Question 1: Is "global warming" happening? A tough question, but one we can answer with enough data.

Question 2: If so, are humans to blame? An even tougher question, but one which could presumably be answered with improvements in the data and computing power for the models. However, our time baseline for really accurate data will always be quite short.

Question 3: If so, what should we do about it? This is not a scientific question, it's a policy question.

"The argument of skeptics is not so much that global warming is not happening...."

'ATORE' of course....

Investing more money at the University systems to understand climate and weather could have tremendous benefits to global agriculture
and therefore to the benefit of people. It would
also (don't laugh) help us understand if
it is in possible to make planets like Mars
inhabitable by humans via massive manmade climate
change.

However, the whole global warming scare (presupposed as truth) and assumed to be of
human origin (rather than part of an interglacial
trend) is a bit of side track to say the
least.

I would Amend Brians Question 3 to read:

3. a. Assume Global Warming, is it malign or benign?

3. b. if it is malign, how malign is it?

3. c. Can we stop, slow it, or meliorate it on a cost effective basis.

Ummm. Has there ever been a time in Earth's history when there wasn't creeping climate change?

And when our moving into a new ice age becomes evident ... as is currently projected ... then what?

If climate is chaotic, in the mathematical sense, then better computer programs and even better data will not improve the ability to predict change in climate.

The amazing thing about Earth's climate is that it wobbles but never falls. Over 4 billion years, it shows no trend. If it did, we'd be icy cold or boiling hot.

(Paleoclimatologists-- some of them -- do think it fell over on the cold side once (Snowball Earth), but if that's corrent then the climate somehow restored itself to its accustomed balance point.

Of all the possible temperatures, why does Earth hover around 59F degrees? What's special about that?

It's what is required.

It all depends on what sort of time scales the system is chaotic. If "sensitive dependence upon initial conditions" only makes a big difference after 100,000 years, and you want to predict the climate behavior over 100 years, you're fine.

I confess I've also always been partial to the anthropic principle answer to "what's so special" about the Earth...

Actually, I favor the view that Earth's climate is antichaotic. No matter how much you vary the inputs, you get the same output.

But nobody's going to fund a computer simulation to show that.

Harry:

But He did.

There's nothing "antichaotic" about that. It just means that the Earth's climate is subject to an "attractor," a term fundamental to chaos theory (which really is a horrible misnomer, in my opinion, which causes considerable difficulty in explaining the idea, but does help to sell books). In fact, it is much more interesting to look at a system that behaves in a very complex way within a limited parameter range (Earth's atmosphere), than one which (in a sense) runs off to plus or minus infinity (Venus/Mars).

Here's a cool site with far more information: http://www.cmp.caltech.edu/~mcc/chaos_new/Lorenz.html

Brian;

Precisely.

The limited range being the obvious key.

Global warming is malign if it means that Manhattan (partially) and the Hamptons will be under water in 100 years.

Other than that, Harry and Jeff seem to have it about right.

Harry Eagar:
I once had a temporary job in a distribution warehouse of the Rowntree chocolate company (Smarties, After Eight, et cetera). The chocolate that was stored for long periods was kept in a large room where the temperature was maintained at 59 degrees Fahrenheit. This may or may not be a clue to the mystery of climate.

Manhattan isn't going to be submerged, even partially, even if global warming is every bit as real as the worst case scenario.

Well, I read Lorenz's Danz Lectures, and I don't recall any examples of "an attractor" for a specific equation, only "many attractors." But I am self-taught, and not very well, in math; so maybe I misunderstood.

So, to my mind, that the Earth seems to have settled on one attractor seems antichaotic.

Lorenz was suitably modest about the implications of his theory, not even asserting that it is certain that weather is chaotic; with climate even more doubtfully chaotic.

Harry:

Many complex systems have states that exist in a locus around an "attractor." Over time, the states move through the same region around the attractor, but not in a repeatable path.

Even your heart works this way. If timed sufficiently accurately, your heartbeats aren't completely regular, but rather have a varying periodicity around a central value.

That's for a healthy heart. Oddly, sick hearts lose that randomized periodic behavior around the attractor, and become very, very, regular.

I wish I'd gone to public school so I could understand that stuff.