[SustainableTompkins] Avoiding the GHG tipping point

[bosak at ibiblio.org]

A scientific opinion widely quoted recently is that we will reach
a "tipping point" where climate change becomes irreversible when
the atmospheric concentration of CO2 reaches 440 ppm.  (We're
currently at about 382 ppm and at present rates of increase will
hit the 440 mark in about twenty years.)

It should be obvious that there are a lot of problems with setting
a specific figure like this, but it's useful in trying to
understand the situation.

What I want to know is, by what percentage would current world
consumption of fossil fuels have to decrease in order to avoid
hitting the tipping point?  There would appear to be a lot of
hysteresis (i.e., time lag) involved, so the amount of reduction
would have to be more than the amount needed to maintain a steady
state once we got there, but how much is that?  Fifty percent?
Sixty?  Eighty?

Has anyone crunched the numbers on this?  For purposes of
discussion, I need just one credible, conservative estimate of the
amount we'd have to reduce global fossil fuel consumption,
starting today, in order to avoid hitting 440 ppm.  (It would be
best to have several estimates, since they're bound to differ
depending on methodology used, but I'd be happy to have just one.)

Jon