[asa] Re: more CO2

From: Joel Moore <redsoxfan1977@gmail.com>
Date: Sat Feb 03 2007 - 20:53:17 EST


I'm speculating that the 20% includes the total effect of
deforestation but the relatively large uncertainties about things
like decreased capacity are the reason that the error bars are so
large for the contribution of land use to CO2. You could probably
look back at the 2001 IPCC report for their methods in estimating
that number. I'd be surprised if the methodology for those estimates
has changed much over the few years.

A couple of things about vegetation and CO2:
1) Existing tropical forests contain a lot of CO2 but they're not
really a sink for atmospheric CO2. CO2 uptake occurs when a forest is
adding biomass but most tropical forests are at a steady state (or
dynamic equilibrium). Planting trees in places that have been
deforested would result in CO2 uptake over a few decades to a century
while the trees and ecosystem were adding biomass and growing toward
a mature ecosystem.

2) A recent NY Times op-ed piece from a climate scientist argues that
planting trees in the temperate zone (on a large scale) is not a
viable option for reducing global warming. He argues that temperate
zone forest likely have a neutral (at best) to negative effect on
warming because while the trees take up CO2, the trees may result in
warming due to a decreased albedo.

3) That said, there seems to be a sink for CO2 in the northern
hemisphere that has resulted in a somewhat slower rate of CO2
increase over the last couple of decades than would be expected from
CO2 emissions. Reforestation in the temperate zone of North America
and elsewhere is may be part of that northern hemisphere sink. Other
possible factors include CO2 fertilization, increased weathering
rates of soil minerals, such as carbonates, that can serve as a short-
term (decadal-scale) CO2 sink, or changing precipitation.

4) One of the many feedback mechanisms that makes things worse is
that tropical deforestation may occur due to climate change. Changes
in precipitation in Amazonia that have caused a drought seem to
result in increased tree mortality. There may be a precipitation
threshold where it becomes too dry for tropical forests and savannah
becomes favored. Certainly a savannah would be a smaller pool for CO2
and so the transition from forest to savannah would result in
increased CO2 input to the atmosphere. Note that the changes thus far
haven't been directly linked to global warming.

5) The feedback mechanisms are what are scary about climate change.
That's why a 3 degrees C increase in warming with a doubling of CO2
is the low end and up to 8 or 9 degrees C is the high end. There are
a lot of thresholds we know qualitatively exist but don't have a good
handle what the exact threshold is. For example, some people have
published papers arguing we've already crossed the threshold for the
melting of the Greenland ice sheet and now the question is does it
melt slowly or quickly.

Hope that's helpful,

On Feb 3, 2007, at 5:29 PM, George Murphy wrote:

> Joel -
> Thanks. I had received a couple of helpful off-list responses.
> There's one thing I'm still not clear on. What I was thinking of
> when I posted my query what I was thinking of was (1) decreased
> ability to take CO2 out of the atmosphere by photosynthesis due to
> elimination of vegetation. When reference is to "emissions
> associated with land use change" (as below), it sounds like
> something different - namely (2) burning of vegetation & consequent
> putting of CO2 into the atmosphere. The total effect of
> deforestation would be additional CO2 emissions _plus_ decreased
> capacity for removing CO2 in the future. Does the 20% figure
> include both effects, or just (2)? & if the latter, how
> significant is (1)?
> Shalom
> George
> http://web.raex.com/~gmurphy/
> ----- Original Message ----- From: "Joel Moore"
> <redsoxfan1977@gmail.com>
> To: <gmurphy@raex.com>; <asa@calvin.edu>
> Sent: Saturday, February 03, 2007 2:24 PM
> Subject: Re: more CO2
> George,
> I didn't see any responses to your question about the effects of
> deforestation on CO2. Deforestation was responsible for about 20% of
> the rise on CO2 during the 1990s according to the current best
> estimate in the most recent IPCC summary for policymakers. Here are
> the relevant sentences from that summary:
> "The primary source of the increased atmospheric concentration of
> carbon dioxide since the pre-industrial
> period results from fossil fuel use, with land use change providing
> another significant but smaller
> contribution. Annual fossil carbon dioxide emissions4 increased from
> an average of 6.4 [6.0 to 6.8] * GtC (23.5 [22.0 to 25.0] GtCO2) per
> year in the 1990s, to 7.2 [6.9 to 7.5] GtC (26.4 [25.3 to 27.5]
> GtCO2) per
> year in 20002005 (2004 and 2005 data are interim estimates). Carbon
> dioxide emissions associated with
> land-use change are estimated to be 1.6 [0.5 to 2.7] GtC (5.9 [1.8 to
> 9.9] GtCO2) per year over the 1990s,
> although these estimates have a large uncertainty. {2.3, 7.3} "
> *"In general, uncertainty ranges for results given in this Summary
> for Policymakers are 90% uncertainty intervals unless stated
> otherwise, i.e., there is an
> estimated 5% likelihood that the value could be above the range given
> in square brackets and 5% likelihood that the value could be below
> that range. Best
> estimates are given where available. Assessed uncertainty intervals
> are not always symmetric about the corresponding best estimate. Note
> that a number of
> uncertainty ranges in the Working Group I TAR corresponded to 2-sigma
> (95%), often using expert judgement. "
> You can download the summary at
> http://www.ipcc.ch/
> Sorry about the late response. I'm not on the list-serv but read the
> archives once or twice a week.
> -Joel
>> From: George Murphy <gmurphy@raex.com>
>> Date: Sun Jan 28 2007 - 17:32:02 EST
>> I appreciate many of the recent posts on global warming. I write
>> a column for Lutheran Partners, a journal for ELCA clergy, on
>> issues of science and technology in ministry, & will probably do
>> my next one on global warming. I've saved some of the recent
>> posts & expect that they'll be helpful for that.
>> One question. 15-20 years ago there was a lot of discussion about
>> loss of rainforests, especially in the Amazon, & the contribution
>> of this to shifting O2 - CO2 proportions in the atmosphere. How
>> significant a contribution is this in comparison with use of
>> fossil fuels & other contributions to greenhouse gases?
>> Shalom
>> George
>> http://web.raex.com/~gmurphy/

Joel Moore
Department of Geosciences
315 Hosler Building
Pennsylvania State University
University Park, PA 16802

(814) 863-8055

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Received on Sat Feb 3 20:43:48 2007

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