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CHAPTER EIGHT
Relationship of U.S. Climate
Modeling to Other International
and National Efforts
T
he field of climate modeling has grown tremendously over the past several
decades, and much of that growth has occurred in the international commu-
nity. In the first Intergovernmental Panel on Climate Change (IPCC) report (IPCC,
1990), only three coupled ocean-atmosphere models were used for estimates of the
transient evolution of global temperature in response to changing greenhouse gases.
Those models were all from the United States (the Geophysical Fluid Dynamics Labo-
ratory [GFDL], the National Center for Atmospheric Research [NCAR], and the Goddard
Institute for Space Studies). Since that time the growth in climate modeling has been
substantial—for the IPCC Fourth Assessment Report in 2007, 23 models were used
from 11 countries around the world (Table 10.4 of the Fourth Assessment Report), and
even more will likely be used in the upcoming Fifth Assessment Report, scheduled
for completion in 2013. These include climate modeling centers in a wide range of
countries, including Canada, the United Kingdom, Germany, France, Norway, Russia,
Italy, China, Japan, Korea, and Australia. Computational resources associated with these
international centers have likewise grown, including facilities such as the Earth System
Simulator in Japan.1
INTERNATIONAL COORDINATION, ESPECIALLY AS IT RELATES TO
THE INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE
Systematic comparison of simulations using these models has proved highly ben-
eficial. Since the 1990s the leading climate modeling efforts around the world have
exchanged information and coordinated their efforts under the umbrella of the World
Climate Research Programme (WCRP), an activity of the World Meteorological Orga-
nization of the United Nations. A number of working groups have sought to facilitate
interactions and coordination of modeling activities. The Working Group on Numerical
Prediction (WGNE) has coordinated activities involving weather prediction models.
The Working Group on Seasonal to Interannual Prediction (WGSIP) has coordinated
1 http://www.jamstec.go.jp/esc/index.en.html (accessed October 11, 2012).
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efforts in developing and using coupled ocean-atmosphere models for seasonal to
interannual prediction, with its primary focus on the El Niño/Southern Oscillation
phenomenon. The Working Group on Coupled Modeling (WGCM) has coordinated
coupled ocean-atmosphere models that are primarily developed and used for the
study of decadal to centennial climate change projections. A subset of the WGCM, the
Working Group on Ocean Model Development, has fostered the development world-
wide of the ocean component of coupled models to improve the representation of
the ocean component of coupled models.
The community as a whole, under the aegis of the WGCM and the WGNE of WCRP, with
links to the International Geosphere-Biosphere Programme, comes to consensus on a
suite of experiments, which they agree would help advance scientific understanding.
The WGCM sponsors the Coupled Model Intercomparison Project (CMIP), a project that
seeks to foster and coordinate the design and execution of simulations using mod-
els around the world that are subjected to a common experimental protocol. Meehl
and Bony (2011), Stouffer et al. (2011), and Doblas-Reyes et al. (2011) describe the
current protocol and how it has evolved. All the major modeling groups participated
in defining the experiments and protocols and have agreed to the CMIP5 suite2 as a
sound basis for advancing the science of secular climate change, assessing decadal
predictability, and so forth (Taylor et al., 2012). The use of this common protocol is de-
signed to facilitate the comparison of the various models used. Model output is freely
available over the Web. The Program for Climate Model Diagnosis and Intercomparison
(PCMDI), sponsored through the U.S. Department of Energy, has played a key role in
archiving this model output and facilitating its wide public dissemination.
These common experiments have evolved significantly over the years. The first ex-
periments were performed in the early 1990s with atmosphere-only models as part
of the Atmospheric Model Intercomparison Project (AMIP). A key aspect of this early
effort that set the tone for future success was an emphasis on making model output
available for use by a wide community of users. This early AMIP effort then spawned a
number of model intercomparison projects, including an Ocean Model Intercompari-
son Project, the Paleoclimate Model Intercomparison Project, and the widely known
CMIP.
In addition to the output of such coordinated experiments, the various working
groups serve as important mechanisms for exchange of information and ideas among
modeling scientists around the world. U.S. scientists have benefited greatly from such
interactions. These working groups sponsor internationally coordinated experiments
2 Currently ongoing at the time of this report.
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Relationship of U.S. Climate Modeling to Other International and National Efforts
with climate models, diagnostic projects across models, and international workshops
to synthesize model results and foster increased understanding.
The intraseasonal to interannual community agrees on similar multimodel approaches
for seasonal forecasting, for example, through the WCRP WGSIP and its Climate System
Historical Forecast Project.3 A globally coordinated suite of experiments is then run,
and results are shared for a comparative study of model results.
The data archives that result from all of these coordinated campaigns have spawned
an entire new field of research in the interpretation of multimodel ensembles (e.g.,
Reichler and Kim, 2008; Santer et al., 2009), including studies of model genealogy and
cladistics (see, e.g., Masson and Knutti, 2011) and uncertainty quantification (Tebaldi
and Knutti, 2007). The data are stored in petabyte-scale (and soon exabyte-scale; see
Overpeck et al., 2011) distributed archives. Providing access to these data, especially
for users who may not be climate experts, is one of the primary challenges of the
decade.
Finding 8.1: U.S. climate modelers are extensively involved in internationally
coordinated activities, including the Coupled Modeling Intercomparison Project,
the IPCC, and a suite of observational and modeling programs that are designed
to advance climate models by improving processes-based understanding of im-
portant aspects of the climate system, such as clouds and their feedback on the
climate system.
Finding 8.2: The U.S. involvement in such international activities contributes sig-
nificantly to advances in U.S. climate modeling through leveraging international
resources that are applied to climate modeling.
Finding 8.3: Modeling intercomparison projects create vast amounts of data that
need to be curated, managed, made readily available, and analyzed.
INTERNATIONAL ACTIVITIES IN PROCESS-BASED STUDIES AND
OBSERVING SYSTEMS THAT CAN LEAD TO IMPROVED MODELS
Under the auspices of WCRP, there are also numerous international activities aimed
at testing the fidelity of model simulations of various specific physical processes, for
instance sea ice, carbon dioxide (CO2) fluxes from vegetated surfaces, aerosol trans-
port, or tropical cirrus clouds. Examples of such activities are the Global Atmospheric
System Study coordinated through the Global Energy and Water Cycle Experiment
3 http://www.wcrp-climate.org/wgsip/index.shtml (accessed October 11, 2012).
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(GEWEX), the GEWEX Atmospheric Boundary Layer Study, and the Year of Tropical
Convection (YOTC).
Typically, such activities evaluate how well a process is simulated by comparing a
relatively new data set with data from a suite of participating models (e.g., from a field
experiment, a ground observing network, or a satellite instrument). Participation is vol-
untary; usually a case leader will specify details of how the models are to be run (what
days, boundary conditions, what fields to output) and then process all the model
output for direct comparison with the observations.
This process is rarely as straightforward as it may appear. A focus on a single process
(e.g., cirrus microphysics) requires other related processes to be constrained (e.g.,
cumulonimbus convection that first creates the cirrus) using observations or a best
guess at the related meteorology; often the model results themselves suggest how to
better do the comparison. An international intercomparison leverages the effort in-
volved in setting up both the observations and the modeling protocol; most modeling
groups can participate with relatively minor effort once the case is defined, and they
get valuable analysis of their results essentially for free.
Many recent U.S.-led field experiments have from the start been designed in part
for such an intercomparison. A partial list of such projects includes DYCOMS-II (the
Second Dynamics and Chemistry of Marine Stratocumulus field study); the Rain in
Cumulus over Ocean project, sponsored by the National Science Foundation (NSF); the
North American Monsoon Experiment (NAME); the Tropical Warm Pool International
Cloud Experiment; and the VAMOS Ocean-Cloud-Atmosphere-Land Study, a part of
the Variability of the American Monsoon Systems (VAMOS) project. Intercomparisons
have also been based around observation networks such as the Atmospheric Radia-
tion Measurement sites, the AERONET aerosol monitoring network (AEROsol robotic
NETwork), or the AMERIFLUX array of CO2 monitoring sites, or using new satellite data
sets (e.g., the CFMIP Observation Simulator Package, within the Cloud Feedback Model
Intercomparison Project [CFMIP] project).
U.S. funding agencies have supported these types of projects (e.g., as part of the
NAME and VOCALS science plans) because of their potential for speeding the pace at
which new observations are used to test and improve process representation in mod-
els. In some cases (e.g., National Oceanic and Atmospheric Administration/NSF Climate
Process Teams), U.S. climate modeling groups such as GFDL, NCAR, and the National
Centers for Environmental Prediction (NCEP) also have received dedicated funding to
aggressively use such intercomparisons to improve their models.
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Relationship of U.S. Climate Modeling to Other International and National Efforts
Model intercomparisons allow modelers to see weaknesses of their simulations in
focused settings, and also to see whether other parameterization approaches clearly
work better. They are only one part of the road to actual model improvement because
different process parameterizations can strongly interact so that a change in one
parameterization (e.g., cumulus convection) may not have the same effect on overall
results when applied to different climate models. However, leading modeling groups
such as GFDL and NCAR in the United States and the U.K. Met Office, Max Planck
Institute, and the European Centre for Medium-Range Weather Forecasts in Europe are
typically participating in many intercomparisons at any one time. Their model devel-
opment teams see great merit in this approach. NCEP has participated less, perhaps
because of a lack of available manpower. Overall, the committee’s assessment is that
voluntary process-oriented international intercomparisons greatly benefit U.S. climate
models rather than being a distracting drain on resources.
Finding 8.4: International model intercomparison projects have proven to be
effective mechanisms for advancing climate models because they leverage the
effort involved in setting up both the observations and the modeling protocols
used for testing, and they allow modelers to see weaknesses of their simulations
in focused settings.
CURRENT CMIP/IPCC EFFORTS
In support of the Fifth Assessment Report of the IPCC, the CMIP has established an
extensive suite of common modeling experiments that many centers around the
world are executing. A goal is to make the output from such experiments widely
and easily available so that scientists from around the world can analyze that model
output in time for the results of such analyses to inform the Fifth Assessment Report
of the IPCC (as of this writing, any paper that will be cited in the next IPCC report must
be submitted by July 31, 2012). However, the CMIP archive will be available for many
years to come, so that additional studies using this archive will likely occur well after
the IPCC. The previous (CMIP3) suite of simulations from 2005-2006 had been used in
595 publications as of January 20124 and is still heavily utilized. There are more than
6,700 registered users of the CMIP3 archive, with new users continually registering for
access; data are being downloaded from the archive at a rate of approximately 160
4 http://www-pcmdi.llnl.gov/ipcc/subproject_publications.php (accessed October 11, 2012).
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TB per year, with over 1 PB of data downloaded since the start of the CMIP3 project in
2005, corresponding to 3 million files.5
The CMIP activity has evolved from common experiments using models of just the
atmosphere to models of the full Earth system, including oceans, interactive aerosols,
and biogeochemical cycles. As described below, the current suite of CMIP experi-
ments involves models of differing levels of complexity; these span a range from
atmosphere-only models to more comprehensive coupled ocean-atmosphere mod-
els that include representations of ecosystems and various biogeochemical cycles,
including the carbon cycle. In addition to model comprehensiveness, the suite of
experiments conducted under CMIP has grown in diversity over the years. While the
initial protocols consisted of very simple, idealized experiments, the full protocol for
CMIP56 is extremely complex, entailing many thousands of years of model simulations
(note, however, that there are several tiers of simulations of different priority levels,
of which the highest-priority tier is less computationally demanding, and there is no
requirement for even a leading modeling center to perform all requested simulations).
This allows a much fuller examination of simulations but also entails significant costs.
This general issue is discussed below. The experimental protocol entails designs for
both long-term and near-term climate change predictions and projections, as well as
a focused effort on evaluating the role of biogeochemical cycles and changes in the
climate system and their potential future change. The model output from this archive
is used to investigate a host of issues. These range from detailed analyses of the physi-
cal processes that operate in models in order to assess their credibility, to using this
model output to assess the impact of projected climate change for various regions to
estimate climate vulnerability and adaptation.7
Finding 8.5: CMIP outputs, including model outputs from models outside the
United States, are a valuable resource for a wide range of activities, including
estimating climate change impacts and adaptation planning.
BENEFITS AND COSTS OF INTERNATIONAL COLLABORATIVE EFFORTS
One of the fundamental challenges in the use of climate models for projections of
future change is the very limited understanding of the uncertainties embedded within
5 Karl Taylor, PCMDI, personal communication, 2011.
6 http://cmip-pcmdi.llnl.gov/cmip5/experiment_design.html?submenuheader=1 (accessed October
11, 2012).
7 See, for example, http://www-pcmdi.llnl.gov/ipcc/diagnostic_subprojects.php for a list of projects
using CMIP3 output (accessed October 11, 2012).
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Relationship of U.S. Climate Modeling to Other International and National Efforts
any single model projection. The construction and use of a climate model represents
a series of choices on many topics, including physical parameterizations and scenarios
of future changes in emissions of radiatively active atmospheric constituents. Each of
these is highly uncertain, and yet climate modelers are constrained in our choices due
to various resource limitations. Thus, a projection based upon one model represents a
single point within a very large parameter space.
A more robust assessment of future climate change arises with fuller coverage of this
parameter space of uncertainty in model formulation and scenarios of future radia-
tive forcing changes. Thus, many recent assessments of future climate change draw
not only on the output of a single simulation, but also on the full suite of possible
outcomes as drawn from the archives of past CMIP experiments. Although this assess-
ment is still very far from a satisfactory estimate of the full range of possible future
climates, it represents an invaluable guide. Thus, participation of modeling centers
around the world in the CMIP suites of experiments contribute both to better esti-
mates of future climate change and to model development and improvement. Such
international coordination and exchange of information provide a vital exchange of
ideas and techniques that improve climate modeling in the United States and around
the world.
Model intercomparison programs, such as CMIP, provide timelines for model develop-
ment and the execution of coordinated experiments. The process of climate model de-
velopment is one in which there are often not obvious ending points. Models can be
changed in an almost continuous fashion, with each change producing new simula-
tions that must be carefully evaluated. This process often has no natural closure points
and generally becomes longer as models become more comprehensive. However,
participation in activities such as CMIP can provide clear schedules for the conclusion
of such model development processes that can be used very effectively by modeling
centers to define completion of the model development cycle. In fact, many of the
model development cycles at centers around the world are now timed to the schedule
of IPCC assessment reports. Although such a schedule can be a benefit by providing
firm deadlines for concluding model development cycles, it can also be a serious detri-
ment by artificially constraining the model development process and placing enor-
mous strain on already underresourced model development efforts.
Participation in CMIP-like activities can also produce a healthy sense of competition
among national and international modeling centers. The output from such coordi-
nated experiments is routinely made available to researchers around the world, who
provide evaluations and comparison among the models. Such activities often show
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which models are among the world’s elite, and this can produce a very positive feed-
back in the ongoing model development process.
Finding 8.6: There are many benefits to the participation of U.S. climate models
in the CMIP process, including defining timelines for model development and
creating healthy competition among modeling centers.
However, there are also costs to participation in such efforts. Bringing closure to the
model development process on any timeline is a difficult task, especially because
modeling centers want to have the best possible physics and numerics in their mod-
els. These typically involve recently developed physical parameterizations based on
new observational and theoretical research, and their behavior in complex models can
be difficult to predict. The often unpredictable nature of newly developed model pro-
cesses can create an environment of intense pressure to finalize a model with simula-
tion characteristics that are superior to its predecessor model and to competitor mod-
els. This pressure can be exacerbated by the CMIP-derived timelines for coordinated
model experiments and can lead to model decisions being heavily influenced by
artificial time pressures rather than the best possible science. The effect of this process
can lead to “burn out” among those most deeply involved in the model development
process.
In addition, as described in Chapter 7, model development is an enormous task re-
quiring substantial human and computational resources, yet the vast majority of this
effort, including the production of model runs for CMIP activities, does not lead to
peer-reviewed publications. Because such publications are usually the metric by which
scientists are evaluated, participation in the model development process can some-
times hurt a young scientist’s career, at least in the short term as measured by publica-
tions. As noted in Chapter 7, a culture of coauthorship with model developers and the
careful citation of model development papers could change this.
The benefits of CMIP-related model experimentation also have to be weighed against
some lost opportunity costs, especially for the scientists directly involved in model de-
velopment. Fundamental advances and new findings are often the result of research
that is curiosity driven or inspired by an idea or question. The more time that a scien-
tist devotes to large-scale science, as embodied by programs and activities such as
CMIP, the less time is available for small-scale or curiosity-driven research. In addition,
the full suite of CMIP simulations requires an enormous computational effort that can
consume a substantial fraction of the computational resources available to a modeling
center for a year or longer.
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Finding 8.7: One cost associated with the effort and time pressure of participat-
ing in the CMIP/IPCC is the reduction of time and computational resources that
model developers have to devote to fundamental research that produces results
on longer time scales.
THE WAY FORWARD
For decades, the United States has sustained the largest climate research enterprise
in the world. The first climate simulation model was developed in the United States,
and the United States continues to support a diverse range of approaches to better
understanding future extreme weather and climate on all space and time scales. A
robust international climate modeling community has evolved, including state-of-the-
art efforts in Europe in regional and global modeling, as well as growing efforts in Asia
supported by large new investments in computing. This has led to Earth system mod-
els that simulate the current climate more accurately and comprehensively than in the
past, and the application of these and finer-scale, more specialized regional models
to many societal and scientific problems, although model-related uncertainties in
future climate projections remain substantial. In response to IPCC-type assessments,
the international community, led by the United States, has pioneered mechanisms for
distributing an ever-growing set of standardized outputs from international suites of
models. These are a major resource for the U.S. climate community.
On balance the CMIP activities are a clear positive for U.S. climate modeling activities.
These activities help to keep U.S. models and model-based research at the leading
edge of activity around the world. However, the costs associated with these activities
imply a need for balance among the various sorts of activities in modeling centers
to achieve some optimal outcome, especially in light of the rapidly growing scope of
CMIP experiments. These activities are important enough to be considered an ex-
pected part of the model development process and thus warrant sustained support.
This includes support for participation in the CMIP/IPCC activities and for the systems
to archive model output in a way that is freely and easily available to users. Such sup-
port would include (a) software specialists for the development and maintenance of
data storage and distribution systems that meet the needs of the climate community
and (b) the required hardware, including storage, transmission, and analysis capabili-
ties. This support would likely include resources at the modeling centers that run the
climate model simulations, as well as support for a centralized capability that coordi-
nates this activity within the United States.
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In addition, it is anticipated that over the coming decades climate change assessments
will be conducted in the United States that focus on both national and regional scales,
with an increasing emphasis on adaptation. The utility of such assessments is greatly
enhanced through the active use of climate models both from the United States and
from institutions around the world. The utility of a large number of models enhances
the credibility of any such assessments by providing the potential for an improved
assessment of the uncertainty of climate change projections. The U.S. participation in
CMIP and related activities greatly facilitates the use of multimodel ensembles incor-
porating U.S. and international models.
U.S. modeling centers should be encouraged to participate in international activities,
including the execution of internationally coordinated numerical experiments such as
CMIP, and to make that data publicly available. In addition, there should be sustained
support and encouragement for the participation of U.S. scientists in international
activities in support of climate modeling and the use of climate models, such as those
organized by WCRP, and for the systems to archive model output from leading U.S.
climate models, and to make that output freely and easily accessible (this is discussed
in Chapter 10).
Recommendation 8.1: To advance in the next 10-20 years, U.S. climate modeling
efforts should continue to strive for a suitable balance among and support for
• the application of current generation models to support climate research
activities, as well as national and international projects such as CMIP/
IPCC;
• near-term development activities that lead to incremental but meaning-
ful improvements in models and their predictions; and
• the investment of resources to conduct and capitalize on long-lead-time
research that offers the potential for more fundamental and transforma-
tional advances in climate modeling.
Recommendation 8.2: The United States should continue to support the partici-
pation of U.S. scientists and institutions in international activities, such as model
intercomparisons, including support for systems to archive model output, be-
cause such activities have proven effective in robustly addressing user needs for
climate information and for advancing U.S. climate models.
Recommendation 8.3: To enhance their robustness, national and regional cli-
mate change/adaptation assessments should incorporate projections from lead-
ing international climate models as well as those developed in the United States.
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