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EXECUTIVE SUMMARY
The climate of Earth is evolving, and understanding this change can
help us to be prepared to deal with the consequences for water resources,
agriculture, energy demand and supply, health, recreation, and ecosystems
(IPCC, 2001b). Climate changes can be initiated by external factors forcing
the climate system. These climate forcings include natural factors such as
changes in energy flux from the Sun, variations in the Earth's orbit, and
volcanic eruptions, as well as human activities, such as production of
greenhouse gases and aerosols and modification of the land surface. Over
the next century it is likely that forcing of the climate system by human
activities will greatly exceed changes in forcing caused by natural events.
Processes in the climate system that can either amplify or damp the
system's response to changed forcings are known as feedbacks. According
to estimates generated by current climate models, more than half of the
warming expected in response to human activities will arise from feedback
mechanisms internal to the climate system, and less than half will be a direct
response to external factors that directly force changes in the climate system
(NRC, 200 la). Moreover, a substantial part of the uncertainty in projections
of future climates is attributed to inadequate understanding of feedback
processes internal to the natural climate system (IPCC, 2001a). Therefore, it
is of central importance to understand, model, and monitor climate feedback
processes.
At the request of the interagency U.S. Global Change Research
Program, the Panel on Climate Change Feedbacks of the Climate Research
Committee was given the following tasks:
1. Characterize the uncertainty associated with climate change feedbacks
that are important for projecting the evolution of Earth's climate over the
next 100 years, and
2. Define a research strategy to reduce the uncertainty associated with
these feedbacks, particularly for those feedbacks that are likely to be
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UNDERSTANDING CLIAl4TE CHANGE FEEDBACKS
important and for which there appears to be significant potential for
scientific progress.
The study looks at what is known and not known about climate change
feedbacks and seeks to identify the climate feedback processes most in need
of improved understanding. This report suggests an approach by which
progress toward better understanding of climate feedback processes can be
measured and accelerated. Such improvements will serve policy makers as
they deliberate on climate-related decisions.
THE NEED FOR CLIMATE FEEDBACKS RESEARCH
In recent years the principal way scientists have sought to understand
changes in climate has been to simulate the record of global mean surface
temperature over the period of the instrumental temperature record from
about 1860 to the present (e.g., Hansen et al., 1981; IPCC, 2001a). Such
comparisons allow testing of our understanding of climate forcing, climate
sensitivity, and heat storage in an integrated global sense, but they are
imperfect. A second approach has been to make model-to-model
comparisons of climate simulations, and this has revealed significant
differences and similarities between models (Gates et al., 1998; Covey et al.,
in press). At this point in time this Panel believes that an effort to refine our
understanding of the key climate feedback processes and improve their
treatment in models used to project future climate scenarios is an effective
way forward in the quest to better understand how climate may evolve in the
future in response to natural and human-induced forcings. An appropriate
strategy for accomplishing this is to make more vigorous comparisons of
models with data and to focus particularly on observational tests of how well
models simulate key feedback processes. A key finding of this report is that
an enhanced research effort is needed to better observe, understand,
and model key climate feedback processes.
Key Observations Needed to Monitor and Understand Climate
Feedbacks
Previous reports by the National Research Council (NRC) have
emphasized the need for stable, accurate, long-term measurements of climate
variables (NRC, 1999a). Because of their important role in determining the
magnitude of climate change, additional variables must be monitored to
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EXECUTIVE SUMMARY
assess the role of feedback processes in climate change. Observation of
feedback processes is needed to better understand these processes, to
identify the causes contributing to observed climate changes as they occur,
and to test and improve simulations of climate. As described in the body of
the report, some variables key to feedback processes are not being
adequately monitored on a long-term basis. To understand and monitor
climate feedback processes requires good observations of the basic state of
the climate system, plus some additional variables that monitor specific
feedback processes.
Recommendation:
An integrated global climate monitoring system must include
observation of key climate feedback processes. Stable, accurate, long-
term measurements should be made of the variables that characterize
climate feedback processes.
To better understand and model climate feedback processes and to
interpret the role of feedbacks in climate changes that may develop in the
future, research efforts must monitor not only traditional climate variables
like temperature and precipitation but also variables that define the feedback
processes. Key long-term measurements that are needed to monitor and
understand climate change feedbacks are:
temperature, humidity, precipitation, and wind;
radiation budget at the top of the atmosphere and at the surface
global cloud and aerosol distributions and properties;
temperature and salinity of the upper ocean and of other portions of the
ocean that affect interannual to decadal climate change;
· terrestrial vegetation, soil moisture, snow extent and its properties, and
sea-ice distribution and thickness; and
· atmospheric C02, 03, O2-N2 ratio, and ocean color.
Several of these variables are being monitored for purposes of weather
analysis and prediction, but none adequately for climate purposes.
As recommended in several previous NRC reports there are advantages
to collecting these observations in the context of an integrated global climate
monitoring system (e.g., NRC, 1999a). Such a system is required for other
aspects of climate change research and applications not addressed in this
report, including for climate change attribution and detection, and for
providing a broad range of climate services (NRC, 2001 e). The collection
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UNDERSTANDING CLI~4TE CHANGE FEEDBACKS
and validation of all these datasets will require international collaboration
and cooperation among U.S. agencies.
In addition to using the observations as climate data records (see NRC,
1999a and 2000b, for a description of the characteristics of climate data
records), they should also be incorporated in 4-D data assimilation. Subject
to important caveats, the resulting integrated datasets will be suitable for
model initialization, model validation, and for multivariate diagnostic
studies on climate time scales.
Evaluating Progress in Understanding Climate Feedbacks
To ensure focused research and to measure progress, we need
observable climate metrics that define the feedbacks sufficiently both to
understand the key processes and to test and improve the simulation of these
processes in climate models. A climate feedback is a set of numbers that can
be derived from both observations and model output, and that characterizes
the nature of a climate feedback process. It is important that this
characterization be useful for better understanding the feedback process and
for assessing the accuracy of its simulation in climate models. Metrics can
use observed past climate trends, but should also use the variability of
climate on other time scales that are better observed and where forcing is
larger, such as seasonal and diurnal time scales. Good metrics must be
focused on objectives that will increase confidence in our ability to usefully
model climate feedback processes, and must be defined in terms of variables
that are well observed. They should evolve as our understanding and
observations improve.
Recommendation:
Both global and regional metrics that focus on feedback processes
responsible for climate sensitivity should be used to more rigorously test
understanding of feedback processes and their simulation in climate
models.
A good set of diagnostic tests for climate feedback processes should
capture the covariation or coupling between the system's components. If
effectively employed, these metrics can be an essential tool to help organize
and stratify diagnostic analyses, as well as to relate model simulations to the
fundamental aspects of observed phenomena. Successful reproduction of
these observed metrics by climate models will not guarantee that climate
models will give reliable projections of future climates, but testing climate
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EXECUTIVE SUMAL1RY
models against a large set of carefully considered metrics is an effective way
forward. They can also be a useful tool for observing the evolution of the
climate system and thus make important contributions to the field of climate
change detection and attribution. The set of metrics will evolve with time as
understanding and simulation of the climate system evolve and improve.
A few examples of possible climate feedback metrics can be given. At
the global or regional scale, the covariability of sea-surface temperature,
clouds, upper-tropospheric water vapor, the vertical profile of atmospheric
temperature, and other observations can be studied over a variety of time
scales, including well-observed natural scales of variability, such as the
diurnal, annual, and El Nino Southern Oscillation (ENSO) signals. These
covariance metrics should then be applied to model simulations to pinpoint
those aspects of the models that appear to represent nature accurately and
those that require further work. A metric that might enable improvement of
feedback processes over land would be observed diurnal and seasonal
variations of temperature, clouds, precipitation, and soil moisture. Many
other possible regional metrics for testing the simulation of climate system
feedbacks can be envisioned, and some are discussed further in Chapters 2
through 8.
A step toward developing widely accepted metrics to evaluate feedback
processes could be for the relevant agencies to organize a workshop or series
of workshops to define a set of observational and diagnostic metrics that can
be used to test understanding and modeling of climate feedback processes.
These workshops could include scientists engaged in observation, diagnosis,
and modeling of climate and climate processes.
Climate Modeling and Analysis for Climate Feedbacks Research
To test understanding and modeling of climate feedback processes using
a set of climate feedback metrics requires a substantial infrastructure and a
proportionate intellectual effort. To undertake a rigorous program of testing
the simulation of climate feedback processes in our most capable climate
models requires that the observations and the expertise in applying them be
brought together with the modeling capability. Previous NRC reports have
stated the need for capable and effective climate modeling facilities (NRC
1998a, 2001 c), and have recommended the development of centralized
operations for climate predictions and ozone assessments (NRC, 2001c). To
advance understanding of climate change feedbacks and their role in climate
sensitivity it is essential that U.S. climate modeling facilities also have the
capability and mandate to test climate feedback processes and their
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UNDERSTANDING CLIMA TE CHANGE FEEDBACKS
interactions using the most discriminating observational constraints. Within
the context of climate feedback processes, this will also address the need for
uniform criteria with which to judge climate models (NRC, 2001 c).
Recommendation:
Climate modeling facilities in the United States must be given the
capability and mandate to test understanding and simulation of climate
feedback processes and their interactions using the best observational
constraints on climate feedback processes. Periodic assessment of the
progress being made by major climate models should be conducted! to
evaluate the ability of these modlels to simulate the processes underlying
key climate system feedbacks.
One interdisciplinary coordination challenge is to lessen the separation
between U.S. observational and modeling research (NRC, 2001 c).
Representation of processes related to climate feedbacks in global climate
models is a complex and challenging undertaking, which often proceeds
without adequate connection to the developing observational basis. It is also
difficult for the observational community, which tends to focus on the
technical aspects of data collection and analysis, to find the time and
resources to assist in the development of Earth system models. While
observations are used to test the climatological statistics derived from
climate simulations, more attention needs to be given to using data to
rigorously test the simulation of feedback processes in these models and
their role in determining climate sensitivity.
Another opportunity to encourage progress in climate feedbacks
research is to reduce the separation between operational numerical weather
prediction centers and climate research centers in the United States. Many
climate feedback processes operate on time scales short enough to be tested
effectively by comparing numerical weather forecasts with instantaneous
measurements of cloud properties, humidity, or other variables that
characterize the fast feedback processes in the climate system. Similar use
can be made of seasonal forecasts, which bring slower feedback processes
into play. Systematic biases in seasonal forecasts of climate often reflect
problems with the treatment of climate feedback processes in the forecast
models.
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EXECUTIVE SUMMARY
PRIORITIES IN CLIMATE FEEDBACK RESEARCH
This report reviews the scientific understanding of key feedback
processes in the climate system and suggests research activities that will
improve our understanding of these processes and our ability to model them
effectively in global climate models. In selecting the priority feedbacks, the
following criteria were applied:
.
the expectation that the feedback process will have a significant effect
on the magnitude, timing, or spatial structure of the climate response to
human-induced climate forcing during the next century;
the likely magnitude of the uncertainty of the effect of the feedback
process; and
the probability that a well-focused research effort could over the next
several years significantly enhance our understanding of and ability to
characterize and perhaps quantify the uncertainties associated with the
feedback process.
In addition to these criteria, discussion is limited to feedback processes
that are likely to have large-scale effects that would appear in global
averages or averages over large areas of at least continental scale. Better
knowledge on these large scales should translate into better understanding
on smaller scales, but additional uncertainties in local climate arise from
local winds, ocean currents, and geography that are not addressed here.
In studying this problem and preparing this report the Panel found that
the scientific understanding, observations, and models necessary to
understand feedback processes and climate sensitivity overlap significantly
with understanding, observing, and modeling climate forcing. Because both
factors are changing over time, the transient response of the climate system
to gradually increasing forcing must also be considered. Partly for these
reasons this report takes a broad view of climate feedback processes and
climate sensitivity. It groups feedback processes into three categories: (1)
those that primarily affect the magnitude of climate change, (2) those that
primarily affect the rate or timing of climate change; and (3) those that
primarily affect the spatial patterns of climate change. These categories are
also helpful for promoting public understanding of the importance of these
processes because they translate into questions like: "How big or important
will climate change be?" "How rapidly will climate change?" and "How
will climate change in my area?"
The Panel has identified the following key climatic processes or closely
related phenomena that it judges to be high-priority research areas, based on
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UNDERSTANDING CLIMATECHANG~FEEDsACKS
the potential contribution to understanding climate evolution over the next
100 years and the potential for rapid scientific progress. The priorities are
organized into three categories based on whether their most important
effects are on the magnitude, timing, or spatial structure of climate change.
More detail supporting these priorities can be found in the body of the
report.
Feedbacks that primarily affect the magnitude of climate change
Cloud, water vapor, and lapse rate feedbacks
Ice albedo feedback
Biogeochemical feedbacks and the carbon cycle
Atmospheric chemical feedbacks
Feedbacks that primarily affect the transient response of climate
· Ocean heat uptake and circulation feedbacks
3. Feedbacks that primarily influence the pattern of climate change
Land hydrology and vegetation feedbacks
· Natural modes of climate system variability
Over the long term all these areas stand to make valuable contributions
to understanding climate change. For the near term the two most important
areas are (1) cloud, water vapor, and lapse rate feedback and (2) ice-albedo
feedbacks, both of which primarily affect the magnitude of climate change.
Feedbacks That Primarily Affect the Magnitude of Climate Change
Feedbacks that primarily affect global climate sensitivity
Cloud, water vapor, and lapse rate feedbacks as a group and ice-albedo
feedback are the feedback processes that seem most important in
determining the global mean climate sensitivity.
Cloud, Water Vapor, and Lapse Rate Feedbacks
Cloud feedback is one of the key uncertainties in projections of future
climates, and is responsible for a large fraction of the model-to-model
variation in climate sensitivity. Significant uncertainties remain in water
vapor and lapse rate feedback, but these are closely coupled to cloud
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EXECUTIVE SUMMARY
9
processes, so we have grouped them together. It is not known whether cloud
feedback will increase or decrease global warming, let alone its magnitude.
An accelerated and focused effort to test the simulation of cloud, water
vapor, and lapse rate feedbacks in climate models, and their role in climate
sensitivity is needed. Such an effort is particularly appropriate now because
new climate models that predict cloud properties show a large range of cloud
feedback strength, new satellite and surface-based measurements exist to test
cloud simulations, and cloud-resolving models have emerged as a tool for
understanding the interaction of clouds, water vapor, and lapse rate.
Effective synergism among efforts to diagnose observations, to model cloud
systems, and to model the global climate is essential. A set of observable
metrics should be defined and used to test our understanding of cloud, water
vapor, and lapse rate feedbacks. Because of its large contribution to current
uncertainty estimates and the potential to make significant progress in the
near term, the Panel feels that cloud, water vapor, and lapse rate feedback is
the highest priority at this time.
Ice Albedo Feedback
Ice and snow in high latitudes, and in particular sea ice, are important
contributors to climate sensitivity through ice albedo feedback, but the
magnitude of this feedback remains uncertain. Ice albedo feedback in polar
regions is coupled strongly to polar cloud processes and ocean heat
transport. Improvements are needed in the parameterization of sea-ice
growth, associated heat and freshwater fluxes, surface albedo variations, and
polar clouds. Better observations of polar ice distributions and associated
atmospheric and oceanic properties is needed. Systematic global
observations of sea-ice thickness, polar clouds, and the surface albedo in ice-
covered areas are especially important, but a system to make ice thickness
measurements is as yet unavailable. Further development and distribution of
satellite and in situ datasets describing variations of polar ice and polar
clouds should be a priority.
Processes That Feed Back on Climate Forcings
As the climate changes, temperature, precipitation, and circulation
changes are likely to change how the climate system deals with the
greenhouse gases, aerosols, and surface modifications produced by humans,
and this will affect the climate forcing. It is likely that climate change will
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UNDERSTANDING CLIMATE CHANGE FEEDBACKS
evoke natural responses in the climate system that will magnify or mute
human-produced climate forcing through alterations in greenhouse gases and
aerosols.
Biogeochemical Feedbacks and the Carbon Cycle
The global carbon and sulfur cycles contain potentially important
feedback processes. There are, however, major gaps in understanding. No
definitive explanation has been given for the apparent vast uptake of CO2 by
the terrestrial biosphere, and no confident prediction can be given of future
biological uptake or release of CO2, particularly over the long term. Few
observations are available to guide the necessary scaling of vegetation-
climate feedbacks from the scale of an individual leaf to a landscape mosaic
of vegetation and soils. In the marine realm the strengths of a wide variety of
potential feedback mechanisms related to CO2 uptake and release of
dimethylsulfide are yet to be determined.
Research into carbon uptake by the land and ocean as outlined in the
U.S. Carbon Cycle Plan (Sarmiento and Wofsy, 1999) and North American
Carbon Program (Wofsy and Harriss, 2002) should be undertaken to
characterize and reduce the uncertainty associated with carbon uptake
feedbacks. The goal is to characterize key atmospheric, biospheric, and
oceanic processes that influence the abundance of CO2, with special
attention given to observations that define large-scale, decadal, and longer-
term sources and sinks, and to define the influences on these processes of
climate, land use, and socioeconomic policies. A high priority is to
understand the nature of the Northern Hemisphere carbon sink, so that the
evolution of this sink and its relationship to the evolving climate can be
better understood. Research outlined in the Surface Ocean Lower
Atmosphere Study Science Plan (Liss et al., 2002) will improve
understanding of climate-dimethylsulfide feedbacks.
Atmospheric Chemical Feedbacks
Improved understanding of atmospheric chemistry feedbacks is
important for producing fixture climate projections, for understanding the
relationship between measured concentrations of greenhouse gases and their
emissions, and for formulating control strategies. Both tropospheric and
stratospheric chemical processes interact with temperature, humidity,
circulation, and air composition changes and may in turn affect Earth's
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EXECUTIVE SUMAL4RY
11
radiative balance. More research on atmospheric chemical feedback
processes is required, with the goal of representing these processes more
comprehensively in projections of future climate.
The physical and chemical processing of aerosols and trace gases in the
atmosphere, the dependence of these processes on climate, and the influence
of climate-chemical interactions on the optical properties of aerosols must be
elucidated. A more complete understanding of the emissions, atmospheric
burden, final sinks, and interactions of carbonaceous and other aerosols with
clouds and the hydrologic cycle needs to be developed. Intensive regional
measurement campaigns (ground-based, airborne, satellite) should be
conducted that are designed from the start with guidance from global
aerosols models so that the improved knowledge of the processes can be
directly applied in the predictive models that are used to assess future
climate change scenarios.
The key processes that control the abundance of tropospheric ozone and
its interactions with climate change also need to be better understood,
including but not limited to stratospheric influx; natural and anthropogenic
emissions of precursor species such as NOx, CO, and volatile organic
carbon; the net export of ozone produced in biomass burning and urban
plumes; the loss of ozone at the surface, and the dependence of all these
processes on climate change. The chemical feedbacks that can lead to
changes in the atmospheric lifetime of CH4 also need to be identified and
quantified.
Feedbacks That Primarily Affect the Transient Response of Climate
Ocean Heat Uptake arid Circulation Feedbacks
Many climate models predict that the rate of warming over the next 30
years will be much larger than the rate of warming observed over the past
century. The rate of warming is important for its effect on human affairs and
natural ecology, but it is also very important in continuing efforts to
understand the relative roles of feedbacks, forcings, and heat storage in
setting the observed warming rate. These efforts are important both for
detection and attribution of climate change and for improving projections of
future climate. The transient response to changed climate forcing involves
important feedback processes, because the evolving climate may alter the
rate of heat uptake by the ocean through increased thermal stratification of
the ocean, or through the effect of changes in surface precipitation and
evaporation on ocean salinity and density.
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UNDERSTANDING CLITIC TE CHANGE FEEDBACKS
To better represent the exchange of heat and carbon dioxide at the air-
sea interface, physical representations of upper ocean processes need to be
improved in climate models based on experimental studies of the vertical
structure of temperature, absorption of solar radiation, and salinity
representative of different ocean environments, including high northern and
southern latitudes. Improved definition of the time-dependent temperature
and salinity distribution in the global ocean is essential, including the air-sea
fluxes of heat and freshwater. This will require full implementation of a
system with the capabilities of the current and planned ocean-observing
satellites, the Argo global array of profiling floats, the in situ tropical ocean
observation networks, and a strategy for monitoring key regions of the ocean
where deep-water formation occurs, such as the Labrador, Greenland-
Iceland-Norwegian, Weddell, and Ross Seas.
Feedbacks That Primarily Influence the Pattern of Climate Change
Although the change in global mean climate is important and in some
ways easier to project, regional changes are of great practical significance
and may provide important clues to understanding the climate system.
Land Hydrology and Vegetation Feedbacks
Feedback processes over land are critically important to understanding
the climate response over land and its effect on humans. Global climate
change may initiate local changes in hydrology and surface albedo that feed
back to produce larger or smaller local changes in temperature, precipitation,
evaporation, soil moisture, and vegetation. The responses of the hydrologic
and energy cycles over land play a critical role in determining the impacts of
climate change on water resources, carbon stocks, and agriculture, yet these
responses vary widely among different climate models. Basic processes such
as the response of the land-atmosphere system to diurnal variations of
insolation are poorly simulated in current climate models. The melting of
snow and ice and associated hydrologic and radiative consequences also
tend to be poorly simulated. Dynamic vegetation modeling is also in its very
early stages.
An integrated analysis is required of the diurnal and annual cycles of the
energy, water, and carbon budgets at the land-surface and through the
atmospheric boundary layer for different ecosystems and climatic regimes,
including managed ecosystems like irrigated cropland. This analysis aimed
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EXECUTIVE SUMA~1RY
13
at improving theoretical understanding and model parameterizations needs
to fully integrate land and atmosphere processes and use carefully designed
observational metrics to test modeled processes. These models must account
for time-varying land surface properties. Sustained multiyear observations of
terrestrial ecosystems, their functioning, and their role in the climate system
that will contribute to the development and improvement of process-oriented
vegetation models for use with climate models should be encouraged.
Natural Modes of Climate System Variability
Radiatively induced greenhouse warming is not the only effect of
greenhouse gas buildup. There is a growing body of evidence that suggests
that human activities may also be capable of changing the time-averaged
states of the natural modes of variability of the climate system, most notably,
the El Nino-Southern Oscillation (ENSO) and the high-latitude northern and
southern hemisphere annular modes. An understanding of these modes and
how they react to anthropogenic forcing is essential for detection and
attribution of global climate change and for interpreting the role of
feedbacks. In addition, the natural variability of these modes on a year-to-
year time scale provides a testbed for model parameterizations of feedbacks.
A tightly integrated effort is needed to close the major gaps in the
understanding and modeling of the relationships between natural modes of
climate variability and climate change. This effort should integrate data
acquisition, analysis, and modeling and should include interactive interfaces
among national and international programs that are pursuing seasonal
forecasting, climate change feedbacks research, climate change simulation,
and climate change detection and attribution (NRC, 2001d).
Chapter 1 provides introductory materials as context for understanding
the need for a national research strategy in climate feedbacks research.
Chapters 2 through 8 provide expanded discussions of the key climate
feedback processes that the Panel believes are most in need of study.
Chapter 9 summarizes the main recommendations from the chapters.
Each chapter is structured somewhat differently, in part because the
research needs are different in each area. However, each discussion is
intended to leave the reader with a sense of the key processes that are
important role in determining the climatic response to a greenhouse gas
forcing. Each discussion outlines some of the most important first steps that
should be taken to better characterize and hopefully reduce the uncertainty
associated with the various feedbacks. These steps include, in general terms,
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UNDERSTANDING CLITIC TE CHANGE FEEDBA CKS
the types of observations and metrics that can be used to improve both
understanding and model representations, as well as to test simulations.
Representative terms from entire chapter:
climate feedback
