A naive question - geoengineering

Message from discussion A naive question

Sam Carana

More options Dec 31 2008, 8:12 pm

From: "Sam Carana" <sam.car...@gmail.com>

Date: Thu, 1 Jan 2009 12:12:17 +1100

Local: Wed, Dec 31 2008 8:12 pm

Subject: Re: [geo] Re: A naive question

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Many of the graphs relating to global warming are exponential, rather
than linear. Additionally, there are several scenarios in which the
combination of several tipping points can lead to a runaway greenhouse
gas effect that feeds on itself through positive feedback mechanisms.
For an example, read the page at:
http://en.wikipedia.org/wiki/Clathrate_gun_hypothesis

For decades, people have warned about this. Back in the early 1990s, a
poll of the world's leading climatologists showed that many feared
that the greenhouse effect could be unstoppable if emissions of
polluting gases were merely frozen and not cut. In December 1991,
Greenpeace asked 400 climate scientists if they thought the greenhouse
effect might reach the point of no return in the near future. Of the
113 scientists who returned their questionnaires, almost half thought
a runaway greenhouse effect is possible, and 13 per cent thought it
probable.

James Hansen, who heads the NASA Goddard Institute for Space Studies,
recently said that human activity is causing greenhouse gas levels to
rise so rapidly that his model suggests there is a risk of a runaway
greenhouse effect, ultimately resulting in the loss of oceans and of
all life on the planet:

"In my opinion, if we burn all the coal, there is a good chance that
we will initiate the runaway greenhouse effect. If we also burn the
tar sands and tar shale (a.k.a. oil shale), I think it is a dead
certainty."
http://www.newscientist.com/blogs/shortsharpscience/2008/12/nasa-scie...

I wrote about how Venus became a victim of such planetary warming
resulting in a runaway greenhouse effect, at:
http://geo-engineering.blogspot.com/2007/11/venus-runaway-greenhouse-...

Even if the risk of such scenarios occurring on Earth were small, it
makes sense to do the following:
- describe the risk and estimate the chances of manifestation, timelines, etc;
- identify tipping points, feedback mechanisms and give estimate
ranges of their combined impact;
- investigate ways to avoid it, mitigate it, etc;
- conduct comparative analysis of the various proposals
- make recommendations

I urge everyone who can make contributions to join this group and post
comments, views, proposals and suggestions here.

For evaluation criteria, to be used in above comparative analysis, see
my post here under "ranking the ideas", at:
http://groups.google.com/group/geoengineering/msg/751aa59e3cc5e8ff

Cheers!
Sam Carana

"We all hope that things will turn out right, but we must think about
what to do, in case it doesn't!"

=========== in response to: ==============

On Thu, Jan 1, 2009 at 7:37 AM, Ken Caldeira <kcalde...@gmail.com> wrote:
> I was somewhat flippant, but my comment holds ...

> Tom indicates that for the ranges expected this century, climate effects are
> largely in a linear domain, and this seems reasonable to me.

> As Mike points out, we know that the climate responds differently to
> different patterns and types of radiative forcing even if they have the same
> global mean radiative forcing. Whether these effects are additive depends
> mostly on whether you are in a range where linear approximation is "good
> enough" for some specific purpose.

> However, if you think the world is full of "tipping points" (where that is
> an irreversibility or perhaps a discontinuity) but neither A nor B would be
> sufficient to push you past the "tipping point" but A + B collectively would
> be enough, then we would expect the linear approximation to fail.

> ___________________________________________________
> Ken Caldeira

> Carnegie Institution Dept of Global Ecology
> 260 Panama Street, Stanford, CA 94305 USA

> kcalde...@ciw.edu; kcalde...@stanford.edu
> http://dge.stanford.edu/DGE/CIWDGE/labs/caldeiralab
> +1 650 704 7212; fax: +1 650 462 5968

> On Wed, Dec 31, 2008 at 11:55 AM, Mike MacCracken <mmacc...@comcast.net>
> wrote:

>> Dear Stuart:

>> The issue is a bit more complicated than Ken indicates:

>> Radiative forcing is measured (well, actually calculated) for the
>> tropopause. It does make a difference in the response the degree to which
>> the resulting forcing results in a change in the surface relative to the
>> tropospheric energy balance, especially for the hydrological cycle
>> influence.
>> The geographic pattern also matters, though perhaps not for the short
>> term. But if one has all the forcing in the Northern Hemisphere versus
>> Southern, one will get a different response due to the land/ocean
>> difference, etc. (this was the case for example during mid-20th century when
>> sulfate forcing in NH led to a slowing of warming or even a cooling, but did
>> not do so in the SH). Also, the orbital changes that drive the ice age
>> cycling actually create (independent of changes at the surface) virtually no
>> annual global forcing—all they do is redistribute energy by season and
>> latitude, so for that case, a zero forcing is causing ice age cycling
>> (through a lot of feedbacks, etc.).
>> There is also the issue of how long the forcing persists—short time ones
>> like volcanic eruptions have a large forcing, but due to short time they are
>> aloft, the response does not reach equilibrium and eventually goes away. A
>> small, persistent change can, however, have a longer term effect as it
>> activates some of the longer term feedback processes.

>> And I am sure there are further nuances.

>> Mike MacCracken

>> On 12/31/08 2:29 PM, "Ken Caldeira" <kcalde...@gmail.com> wrote:

>> You can of course add radiative forcing "linearly". (What other kind of
>> addition is there?)

>> An important question is whether climate response to the sum of radiative
>> forcings is the same as the sum of the climate responses to individual
>> radiative forcings.

>> The answer to this question depends on the size of the perturbation and
>> your tolerance for approximation.

>> Recall the maxim: "To first order, everything is linear !!"

>> ( More strictly speaking, "To first order, differentiable functions are
>> linear." )

>> On Wed, Dec 31, 2008 at 11:13 AM, Stuart Strand <sstr...@u.washington.edu>
>> wrote:

>> Is radiative forcing additive linearly?

>> = Stuart =

>> Stuart E. Strand
>> 167 Wilcox Hall, Box 352700, Univ. Washington, Seattle, WA 98195
>> voice 206-543-5350, fax 206-685-3836
>> http://faculty.washington.edu/sstrand/

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