Re: Democratic candidates address the issue of global warming while Republicans snooze...

Rumpelstiltskin wrote:
On Fri, 4 Jan 2008 09:00:29 -1000, "Alvin E. Toda" <aet@xxxxxxxx>
wrote:

On Fri, 4 Jan 2008, Rumpelstiltskin wrote:

On Fri, 04 Jan 2008 01:18:27 GMT, Rumpelstiltskin
<PleaseDoNotReplyByEmail@xxxxxxxxxxx> wrote:

On Fri, 04 Jan 2008 01:07:27 GMT, Rumpelstiltskin
<PleaseDoNotReplyByEmail@xxxxxxxxxxx> wrote:

<snip>


I agree the effect is not very significant. The
eccentricity of Earth's orbit is one part in 60, so
the difference in luminosity from the sun between
apogee and perigee is 1.016 squared or about
1.032, three percent. More important, I'd say,
though still not very important, is the fact that the
Earth is moving more slowly at aphelion which
occurs in early July, when the Southern
hemisphere is in its winter, so the Southern
hemisphere winters are slightly longer than the
Northern, I guess by the same three percent
since that ratio is also a square. See the cute
moving illustration at:
http://tinyurl.com/2fzt4y


I think I screwed up the calculation. The article
at the URL above says the earth is 3.2% farther
from the sun at aphelion and at perihelion. Maybe
the eccentricity works on both sides, so that the
earth is BOTH 1/60 closer than a circle to the sun
at perihelion AND 1/60 further away than a circle
at aphelion. In that case, the difference in
luminosity would be 6% instead of 3%, and also
the difference in the length of winter would be 6%.
That's a bit more significant.


That is indeed the case. See the geometry
of ellipses, noting the position of a focus with
respect to the closest and furthest distances
to the path of the elliptical orbit, at:
http://tinyurl.com/357se4
You do have to both add the distance determined
by the eccentricity number from the perihelion
distance and also add it to the aphelion distance.
(The sun is at one focus of a planet's elliptical orbit.
Take your pick which focus, it doesn't matter.)
Here you assume that the difference in the orbit is significant-- ie more than the effect of changes in the earth's tilt.


I was writing only of year-to-year differences
between the Northern and Southern hemisphere, which was a based on Islander's post which was
Changes in the earth's tilt are almost surely more
significant, but they take place over ages, not within a year.




Problem with this reasoning is that no one has noticed a change in the average over a year of the solar constant. Ie the average energy from the sun remains the same from year to year. Picking the average out of a signal with so much noise is hard but IIRC over a century of efforts and with recent independent satellite data finally coming into agreement, I would think that this issue is closed.


The gross solar constant likewise changes over ages, not in the short term.


Even with the tilt change, I can see that a new section at the poles become cooler, but that just means that the previous cool spot before the change becomes warmer. The exchange of total energy still remains the same on the average. This is why I think that change which is observed from changes in tilt must be secondary or of the order (1/60)**2. With so small a result, it would be prudent to wait and see if someone else can independently come up with the same number.


OK, I think that the two of you are talking about different things. My response to Jean had to do only with the change in the tilt of the earth's axis, not the difference in the distance of the earth from the sun.

The change in the tilt is a maximum of only 3 degrees and that occurs over a period of about 41K years. The specific cycles that Jean referred to happen over shorter cycles, but are still pretty long.

The change of distance between the earth and the sun is only about 4 million miles (95M at aphelion and 91M at perihelion. But, don't forget that the intensity of the sun's rays varies as a cube law (space is 3D). So, the difference in intensity between perihelion and aphelion as a percentage is (95M**3 - 91M**3)/95M**3 = 13%

So, summers in the Northern hemisphere will be at most 13% cooler than summers in the Southern hemisphere, just due to the elliptical orbit of the earth, if all other things are equal. Conversely, winters in the Northern hemisphere will be on average 13% warmer than winters in the southern hemisphere. Note, for example, that the south pole has colder temperatures than the north pole.

But, all things are not equal. There is mixing of the atmosphere between the northern and southern hemispheres which tends to dampen the temperature extremes. Secondly, the south pole, largely over land, is also larger in area than the north pole. This larger snow and ice mass reflects the suns rays to a larger extent than at the north pole. Thirdly, since the north pole is over sea water, there is thermal mixing occurring below the pole, increasing it's temperature. Finally, there are more land masses in the northern hemisphere resulting in more dynamic meteorology and more melting at the north pole than at the south pole.

Now, add to this the change in the tilt of the earth's axis over time and you can get some appreciation for how complex the models for global warming are. You can also get some appreciation for how the scientifically naive might be "convinced" that any one of these effects might refute claims about increasing global warming.

There is nothing that explains the rapid increase in global temperature other than the increase in CO2 due to man's burning of fossil fuels.
.

Relevant Pages

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