[Prev][Next][Index][Thread]

Re: Human vs. natural influences on the environment



charliew@hal-pc.org (charliew) wrote:
>Let's get real here.  How many things in nature have 
>positive feedback loops associated with them?  Not many.

Climate is one of them.  The Earth's climate has at least
two metastable states (glaciated and temperate), and may have
more on the hotter side of temperate.  Changes between them
appear to be driven by short-term positive feedback and long-term
negative feedback, forced by inputs such as orbital and axial
variations.

>If 
>you knew anything about positive feedback, you would realize 
>that it is inherently unstable.

Exactly.  You wind up with runaway, periodic or chaotic behavior.

>  The mere fact that the 
>atmosphere and oceans have not turned into a frozen puddle 
>or boiled away strongly implies that no positive feedback is 
>involved.

How do you explain the period appearance and disappearance of
glacial cycles in Earth's history, if not positive feedback
leading to oscillation?

>  The same argument applies for variables with no 
>limits.  As far as all of the physicists can tell, there are 
>no absolute zeroes or infinities in nature.  Everything has 
>a limit.

I admit that I was not as careful as I could have been when
choosing my words.  However, a limit of the conditions now
prevailing on Venus is as good as no limit at all.

We don't know the characteristics of climate well enough to
be able to say whether or not continued addition of CO2 into
the atmosphere will push our climate into a much warmer
state.  If you know anything about feedback systems, you
know that changing the level of a threshold can lead to
sharp and discontinuous changes in the system characteristics.

Until we know where these thresholds are for Earth's climate,
we ought to be careful about how far we push it.  It may turn
out that the system is stable albeit a bit warmer at 1000 ppm
CO2, in which case we have little to worry about.  It may turn
out that at 770 ppm CO2 the system acquires a new metastable
state where tropical conditions prevail up to the 40th parallel,
and a good disturbance can push the system over.

We don't know.  We should be careful what we do until we do.

Is that enough of an antidote to "chicken little" for you?


Follow-Ups: References: