Report of the CCSM Paleoclimate Working Group Meeting
14 and 15 March 2005
NCAR, Damon Room
At the spring meeting in Boulder on 14-15 March 2005, Paleoclimate Working Group
(PaleoWG) members discussed a strategy for new initiatives to provide scientific focus to the
PaleoWG.
Several areas of broad appeal and scientific importance were identified for further discussion at
the CCSM summer workshop.
It was decided that significant interesting science that would entrain a significant fraction of
PaleoWG members, as well as the broader paleoclimate and climate change communities, could
be accomplished by focusing efforts over the next several years. Questions that can be addressed
by studying paleoclimate with CCSM include the mechanisms behind major climate changes and
their relationship to major events in the history of life, including mass extinctions.
Three strategic initiatives were identified by combining these two themes with additional
considerations, such as the ability of the initiative to be of use to, and more importantly, directly
involve a substantial portion of PaleoWG members. To ensure organized progress, a tentative
organizer for each initiative was identified.
The initiatives are:
(A) Transient, fully coupled long Quaternary simulations
(B) A crosscutting modeling approach to the Cretaceous (including carbon cycle modeling, and
intermodel comparisons)
(C) An intercomparison of CCSM simulations across different time intervals
Our intention is to move forward in the areas in which sufficient enthusiasm for intramural
working group-wide collaborations exist and for which substantial groundwork has already been
laid. The future directions of the PaleoWG are always drawn from the interests of its
membership. Other intervals/problems identified as having intrinsic interest were the Permian,
Jurassic, PETM, Eocene, Eocene-Oligocene, Oligocene-Miocene, Miocene, and Pliocene. If
sufficient interest exists to focus PaleoWG activities into these intervals or other subject areas,
then the working group’s strategic direction will be revisited at future PaleoWG meetings or via
discussion with the co-chairs.
A description of the three initiatives follows:
Transient, fully coupled, long Quaternary simulations (Organizer, Bette Otto-Bliesner,
NCAR, ottobli@ucar.edu)
Scientific Merit. Coupled climate simulations are usually snapshot equilibrium simulations
because of limited computing resources. Yet, most proxy data of past climate change is
collected downcore making climate simulations utilizing temporally changing forcings important
for model-data comparisons. Our understanding of climate change in the Quaternary, including
glacial inception ca. 115,000 years ago (115 ka), the meltback of Greenland ca. 130 ka, and the
deglaciation from ca. 20-6 ka, will be enhanced by transient simulations with CCSM. The first
Quaternary simulation, already identified and budgeted in the current PaleoWG CSL allocation
is the mid-Holocene (8 ka to 3 ka), a key time period for climate model-data comparisons.
Globally distributed records show that many regional environments were substantially different
from present during the mid-Holocene, with key archeological transitions linked to climate
change in many areas. Beginning with the presumed meltwater event at 8.2 ka, several key
climatic transitions occurred during the mid-Holocene, including the observed mid-Holocene
onset of the “modern” ENSO regime post-6 ka, the observed global monsoon collapse ~5.5 to
5.0 ka, and the broadly observed late-Holocene transition ~4 ka. The primary difference in
climate forcing during the mid-Holocene (relative to present) was the altered latitudinal and
seasonal distribution of insolation. While atmospheric carbon dioxide concentrations were
relatively close to their pre-industrial values, recent geologic evidence suggests that atmospheric
methane concentrations could also have modulated net radiative forcing of the climate system,
although the potential role of this methane forcing is not well understood.
Approach. Almost all numerical efforts to understand mid-Holocene climate have focused on
time-slice experiments at 6 ka, and a series of these experiments is slated to be included in the
IPCC AR4. However, the response of the climate system to transient mid-Holocene variations in
climate forcing has not yet been investigated with CCSM. The Transient Mid-Holocene
Integration (TMHI) will extend the CCSM3 6 ka integration forward in time 2400 years, with
values of orbital parameters, solar irradiance, and greenhouse gas concentrations varying
transiently from their values at 6 ka to their values at 3.6 ka. This long transient integration will
build on the CCSM3 6 ka equilibrium studies to be undertaken in the IPCC and PMIP2
frameworks, and will help to test non-linearities in the climate system induced by century- to
millennial-scale transient changes in external forcing. For computational efficiency, CCSM3 will
be run at T31x3 resolution. The TMHI will be initialized from the results of the CCSM3 6 ka
integration (with interactive vegetation), with 100 years allowed for equilibration after the
transition from T42x1 to T31x3 resolution. Because atmosphere-vegetation feedbacks have been
proposed as a key element of mid-Holocene climate variations, interactive vegetation will be
used in the TMHI. The TMHI will be preceded by six shorter equilibrium integrations using
CAM/SOM (without interactive vegetation): 1780 AD, 3.6 ka, 4.2 ka, 4.8 ka, 5.4 ka, and 6.0 ka.
These integrations will be 50 years each and will test the sensitivity of the atmosphere and land
surface to the progression of forcings to be applied in the TMHI.
Deliverables. The end product will be a transient Holocene simulation available to the
community. The results will provide data for regional comparisons to the proxy record and a
basis for sensitivity simulations to explore mechanisms of temporal change during the Holocene.
A crosscutting approach to understanding extreme conditions in the Cretaceous
(Organizer: Chris Poulsen, University of Michigan, poulsen@umich.edu)
Scientific Merit. The nature of the Cretaceous climate has posed fundamental questions about
how the climate system works under extreme greenhouse conditions. The causes and character of
the extreme warmth, low meridional thermal gradient, warm continental interiors, accumulation
of widespread organic-rich sediments, and the terminal mass extinction remain largely
unresolved despite decades of study. Progress in our understanding of these problems over the
last 25 years can be linked to the evolution of climate modeling, from the early version of
NCAR’s CCM (Barron and Washington, 1982). With the recent development of the CCSM as an
earth system model, the opportunity exists to make scientific breakthroughs in many longstanding problems.





Mechanisms of Cretaceous climate change. What was the cause of the extreme
Cretaceous warmth? Can high CO2 levels account for the extreme conditions or are other
mechanisms (methane, vegetation, dynamical changes in heat transport, etc.) required?
What effect did paleogeography have on Cretaceous climate?
Heat transports and thermal gradients. What is the relationship between heat transports in
the atmosphere and oceans and low meridional temperature gradients in the Cretaceous?
Was there a tropical thermostat?
Ocean circulation, biogeochemistry, and OAEs. Was the Cretaceous ocean circulation
substantially different than modern? Were regions of the ocean susceptible to anoxia?
Warm continental interiors and Arctic Ocean. Is this still a problem? Does the addition of
ecosystem (vegetation) models and/or finer grid resolution ameliorate this problem?
K-T mass extinction. The terminal mass extinction is most commonly associated with a
bolide impact near the Yucatan Peninsula. What are the climatic/environmental
consequences of an impact and its aftermath?
Approach. To make scientific breakthroughs in these areas will require expertise from the
geological and climatological communities. We plan to involve both communities in the
development of key simulations that can be productively compared with aspects of the
geological record. We aim eventually to include important diagnostic parameters (carbon and
water isotopic ratios, vegetation) in the simulations to aid model-data comparisons. In addition,
we will organize and conduct model intercomparisons with other modeling groups to pinpoint
model-dependant aspects of the simulations and their consequences for model-data comparisons.
Deliverables. The final products of this project will be a suite of Cretaceous simulations
available to the scientific community; detailed sets of boundary conditions (bathymetry,
topography, vegetation, continental distributions, sea level, etc.) for the late Cretaceous; a
catalog of geological climate/environmental indicators for specific time slices in the late
Cretaceous; final reports (in scientific journals) summarizing the results of model-data and
model-model comparisons, as well as aspects of the Cretaceous climate dynamics and physics
and their sensitivity to important climatic factors (greenhouse gas concentrations, etc.).
A cross-time intercomparison of idealized CCSM simulations (Organizer: Henk Dijkstra,
Colorado State University, dijkstra@atmos.colostate.edu)
Summary: To identify climate changes during the Mesozoic and Cenozoic due to pure changes in
continental geometry using a suite of idealized simulations with CCSM3 and leveraged off of
existing paleo-modeling initiatives of the PaleoWG is proposed.
Approach. Equilibrium climate states will be calculated with the CCSM for time slices—present
using idealized boundary and forcing conditions, such as constant greenhouse gas
concentrations, fixed solar forcing, idealized bathymetry and simple vegetation distributions.
The choice of the different time slices will be based on the observational record, and some of
them will be chosen to bound intervals of strong climate change. An advantage of these
simulations is that they can be easily set-up and run (e.g., no complicated paleotopography is
needed) by different groups. The idea is to leverage off of existing research thrusts of members
of the PaleoWG and add value to those simulations by encouraging and enabling cross-time
comparisons.
Scientific Merit: Focus will be on the changes in the large-scale oceanic and atmospheric
circulation and on the physical mechanisms of how these changes are induced by continental
shifts. Of particular interest is the study of the changes in the global thermohaline circulation
(THC) due to the opening and closure of ocean gateways and changes in surface forcing (wind,
buoyancy). As changes in surface forcing are expected to be relatively limited, the effects of the
different interbasin exchanges of heat and salt on the THC can be unraveled. The proposed
equilibrium simulations will, in addition, serve as reference (and benchmark) solutions for
simulations with more realistic boundary and forcing conditions. In this way, they will help to
assess the sensitivity of past climate states to different physical processes (e.g., greenhouse gases
versus continental changes).
Deliverables. The end product will be an extensive model dataset of equilibrium climate states
for different continental geometries. These data will be made freely available to the community
and will serve as a baseline for comparison of paleo-simulations using CCSM.
Esther Brady
Bruce Briegleb
Gokhan Danabasoglu
Noah Diffenbaugh
Henk Dijkstra
Ian Eisenman
Peter Gent
Matthew Huber
Steve Jayne
Jeff Kiehl
Thomas Laepple
Jean-Francois Lamarque
Bill Large
Attendees
Natalie Mahowald
Carrie Morrill
Bob Oglesby
Bette Otto-Bliesner
Chris Poulsen
Zack Powell
Clint Rowe
Karen Shell
Cindy Shellito
Christine Shields
Sanj-ik Shin
Bob Tomas
Roderick van de Wal
Ann Winguth