2011-01-20 09:54:42Annual solar cycle varies by 96 W.mˆ2???
Rob Honeycutt



Found this one today coming from a retired German weatherman.  It popped up on NoTricksZone.

"[Dr.]Thüne explains how many people do not even understand the solar constant, which is not constant at all. Radiation is not constant at 1368 watts/m²."

"In reality, because of the earth’s varying distance from the sun during its orbit, the ‘solar constant’ is between 1320 and 1416 W/m2. That means that at the start of January the earth receives 96 W/m2 more in solar radiation then it does at the beginning of July.”

 It prompted me to look at the ellipse of the earth's orbit...  It varies between 152 million km to 147 million km, or about 3.2%.  Thune is claiming the solar constant varies by 6.7%, which is obviously stupid.  But I don't understand, and can't find anything explaining, why TSI varies less than 0.07% over the 11 year cycle.  

Why does the earth's annual change in distance from the sun not have a larger effect? 


2011-01-20 10:37:27


The energy from the sun depends on the square of distance, it's correct that the solar constant varies about that amount. What the earth surface absorbs, though, is less; you need to multiply the 96 W/m2 by 0.7 to account for albedo and divide by 4 to average the flux over the surface of the earth. You are left with about 17 W/m2, which is still a lot. But when you have a periodic forcing the effect strongly depends on the thermal inertia (response time) of the system. Even the 11 years solar cycle is too fast to have the full effect. With a difference of about 1 W/m2 from minimum to maximum you should get 0.85 °C of temperature change; in reality it is about 0.1 °C.

2011-01-20 10:50:54
Rob Honeycutt


Thanks Riccardo.  

It's an interesting denier crock that I've never seen before.


2011-01-20 18:14:32
Glenn Tamblyn



When I first read the title of this in the General Chat list I had a true What the F... moment.

Yes, over the course of a year of course the Solar intensity here at the Earth varies - I never new the exact figure. But it all evens out in the wash. And the oft quoted figure of 1366 (give or take) allows for this.

Interesting question/observation. Is this German guy a denialist or simply a pedant (because he is right in principle, although I can't confirm his exact numbers) on his pet subject that then plays into the denialists hands. What was the comment from the war. 'Loose Lips Sink Ships!'

2011-01-20 19:47:03
Paul D


Probably helps to imagine the surface of a sphere. As you increase the radius the surface area increases at a faster rate than the radius.

Electromagnetic radiation expands outwards in a sphere from its source. But as the sphere expands, the amount of radiation at a specific radius doesn't increase in proportion, hence the fixed 'amount' spreads out over the sphere surface, or rather gets weaker.


2011-01-21 01:05:42


The physics of what he's saying, for what it's worth:

Intensity(R) = solar power output / R^2

ln(I) = constant - ln(R^2 ) = constant - 2*ln(R)

dI/I = dln(I) = -2*dln(R) = -2*dR/R


%variation in I is proportional to dI/I = -2 * dR/R = -2 * %variation in R

But the essential point is that this is a seasonal variation, and we automatically average over a year (or 30 years) to suppress seasonal variations. 

So this point is no more of interest than is the fact that there's no sunlight at night. Guess what? We never thought there was!