02 January 2009

Happy Perihelion

Not quite yet, but as everyone else is wishing happy New Years that seems covered. On the 3rd (usually at least) of January we are closest to the sun. That the calendar year starts near then is coincidence. Due to orbital variations (Milankovitch cycles), our date of closest approach varies through the calendar year. This is the longitude of perihelion.

Back with the simplest climate model posts we looked some at how our distance to the sun could affect the global mean temperature. In honor of our passage, a project to see how much difference our being 0.0167 astronomical units closer to the sun than normal (i.e., we'll be at 0.9833 AU instead of the average 1.0000) would make. At least, would make if the assumptions of the simplest model were all true.


Anonymous said...

Great Blog and topic.

Only conicidentally, I've started a blog of my own on Earth Climate a did a bit on this subject. Albedo and emissivity should be included to get results resembling actual surface temperatures.


Robert Grumbine said...

Albedo is indeed included.

The emissivity you refer to is something I intentionally did not use -- it is not an observed quantity. In the simplest meaningful model, everything is observable -- earth's albedo, solar constant, and earth's blackbody temperature. It matches up well given any two of the three observables.

The emissivity you refer to is a kludge* to cover the fact that what we care about (more) is the earth's surface temperature, rather than blackbody temperature. The only way to determine that figure is to first observe everything else, and then say that the emissivity is whatever is needed to make the equation balance. This doesn't tell us how the emissivity comes about. It does give the right sense of things -- adding greenhouse gases to the atmosphere decreases the surface emissivity, which leads to warming. So it's ok in that sense.

Good luck with the blog. The more of us trying to explain things, each in our different way, the better chance that one of us will do it in a way useful for a given reader.

*kludge: Used by scientists and engineers. It means a fix that works, but is not pretty and may not be working for the right reason. As we're not all scientists and engineers, it occurs to me that a kind of vocabulary guide would be a good idea here. More later.