Re: Global warming deniers
- From: "Tracy P. Hamilton" <t_p_hamilton@xxxxxxxxx>
- Date: 4 May 2006 07:21:20 -0700
Robert Grumbine wrote:
In article <1146694711.026435.156860@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Puppet_Sock <puppet_sock@xxxxxxxxxxx> wrote:
Tracy P. Hamilton wrote:
[snip]
The graph in http://www.giss.nasa.gov/data/simodel/solar.irradiance/
has oscillations of about 0.15 from peak to trough in the
instantaneous forcing. Changes in the solar irradiance
(or any other effect) are not simply taken as the forcing.
Well, that was sort of my question.
The graph of solar irradiance in that page has oscillations
of about 1W/m^2. The forcing function is taken to be
about 1/6 or less of the change. Why? Something is
going on here.
Indeed. Two things, both of which are taught in elementary
meteorology courses:
1) The earth is a sphere, not a disk.
2) The earth reflects some of the incoming solar radiation
The 1 W/m^2 is the observed change in the solar output over a
solar cycle -- as received by a plane perpendicular to the sun's
rays, at a distance of 1 Astronomical Unit (AU).
If the earth were a disk squarely facing the sun, and we cared
only about the sunlit side, the area intercepting solar radiation
would be pi * r^2. But the earth is, in fact, a sphere(-oid), whose
area is 4*pi*r^2. A change to the disk of 1 W/m^2 is equivalent
to a change of 0.25 W/m^2 averaged over the surface of the sphere.
I just thought of a cool problem for Pratchett Discworld fans.
Compute the W/m^2 of a disk with the sun going around it,
which has an output of 345 W/m^2.
The earth's albedo (portion of solar radiation reflected back
out of the system) is about 0.3. 30% of those 0.25 W/m^2 are
immediately bounced out and never have a chance to affect earth's
climate.
That leaves about 0.17 W/m^2 to participate in earth's climate,
or about 1/6th of the solar variation -- as measured by solar people
above the atmosphere on planes perpendicular to the sun and referenced
to 1 AU.
Doubled CO2 is estimated to lead to about 4 W/m^2 -- over the sphere,
with no albedo to diminish the response. So, roundly, 25 times the solar
term. Hence climate folks, as a group, are more interested in greenhouse
gases than solar variation. Some find the solar variation more interesting,
due to interest in paleoclimates, so work does continue on solar forcings.
Plus we aren't yet to that doubled CO2, so the solar is still some 10-30%
of the forcing, last I heard, and needs to be (and is) considered when
trying to understand details of the last century's temperature changes.
Tracy P. Hamilton
.
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