

Oh wow, we better stop the economy now just in case! Lol.

From

http://en.wikipedia.org/wiki/Idealized_greenhouse_model

and

http://www.geo.utexas.edu/courses/38...ures/chap2.pdf

that 250,000 times as much CO2 on Venus results in a magnification of
warming from

132 *Watts /m^ 2 to *about 16,127 watts/M^2 or an effective 16,127/132
= 122 atmospheres.

122 venus atmospheres *1earth atmosphere/250,000 venus atmospheres =
0.000488 atmospheres warming, or about *0.18 watts.

Comparing the global warming caused by Venus' atmosphere to global
warming caused by Earth's CO2 doesn't cut the mustard. *Obviously
many bands are saturated, making increases in CO2 much less effective
than a linear ratio would imply.

Incidentally the general consensus is that , a doubling of CO2 would
increase the flux by about 3.7 watts.

From Trenbeth's figures

http://www.cgd.ucar.edu/cas/Trenbert...TFK_bams09.pdf

the flux at earth's surface is about 492 watts, giving us a
temperature of about 288 K.

Plugging in that 3.7 watt increase, and remembering that temperature
is roughly proportional * *to the 4th root of the wattage flux,

(493.7/490)^0.25 = * 1.00188

for a temperature increase of 288*0.00188 = 0.54 K

- A. McIntire