----- Original Message -----
From: "Alastair McDonald" <[EMAIL PROTECTED]>
Newsgroups: gmane.science.general.global-change
Sent: Wednesday, September 06, 2006 5:14 PM
Subject: [Global Change: 578] Re: "Tipping points" again
(snip)
Hansen also wrote:
"The CO2 amount of 440 ppm is a critical threshold. Other things being
"normal", it is the peak CO2 amount in a scenario (1) that would keep
additional global warming from exceeding 1°C (1.8°F). Based on the history
of the Earth, it is estimated that warming greater than 1°C above today's
level would weaken the ice sheets that cover Greenland and Antarctica,
causing eventual rise of sea level by several meters. "
See http://www.giss.nasa.gov/research/briefs/hansen_11/
Related Article: The Climate Change Commitment
Wigley, T.M.L. (2005), "The Climate Change Commitment," Science, vol. 307,
pp. 1766-69.
Abstract
Even if atmospheric composition were fixed today, global-mean temperature
and sea level rise would continue due to oceanic thermal inertia. These
constant-composition (CC) commitments and their uncertainties are
quantified. Constant-emissions (CE) commitments are also considered. The CC
warming commitment could exceed 1°C. The CE warming commitment is 2° to 6°C
by the year 2400. For sea level rise, the CC commitment is 10 centimeters
per century (extreme range approximately 1 to 30 centimeters per century)
and the CE commitment is 25 centimeters per century (7 to 50 centimeters per
century). Avoiding these changes requires, eventually, a reduction in
emissions to substantially below present levels. For sea level rise, a
substantial long-term commitment may be impossible to avoid.
---------
(end quote)
The two most aggressive emission reduction scenarios in SRES/TAR (A1T and
B1) would stabilize CO2 between 500 & 600 ppm by 2100. Both of these
scenarios assume a reduction in emissions to substantially below present
levels by 2100, yet both would yield peak concentrations above Hansen's 440
and current CC 380. Damned if we do and damned if we don't, so why bother?
(Hint: what CC commitment level avoids 60 meter sea level rise by 6000 AD?)
-dl
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