**Prospectus for Synthesis and Assessment Product 3.4**

*Abrupt Climate Change*

Lead Agency: USGS

Supporting Agencies: NOAA, NSF

**\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_**

**1. Overview: Description of Topic, Audience, Intended Use,\
and Questions to be addressed**

1.  **Description of Topic**

This prospectus provides an implementation plan for developing and
producing Climate Change Science Program (CCSP) Synthesis and Assessment
Product 3.4, "*Abrupt Climate Change.* " Paleoclimate records of climate
and environmental change derived from archives such as tree rings, ice
cores, corals, speleothems and sediments indicate that global and
regional climate has experienced repeated abrupt changes, many occurring
over a time span of decades or less. The National Research Council (NRC)
report "Abrupt Climate Change" (Alley, et al., 2002) offers two
definitions of abrupt climate change. A mechanistic definition defines
abrupt climate change as "*Transition of the climate system into a
different state (of temperature, rainfall, and other aspects) on a time
scale that is faster than the responsible forcing*." This definition
implies that abrupt climate changes involve a threshold or non-linear
feedback within the climate system from one steady state to another. An
impacts-based definition defines abrupt climate change as "*Change of
the climate system that is faster than the adaptation time of social
and/or ecosystem*s." Abrupt climate changes might have a natural cause
(such as volcanic aerosol forcing), an anthropogenic cause (such as
increasing carbon dioxide in the atmosphere), or might be unforced
(related to internal climate variability). Regardless of the cause,
abrupt climate change presents potential risks for society that are
poorly understood. An improved ability to understand and model future
abrupt climate change is essential to provide decision-makers with the
information they need to plan for these potentially significant changes.

Current research on abrupt climate change is focused on documenting
evidence of past abrupt climate change, refining the temporal and
geographic extent of the change, proposing mechanisms to explain the
change, and performing atmosphere-ocean model experiments. Examples of
abrupt climate change that have received special attention include the
Younger Dryas cold reversal event that occurred during the last
deglaciation, the rapid onset of widespread periods of drought that have
been documented in hydrologic records of the past 2,000 years in the
American West, and abrupt shifts in modes of ocean-atmosphere
interaction (e.g., El Nino-Southern Oscillation and the Arctic
Oscillation) seen in both paleo and instrumental records. Although the
Younger Dryas event occurred during a time when a large ice sheet was
present on North America, causing some to question whether an event of
comparable magnitude could occur in the near future, understanding its
cause is critical if we are to evaluate scenarios of future climate
change.

Abrupt climate changes can affect regions or the entire globe. Many of
the abrupt changes examined so far are regional rather than global in
extent. Such abrupt changes reflect reorganizations of the climate
system from one stable state to another, and are characterized by
changing patterns in the transfer of heat and energy with accompanying
shifts in temperature, precipitation, winds, and other variables. For
example, one region may warm as another cools, or become drier as
another becomes wetter. Regional impacts could be large while the global
mean change is small. In the modern world of increasing population and
limited resources, regional changes are particularly significant in
terms of the challenges or risks that they pose to society. Abrupt
climate changes with either regional or global impacts will be
considered in this assessment.

Much debate exists as to what types of climate change should be
considered under the umbrella of abrupt climate change. The El
Nino-Southern Oscillation (ENSO) and Arctic Oscillation (AO) are
examples of climate processes that appear to have different stable
modes. Typically, as these processes develop, climate variables such as
temperature and sea level pressure remain in one mode for a period of
time, change to a different mean state, and change back again.
Proponents argue that different stable modes exist, and that the shift
between modes constitutes an abrupt climate change. The contrary view
questions whether stable modes exist at all, or argues that these
changes do not fit the definition of an abrupt climate change event.
This assessment follows the lead of the NRC report in including these
processes under the umbrella of abrupt climate change, and supports the
NRC recommendation to examine in more detail the processes that could
lead to different modes of ocean-atmosphere interaction.

Abrupt climate changes can be accompanied by a change in the frequency
of extreme events such as hurricanes, heat waves, droughts and floods.
In a changing climate, an abrupt increase in extreme event frequency
might be far more difficult to adapt to compared to a gradual increase.
For this reason, we include abrupt changes in extreme event frequency
within report 3.4 and exclude gradual changes in extreme event
frequency. Climate extremes are the subject of Synthesis and Assessment
Product 3.3, "*Weather and Climate Extremes in a Changing Climate."*

2.  **Intended Use and Audience**

This CCSP Synthesis and Assessment Product will be in the form of a
report that (a) summarizes the present status and key findings of
national and international research on abrupt climate change, and (b)
discusses the strengths and limitations of existing knowledge for
describing and analyzing the risks of abrupt climate change. Abrupt
climate change research is in an immature stage: processes are still
being identified; gaps exist in the archive of paleoclimate data; new
records are currently being developed; and modeling efforts are
evolving. The analysis of probability (of a specific change occurring),
recently pioneered for global warming predictions (Knutti, et al.,
2002), has yet to be applied to abrupt climate change. For these reasons
we expect the report to be most useful to those seeking to understand
what is currently known and, conversely, what is not known about the
climate processes that can lead to abrupt climate change.

**1.3 Questions to be Addressed**

The paleoclimate record contains many examples of abrupt climate change.
The NRC report on Abrupt Climate Change considered a broad array of
processes and impacts to explain past and potential future instances of
abrupt climate change. This Synthesis and Assessment Product will
consider four types of change documented in the paleo record that stand
out as being so rapid and large in their impact that they pose clear
risks to society in terms of our ability to adapt. They are supported by
sufficient evidence that hypotheses can be tested and risks
investigated, and the research indicates that the changes could occur in
the future. These changes are i) alterations of the ocean meridional
overturning circulation; ii) widespread and sustained hydrologic changes
to the hydrologic cycle; iii) rapid release to the atmosphere of methane
trapped in permafrost and continental shelves; iv) rapid change in ice
sheet mass. Note that the processes listed above closely correspond to
question 4.3 from the CCSP Strategic Plan, which reads: "What is the
likelihood of abrupt changes in the climate system such as the collapse
of the ocean thermohaline circulation, inception of a decades-long
mega-drought, or rapid melting of the major ice sheets?"

***i. Meridional Overturning Circulation change and influence on
climate***

The Atlantic Ocean is characterized by a meridional overturning
circulation (MOC) that has an important effect on the climate of the
surrounding continents. The wind-driven surface circulation transports
water northward in the North Atlantic, where it loses heat to the
atmosphere, becomes denser and sinks in the Nordic and Labrador Seas,
forming a southward-flowing subsurface water mass that eventually fills
the North Atlantic as the North Atlantic Deep Water mass between
2,000-4,000 m (Talley, 1996). Much of this deep water eventually returns
to the surface Atlantic via the Pacific and Indian Oceans, forming what
has been alternately termed the meridional overturning circulation, the
global conveyor belt or thermohaline circulation (THC). There is
evidence that the overturning in the Atlantic is not independent but is
instead coupled to other aspects of the climate. Independent of possible
coupling, there is some evidence and ongoing research to investigate
whether the impacts of the Atlantic MOC are global in extent.

One of the most remarkable discoveries in the field of paleoclimatology
has been the observed coincidence between the glacial climate state and
reduced or altered MOC in the North Atlantic, as observed in paleo
proxies for the deep ocean circulation (McManus, et al., 2004). In
contrast, the present interglacial climate state is characterized by
vigorous meridional overturning, which through global feedback promotes
a vigorous northeastward flow of warm surface waters of the Gulf Stream
(and North Atlantic Drift) across the North Atlantic, which, in turn,
sustains more equable climates in Europe. While the paleo record is
still incomplete, evidence exists that some of the abrupt climate
changes that occurred during the last glacial interval, such as the
Dansgaard-Oeschger cycles, and during the deglacial period, such as the
Younger Dryas, were caused or amplified by changes in overturning
circulation. These observations motivate the paleoclimate community to
improve our understanding of the overturning circulation, how it has
varied in the past, what the dominant controls have been, and how
widespread are the impacts.

Key parameters that influence ocean circulation are expected to change
in the future. Most simulations of future climate predict a slowdown of
the thermohaline circulation in response to increasing greenhouse gases
in the atmosphere. As the climate warms due to the increase in
greenhouse gases, the North Atlantic surface ocean will also warm and
become fresher due to melting of margins of the Greenland ice sheet and
precipitation increases in the mid-latitude North Atlantic. Both
increasing temperature and decreasing salinity reduce the density of the
North Atlantic surface ocean, hindering the convective sinking of the
water and thus inducing a slowdown of the MOC. Some climate models
predict an eventual complete shutdown of the overturning simulation, a
change that might be permanent if the circulation has different stable
states. But many different processes and feedbacks affect this
circulation, leading to considerable uncertainty in these projections
that must be evaluated from a combination of observational,
process-based, and modeling studies. From the perspective of risk, one
of the most important aspects of the MOC is the existence of a threshold
level, beyond which North Atlantic surface water becomes too buoyant to
convect, providing a sound conceptual basis for abrupt climate change.

The primary questions to be addressed in this section of the report are:

-   What are the factors that control the overturning circulation?

-   How well do the current ocean general circulation models (and
    coupled atmosphere-ocean models) simulate the overturning
    circulation?

-   What is the present state of the MOC?

-   What is the evidence for change in the overturning circulation in
    the past?

-   What are the global and regional impacts of a change in the
    overturning circulation?

-   What factors that influence the overturning circulation are likely
    to change in the future, and what is the probability that the
    overturning circulation will change?

-   What are the observational and modeling requirements required to
    understand the overturning circulation and evaluate future change?

The primary value of this section for decision-makers and policy-makers
will be to provide a summary of the present level of scientific
understanding and remaining uncertainties in identifying and describing
the factors that influence the thermohaline circulation and its regional
and global impacts.

***ii. Rapid changes to the hydrologic cycle***

Accurate forecasts of seasonal precipitation change are critical for
managing water resources throughout the world, especially in
water-stressed regions such as the American West and northern Africa.
Such forecasts have also proven to be crucial in the mitigation of
floods and landslides. Measurements from satellites and oceanic and
atmospheric monitoring are used in developing and testing sophisticated
model forecasts. In recent years, significant advances have been made in
predicting precipitation in the western US using Pacific and Atlantic
sea surface temperatures. These analyses assume that future variability
in these parameters will be similar to that which has been experienced
during the past 100 years (the reanalysis period).

During 2002, more than 50 percent of the coterminous US experienced
moderate to severe drought conditions. Detailed study of the North
American paleoclimate record of the past 2,000 years, however, has
revealed numerous periods of extended drought exceeding in duration and
geographic extent the 7-year-long, epic drought of the 1930s Dust Bowl.
For example, a prolonged dry event occurred throughout much of North
America between AD 1575 and 1595, the so-called 16th Century
Megadrought. This 20-year long megadrought, however, pales in comparison
to a 400-year long period of elevated aridity and epic drought that was
experienced throughout the western United States between AD 900 to 1300
during the Medieval Climate Anomaly (MCA). The MCA megadrought likely
resulted from oceanic and atmospheric conditions unlike those that we
have experienced during the past 100 years, possibly from a prolonged
and sustained La Niña state in the Pacific. A sustained La Niña favors
drought conditions in the American Southwest, while the Northwest
experiences increased precipitation and a greater likelihood of floods.
The debate continues as to whether climate processes such as ENSO will
be affected by human induced global warming. Understanding the causes
and impacts of past megadroughts and associated oceanic/atmospheric
conditions is therefore crucial to assessing the risk of abrupt
hydrologic change that we might experience in the future.

Questions to be considered in this section are:

-   What is our present understanding of the causes of major drought and
    hydrologic change over the historical record, including the role of
    the oceans or other natural or non-greenhouse gas anthropogenic
    effects as well as land-use changes?

-   What is our present understanding of the duration, extent and causes
    of megadroughts of the past 2,000 years?

-   What states of oceanic/atmospheric conditions and the strength of
    land-atmosphere coupling are likely to have been responsible for
    sustained megadroughts?

-   How might such a state affect the climate in regions not affected by
    drought? (For example, enhanced floods or hurricanes in other
    regions.)

-   What will be the signatures of change in the state of natural
    variability of the ocean and atmosphere that will signal the abrupt
    transition to a megadrought?

The primary audience for this section is policymakers, who require an
improved basis for ascertaining the present state-of-knowledge, as well
as uncertainties, in our scientific understanding of the causes of past
major droughts and the likelihood future ones. This understanding will
allow the early implementation of programs to limit the impact of major
droughts such as a those outlined by the National Drought Mitigation
Center at the University of Nebraska, Lincoln. These impacts include
loss or damage to agriculture, forest production, and fisheries;
increased energy costs; loss to tourism industry; decreased water supply
for public consumption; damage to ecosystems and biodiversity; increased
fires; decreased air quality (dust, fires); increased health risk, both
through rise of drought-related diseases and from diseases associated
with famine and poorer nutrition; and increased political unrest. At the
same time, this report will also benefit policymakers attempting to
implement programs that limit societal losses in other areas where
increased precipitation and/or increased likelihood of floods may be
forecast from abrupt hydrologic change.

**iii. *Rapid release of methane from hydrates***

Methane is produced naturally during the anaerobic decomposition of
organic matter by bacteria and is regularly released to the atmosphere.
Vast amounts of methane, however, are stored in the frozen form of
methane hydrate (molecules containing methane and water in the solid
state) in Arctic permafrost and in sea floor sediments below a depth of
about 250 m. It is estimated that there are 1000-6000 Gigatons (Gt) of
carbon stored as methane hydrates in ocean sediments and about 400 Gt
stored in sediments under permafrost regions (Buffett and Archer, 2004).
For comparison, the atmosphere currently contains \~730 Gt of carbon. A
warming of bottom ocean waters or the land surface caused by greenhouse
gases could cause the hydrates to melt and release methane to the
atmosphere. Methane is a powerful greenhouse gas, and is about 24 times
more effective on a mass basis at absorbing long wave radiation than is
carbon dioxide. Such a release of methane to the atmosphere could
amplify the initial warming. Following its rapid oxidation, carbon
initially released as methane will persist for centuries as carbon
dioxide.

Questions to be addressed in this section of the report are:

-   What is the volume of methane in terrestrial and marine sources and
    how much of it is likely to be released in various climate change
    scenarios?

-   What is the impact on the climate system of the release of varying
    quantities of methane over varying intervals of time?

-   What is the evidence in the past for abrupt climate change caused by
    massive methane release?

-   How much methane is likely to be released by thawing of the topmost
    layer (3 m) of permafrost? Is thawing at greater depths likely to
    occur?

-   What conditions (in terms of sea level rise and warming of bottom
    waters) would allow methane release from hydrates locked up in sea
    floor sediments?

-   What are the observational and modeling requirements necessary to
    understand methane storage and its release under various future
    scenarios of abrupt climate change?

**iv. *Rapid change in ice sheet mass balance***

Glaciers and ice sheets grow or recede due to differences between
accumulation and ablation. Traditionally these processes were thought to
change slowly, over centuries to millennia. Recent observations (Rignot,
et al., 2006) and process-based studies (Zwally, 2002) indicate that ice
loss can occur much more rapidly, within decades, driven by mechanical
processes that include the formation of meltwater at the surface and
subsequent flow to deeper layers, seasonal cycles in melt and flow,
warming and lubrication at the base, acceleration of outlet glaciers,
and disintegration of ice shelves. Some observations indicate that ice
loss of the Greenland Ice Sheet and West Antarctic Ice Sheet has
accelerated in the last decade. If these rates continue to increase, sea
level rise will occur much faster than predicted in the
Intergovernmental Panel on Climate Change (IPCC) 2001 assessment, which
did not consider mechanical processes in the ice sheet models. New data
from process-based studies and observations of ice sheet mass balance
could significantly shift predictions in future climate models.

The paleoclimate record provides evidence that ice melt is an abrupt
climate change that can occur much faster than the forcing. The rapid
melting of the large Northern hemisphere ice sheets at the end of the
last Ice Age, observed as the rate of sea level rise, occurred much
faster than the orbital forcing thought to drive these changes. It is
likely that individual ice sheets and glaciers responded even faster.
Sea level records integrate the effects of many ice sheets melting at
different rates in both hemispheres. Improved paleoclimate records of
ice sheet and glacier melt can contribute to our understanding of abrupt
melting of the remaining ice sheets and glaciers. Some of these topics
are also considered under the CCSP report titled "Past climate
variability and change in the Arctic and at high latitudes" (Synthesis
and Assessment Product 1.2).

Questions to be addressed in this section of the report are:

-   What is the paleoclimate evidence regarding rates of rapid ice sheet
    melting?

-   What are the recent rates and trends in ice sheet mass balance?

-   What is the impact on sea level if the recently observed rapid rates
    of melting continue?

-   What is needed to model the mechanical processes that accelerate ice
    loss?

**2. Contact Information for Responsible Individuals at Lead and
Supporting Agencies**

The US Geological Survey (USGS) is the lead agency for this CCSP
Synthesis and Assessment Product, with the National Oceanic and
Atmospheric Administration (NOAA) and National Science Foundation (NSF)
as the supporting agencies. Because USGS is the lead agency, the product
will be subject to USGS guidelines implementing the Information Quality
Act (IQA). Contact information for responsible individuals at lead and
supporting agencies is:

USGS (Lead) John McGeehin

> U. S. Geological Survey
>
> MS 926A
>
> 12201 Sunrise Valley Drive
>
> Reston, VA 20192
>
> Email: <mcgeehin@usgs.gov>
>
> Phone: 703-648-5349
>
> John Barron
>
> U. S. Geological Survey
>
> 345 Middlefield Road
>
> MS 910
>
> Menlo Park, CA 94025
>
> Email: <jbarron@usgs.gov>
>
> Phone: 650-329-4971

NOAA David M. Anderson

NOAA Paleoclimatology Program

325 Broadway, E/CC23

Boulder, Colorado, 80305

Email: <david.m.anderson@noaa.gov>

Phone: 303-497-6237

NSF Dave Verardo

> National Science Foundation
>
> Paleoclimate Program
>
> 4201 Wilson Blvd, Room 725
>
> Arlington, VA 22230
>
> Email: <dverardo@nsf.gov>
>
> Phone: 703-292-8527

**3. Lead Authors**

The following individuals are proposed as lead authors:

Dr. Peter Clark

Dr. Ed Cook

Dr. Thomas Delworth

Dr. Carrie Morrill

Dr. Daniel Muhs

Dr. Jonathan Overpeck

Dr. Richard Seager

Dr. Konrad Steffen

Dr. Andrew Weaver

Dr. Robert Webb

Appendix A provides brief biographies for each of the authors proposed
thus far. It is anticipated that additional authors will be added to the
team in order to ensure comprehensive and balanced subject matter
expertise, in conformance with requirements for the Federal Advisory
Committee Act (FACA). The author team will also depend extensively on
solicitation of relevant information from experts in the Federal and
academic research community during the preparation of this report.

**4. Stakeholder Interactions**

Stakeholder input will be solicited through the public comment period
tied to the development of this prospectus and all subsequent draft
documents put forth by the authors of SAP 3.4. The authors in
collaboration with the lead and supporting agencies may call upon a set
of stakeholders to broaden the input for this study as necessary. The
process of drafting and incorporating public comment will comply with
the rules set forth in the Federal Advisory Committee Act.

**5. Drafting**

The lead authors will draft answers to the key questions in their
respective sections. They will also prepare an introductory section to
describe the topic, the audience, and the intended use of this product.
The lead author for each section may assign primary responsibility for
drafting to a specific contributing author. The scientific/technical
synthesis of the document will utilize published, peer-reviewed
scientific literature.

Two workshops are envisioned as a means to help the research community
provide input and identify divergent opinions on as many as four abrupt
climate change topics, 1) meridional overturning circulation, 2) abrupt
hydrologic change, 3) methane hydrate release, and 4) rapid ice melt.
The lead authors will be responsible for incorporating materials from
contributing authors and from the workshop participants in the draft
product.

After the product is drafted, the lead authors (or coordinating lead
author and the authors responsible for each of the sections) will write
a non-technical summary. Lead and contributing authors will base their
writing on published, peer-reviewed scientific literature. Where
appropriate, the product and its non-technical summary will identify
disparate views.

**6. Review**

USGS will ensure that Synthesis and Assessment Product 3.4 is reviewed
at all stages as specified in the *Guidelines for Producing CCSP
Synthesis and Assessment Products* and consistent with the *Information
Quality Act* and *Information Quality Bulletin for Peer Review*. All
comments and responses will be documented and made publicly available.

The public is invited to nominate Expert Reviewers to participate in the
peer review of the draft of CCSP Synthesis and Assessment Product 3.4.
Nominations should be sent to John McGeehin, USGS lead, at the address
given in Section 3 of this prospectus. Nominations are due by 30
November 2006. All nominations will be forwarded to the USGS IQA
representative for consideration. Nominations must include an up-to-date
curriculum vitae and listing of publications. As IQA Lead Agency, USGS
will ensure that selected reviewers are technically qualified, as
demonstrated by scientific experience, published work, and stature
within and across the scientific community. USGS will ensure that the
slate of reviewers reflects a balance of scientific and technical
perspectives. USGS will also be responsible for screening the nominees
for real or perceived conflict of interest and independence. Peer
reviewers who are Federal employees will be subject to Federal
requirements governing conflict of interest \[see 18 U.S.C. 208, 5
C.F.R. Part 2635 (2004)\]. Reviewers who are not Federal employees will
be screened pursuant to the National Academy of Sciences policy for
committee selection with respect to conflict of interest.

The Expert Review will consist of technical experts who will submit
comments similar to those solicited as part of a journal peer review. In
addition, independent reviews may be obtained from non-climate
scientists, selected by USGS, to comment on how understandable and
useful the draft product is to non-specialists.

USGS will provide a charge statement for reviewers, which will be
distributed with the draft product and posted at USGS's IQA website, and
linked and/or replicated on the CCSP web site
\<http://www.climatescience.gov\>. The charge statement will be posted
on the above sites as soon details become available. The names and
affiliations of the reviewers that are selected will be posted on these
sites.

Following the Expert Review, the lead authors will revise the draft
product by incorporating comments and suggestions from the reviewers.
USGS will prepare a written response to the peer reviewers' comments
explaining its agreement or disagreement with the views of the peer
reviewers, and the actions taken in response to the peer review. The
draft product will then be released for a 45 day public comment period
following CCSP guidelines. The lead authors will prepare a third draft
of the product, taking into consideration the comments submitted during
the Public Comment Period. The scientific judgment of the lead authors
will determine responses to the comments.

Once USGS, as IQA Lead Agency, determines that the report conforms to
CCSP and IQA guidelines, it will submit a draft of the product and a
compilation of the comments received to the [CCSP Interagency
Committee](http://www.climatescience.gov/about/officials.htm). If the
[CCSP Interagency
Committee](http://www.climatescience.gov/about/officials.htm) determines
that further revision is necessary, their comments will be sent to USGS
and supporting agencies for consideration and resolution by the lead
authors. If needed, USGS may ask an independent science advisory group
to provide additional scientific analysis to help resolve scientific
uncertainty associated with specific issues. Once the [CCSP Interagency
Committee](http://www.climatescience.gov/about/officials.htm) has
determined that the report has been prepared in conformance with the
CCSP guidelines, the IQA and FACA, it will submit the report to the
National Science and Technology Council (NSTC) for final review and
approval. The CCSP Interagency Committee in consultation with the lead
and supporting agencies and the lead authors will address issues raised
during the NSTC review.

**7. Communications**

Once NSTC clearance has been obtained, USGS will coordinate publication
and release of the Synthesis and Assessment Product. The published
report will follow the standard format for all CCSP Synthesis and
Assessment Products.

**8. Proposed Timeline**

[Prospectus]{.underline}

Drafting, January ***-*** October 2006

Stakeholder interactions (including other scientists), Ongoing during
prospectus development

CCSP review, September 2006

Draft Prospectus public review, October/November 2006

Revised draft, November/December 2006

Final, December 2006

[Report]{.underline}

Drafting, March 2007-August 2007

Workshop 1. March 2007

Workshop 2. August 2007

Stakeholder involvement ongoing during drafting process and public
comment period

Draft 1 provided to expert reviewers, September 2007

Draft 2 made available for public comment, February 2008

Draft 3 submitted to CCSP for review April 2008

Product released June 2008

**References**

Alley, R. B., et. al. (2002). Abrupt climate change: Inevitable
Surprises. Washington, D. C., National Academy Press.

Buffett, B., and D.E. Archer. (2004). Global inventory of methane
clathrate: Sensitivity to changes in environmental conditions. Earth and
Planetary Science Letters 227: 185-199.

Knutti, R., T. F. Stocker, F. Joos and G.-K. Plattner (2002).
\"Constraints on radiative forcing and future change from observations
and climate model ensembles.\" Nature 416: 719-723.

McManus, J. F., R. Francois, et al. (2004). \"Collapse and rapid
resumption of Atlantic meridional circulation linked to deglacial
climate changes.\" Nature 428: 834-837.

Rignot, E., and P. Kanagaratnam. (2006). \"Changes in the velocity
structure of the

Greenland Ice Sheet.\" Science 311: 986-990.

Talley, L. D. (1996). \"North Atlantic circulation and variability.\"
Physica D 98: 625--646.

Zwally, H. J., W. Abdalati, et al. (2002). \"Surface melt-induced

acceleration of Greenland Ice-Sheet Flow.\" Science 297: 218-222.

**Appendix A. Biographical Summaries for Proposed Authors**

**Peter Clark** is a professor in the Oregon State University department
of Geosciences. His research interests include the history and dynamics
of former glaciers and ice sheets, and paleoclimatology. Dr. Clark
co-edited the American Geophysical Union Monograph titled "Mechanisms of
Millennial-Scale Global Climate Change" published in 2002.

**Ed Cook** is a Senior Scholar in the Lamont-Doherty Earth Observatory
and director of the Lamont Tree Ring Laboratory. Dr. Cook's research
focuses on tree-ring records of drought throughout the world.

**Tom Delworth** is a research scientist in the climate dynamics and
prediction group of NOAA's National Geophysical Fluid Dynamics
Laboratory. Dr. Delworth's research involves both models and
observations, and focuses on the ocean's role in climate, in particular
decadal to centennial scale climate variability and change.

**Carrie Morrill** is a CIRES scientist at the University of Colorado
and a staff member of the NOAA Paleoclimatology Branch of the National
Climatic Data Center, and a former NCAR post-doctoral scientist. Dr.
Morrill's research focuses on the paleoclimate record of abrupt climate
change and the role of sea ice in the Atlantic meridional overturning
circulation.

**Daniel Muhs** is a research geologist for the United States Geological
Survey (USGS) at the Denver Federal Center in Lakewood, Colorado. His
research focuses on the paleoclimate of the last interglacial period and
paleoclimatic records of eolian deposits.

**Jonathan Overpeck** is a Professor and Director of the Institute for
the Study of Planet Earth at the University of Arizona. Dr. Overpeck's
research focuses on global change dynamics. Dr. Overpeck is one of two
coordinating lead authors of the paleoclimate chapter of the 2007
Intergovernmental Panel on Climate Change assessment.

**Richard Seager** is a senior research scientist at Lamont-Doherty
Earth Observatory, The Earth Institute at Columbia University. His work
involves how coupling between the atmosphere and ocean causes climate
variability and change around the world on timescales ranging from
seasons to glacial--interglacial cycles. . Dr. Seager is a principal
investigator with the Consortium on the Ocean's Role in Climate Abrupt
Climate Change Studies (CORC-ARCHES).

**Konrad Steffen** is director of the Cooperative Institute for Research
in Environmental Sciences (CIRES) at the University of Colorado,
Boulder. His work involves the study of processes related to climate and
cryosphere interaction in polar and alpine regions based on in-situ and
satellite measurements, and using climate system modeling to study their
sensitivity.

**Andrew Weaver** is a Professor at the School of Earth and Ocean
Sciences at the University of Victoria. Canada. Dr. Weaver\'s research
focuses upon the large-scale ocean circulation and the role of the
oceans in climate, with a special emphasis upon three-dimensional
numerical modelling. Dr. Weaver is co-chair of the CLIVAR/PAGES
Intersection working group, where one of the focus areas is abrupt
climate change.

**Robert Webb** is a physical scientist and interim Leader of the
Climate Diagostics Branch of NOAA's Earth System Research Laboratory.
Dr. Webb is the co-editor of the American Geophysical Union Monograph
titled "Mechanisms of Millennial-Scale Global Climate Change" published
in 2002. Dr. Webb's research interests include paleoclimatology, climate
variability, and change.