Path: utzoo!mnetor!uunet!husc6!bbn!bbn.com!eli
From: eli@bbn.com (Steve Elias)
Newsgroups: sci.misc
Subject: Re: the "greenhouse effect" theory
Message-ID: <22286@bbn.COM>
Date: 18 Mar 88 13:53:47 GMT
References: <22138@bbn.COM> <3851@bloom-beacon.MIT.EDU> <22277@bbn.COM> <3862@bloom-beacon.MIT.EDU>
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Reply-To: eli@BBN.COM (Steve Elias)
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In article <3862@bloom-beacon.MIT.EDU> jfc@athena.mit.edu (John F Carr) writes:

>In article <22277@bbn.COM> eli@BBN.COM (Steve Elias) writes:

>:	but detailed models of any long-term atmospheric effect
>:	just aren't around yet.  you are a planetary scientist, right?
>:	do you admit that we don't have enough data to really be able
>:	to predict any planetary atmosphere?
>
>:	as a computer dude who has studied astronomy, i'll say that even if
>:	we did have enough data to learn how to predict atmospheres, we
>:	don't have .001 the processing power available to do the job.
>
>It depends on what level of detail you want.  I mentioned an article I

	i want lots of detail -- that's what gives confidence that the
	simulations & predictions are correct on every level.	

>read in Scientific American which discussed computer simulations of 
>the Ice Ages.  Today's computers, as far as I know, do a good (but not
>great) job of predicting gross effects (like the Ice Ages and
>approximate global temperature distributions).  They do an acceptable
>job of calculating details.  

	weather forecasters aren't helped much by their computer predictions.
	they still end up being right about half the time -- the same ratio
	one would get if one just predicted that today's weather would 
	continue tomorrow.  i think there's a long way to go towards
	accurate (acceptable?) predictions of detailed atmospheric effects.

>In both cases the models can probably be
>improved, especially as more computer power gives more accurate
>models.  In all cases, more power will help as long as we have the
>observational detail to get data accurate enough (which means, there
>is a lot of room for improvement).  The statement above is just too
>general.

	point taken...  my statement may understate what current
	simulations can do -- but i do still contend that we have
	an incredibly long way to go, both in sensing and computing,
	before we can get very accurate predictions & understanding
	of at least one planetary atmosphere (ours).

>:	you bet.  enough heat could start thermal runaway, just as enough
>:	CO2 could.  i read in today's paper that the current guess about
>:	the extinction of the dinosaurs involves 3x to 5x increase in 
>:	atmospheric CO2 levels around the time they disappeared.
>
>The point is, enough heat is vastly more than we are producing now
>(another article said 10^-7 of the Earth's energy budget is man-made).
>Enough CO2 seems not to be much more than we are producing now.

	we may be producing too much CO2 -- that just isn't clear yet.
	i think your 10e-7 number is not correct -- but i will try to 
	find Frank Drake's notes and provide folks with some numbers
	of my own.  his bottom line is that we will have to worry more
	about thermal pollution as our power needs grow.
 
>Timescales to consider:
> The atmosphere heats and cools in hours (now that I think about it, I
>seem to remember deriving a cooling time for atmospheres in a course I
>took on planetary science.  I'll try to find this).  Looking at daily
>temperature curves I have estimated a time constant of no more than a
>day.  (i.e. the atmosphere effectively follows man-made inputs with no lag)

	this is interesting -- you aren't contending that the atmosphere
	would cool in a matter of days if the greenhouse effect caused
	an 1 or 2 degree temperature rise, are you?  i see your point, 
	on a local scale, at least.  radiational cooling is fast.

> The surface is a trickier matter, as is the ocean.  I would guess the
>time constant from the lag of seasons behind the driving force: the 
>Earth's axial tilt.   The lag is slightly less than 3 months; this 
>would make the best-fit timescale [i.e. if one treated the earth's 
>temp. as a driven 2nd order system] about the same.

	good points.  the multiple order effects (beyond 2) are a good
	example of aspects of the greenhouse effect or any atmospheric
	effect, that aren't understood that well yet...