Wednesday, January 18, 2012

2031.txt

cc: Martin Manning <mmanningatXYZxyznoaa.gov>, WGI-chap6-ar4 <wg1-ar4-ch06atXYZxyzs.ucar.edu>
date: Tue, 5 Sep 2006 10:41:18 -0600
from: Jonathan Overpeck <jtoatXYZxyzrizona.edu>
subject: Re: [Wg1-ar4-ch06] urgent IPCC need
to: Fortunat Joos <joosatXYZxyzmate.unibe.ch>

Thanks Fortunat - not sure how we missed these, but the good news is that we've started a
new tracking system for all the input we're getting. Eystein, �yvind and I are getting back
to full time IPCC, so there will be more to look at soon.

All Chap. 6 LAs - attached is the latest draft of the Chap. 6 Executive Summary - please
read and let us know if there are any errors or ways to make our points more clear
(remember, we're not adding new points at this time. and that our latest edits are in
yellow). As before, the CLA edits have been made with the intent of clarification only, not
changing the meaning. Note that there is a new bullet suggested by Fortunat:

o It is unlikely that CO2 variations have triggered the end of glacial periods.
Antarctic temperature started to rise several centuries before atmospheric CO2 during past
glacial terminations.

This bullet is intended to clarify what is meant by "close relationship between climate and
the carbon cycle " in the previous bullet.

OUTSTANDING ISSUE - there is still concern, voiced by me, Eystein and Fortunat, that the
bullet"

o It is very likely that the global warming of 4 to 7 �C since the Last Glacial
Maximum (ca. 21,000 years ago) occurred at an average rate about ten times slower than the
warming of the 20th century.

is still to vague. Two other LAs that we've checked with feel this cannot be deleted - per
our agreement in Bergen. The question is whether we can clarify it somehow. Please suggest
ideas!

Thanks, Peck

Hi Peck and all,
Sorry was not in over the weekend.
It seems that my earlier comments and suggestions for the ES got overlooked. All my
changes are detailed in the attached revised ES file. Please refer to this file for my
detailed comments.
The most important proposals are given in ascii below for those that do not want to open
the attached file(s).
I copy this also to Martin Manning for information.
Finally, all authors of the chapter should definitly see the latest version and give
their agreement.
With best wishes, Fortunat
Here first my earlier suggestions also in the file from August 15.
1. bullet
"The sustained rate of increase over the past century in the combined radiative forcing
from the three well-mixed greenhouse gases carbon dioxide (CO2), methane (CH4), and
nitrous oxide (N2O) is very likely unprecedented in at least the past 16,000 years.
Pre-industrial variations of atmospheric greenhouse gas concentrations observed during
the last 10,000 years were small compared to industrial era greenhouse gas increases,
and were likely mostly due to natural processes."
1. bullet in 2000 year section:
"It is very likely that the average rates of increase in CO2 and in the combined
radiative forcing from CO2, CH4 and N2O concentration increases have been at least five
times faster over the period from 1960 to 1999 than over any other 40-year period during
the past two millennia prior to the Industrial Era."
1. bullet in feedback section:
What does the original bullet mean to a non-specialist? Non-linear can be anything
(exponential decay? An oscillation?). Why should the small size of the orbital forcing
suggest non-linearity? What about GHGs? Bullet seems very verbose. What does the last
sentence mean? Is this not a contradiction to the figure showing the LGM forcing? In
this figure, a consensus view is given on the magnitude of past forcing. Dust loading
and vegetation albedo feedback/forcing are generally considered to be much smaller than
ice sheet feedbacks/forcing.
What should be said is something like:
"The widely-accepted orbital theory suggests that glacial-interglacial cycles occurred
in response to globally small changes in orbital forcing. The large response of the
climate system to a globally small forcing implies a strong positive amplification of
this forcing. Changes in greenhouse gas concentrations, ice sheet growth and decay,
ocean circulation and sea ice changes, biophysical feedbacks, and aerosol (dust) loading
have very like contributed to this amplification."

The points are
- refer to the orbital theory to caveat the statement as Milankovitch theory is not yet
proven.
- strong amplifications occurred.
- list the factors that contributed to the amplification.
Now to the more recent discussion. Suggestions are again in the attached file in green.
- I agree with Peck that we should say something about 1998 issue.
- I think merging the first and last section would overcome some of the weaknesses of
the previous draft in particular with respect to amplification and orbital theory:
What is the relationship between past greenhouse gas concentrations and climate and the
role of biogeochemical and biophysical feedbacks?
* The sustained rate of increase over the past century in the combined radiative
forcing from the three well-mixed greenhouse gases carbon dioxide (CO2), methane (CH4),
and nitrous oxide (N2O) is very likely unprecedented in at least the past 16,000 years.
Pre-industrial variations of atmospheric greenhouse gas concentrations observed during
the last 10,000 years were small compared to industrial era greenhouse gas increases,
and were likely mostly due to natural processes.

* It is very likely that the current atmospheric concentrations of CO2 (380 ppm)
and CH4 (1760 ppb) exceed by far the natural range of the last 650'000 years. Ice core
data indicate that CO2 varied within of 180 to 300 ppm and CH4 within 320 to 790 ppb
over this period. Over the same period, Antarctic temperature and CO2 concentrations
co-vary, indicating a close relationship between climate and the carbon cycle.
* The widely-accepted orbital theory suggests that glacial-interglacial cycles occurred
in response to globally small changes in orbital forcing. The large response of the
climate system to a globally small forcing implies a strong positive amplification of
this forcing. Changes in CO2 and other greenhouse gases, ice sheet growth and decay,
ocean circulation and sea ice changes, biophysical feedbacks, and aerosol (dust) loading
have very like contributed to this amplification.
* It is unlikely that CO2 variations have triggered the end of glacial periods.
Antarctic temperature started to rise several centuries before atmospheric CO2 during
past glacial terminations.
* It is very likely that marine carbon cycle processes were primarily responsible for
the glacial-interglacial CO2 variations. The quantification of individual marine
processes remains a difficult problem.
* It is virtually certain that millennial-scale changes in atmospheric CO2 associated
with individual Antarctic warm events were less than 25 ppm during the last glacial
period. This suggests that the associated changes in North Atlantic Deep Water formation
and in the large-scale deposition of wind-borne iron in the Southern Ocean had limited
impact.
* Paleoenvironmental data indicate that regional vegetation composition and
structure are very likely sensitive to climate change, and can, in some cases, respond
to climate change within decades.
* It is likely that earlier periods with higher than present atmospheric CO2
concentrations were warmer than present. This is the case both for climate states over
millions of years (e.g., in the Pliocene, ca. 5 to 3 million years ago) and for warm
events lasting a few hundred thousand years (i.e., the Paleocene-Eocene Thermal Maximum,
55 million years ago). In each of these two cases, warming was likely strongly amplified
at high northern latitudes relative to lower latitudes.
--
Climate and Environmental Physics,
Physics Institute, University of Bern
Sidlerstr. 5, CH-3012 Bern
Phone: ++41(0)31 631 44 61 Fax: ++41(0)31 631 87 42
Internet: http://www.climate.unibe.ch/~joos/

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Attachment converted: Macintosh HD:Ch06_FinalDraft_Exec#150AFD.doc (WDBN/�IC�)
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--

Jonathan T. Overpeck
Director, Institute for the Study of Planet Earth
Professor, Department of Geosciences
Professor, Department of Atmospheric Sciences
Mail and Fedex Address:
Institute for the Study of Planet Earth

715 N. Park Ave. 2nd Floor
University of Arizona
Tucson, AZ 85721
direct tel: +1 520 622-9065
fax: +1 520 792-8795
http://www.geo.arizona.edu/
http://www.ispe.arizona.edu/

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