**National Academy of Sciences' Statements on**

**Natural Variation and Multiple Climate Mechanisms**

**Abstracted by David Wojick, Ph.D.**

[**dwojick@climatechangedebate.org**](mailto:dwojick@climatechangedebate.org)

Statements on the importance of natural variation.

\"The evidence of natural variations in the climate system \-- which was
once assumed to be relatively stable \-- clearly reveals that climate
has changed, is changing, and will continue to do so with or without
anthropogenic influences.\" (Dec-Cen Variability, Summary)

\"Climate research on decade to century (\"dec-cen\") timescales is
relatively new. Only recently have we obtained sufficient
high-resolution paleoclimate records, and acquired faster computers and
improved models allowing long-term simulations, to examine past change
on these timescales. This research has led to genuinely novel insights,
most notably that the past assumption of a relatively stable climate
state on dec-cen timescales since the last glaciation is no longer a
viable tenant. The paleorecords reveal considerable variability
occurring over all timescales, while modeling and theoretical studies
indicate modes of internal and coupled variability driving variations
over dec-cen timescales as well.

\"Thus, dec-cen climate research is only at the beginning of its
learning curve, with dramatic findings appearing at an impressive rate.
In this area even the most fundamental scientific issues are evolving
rapidly. Adaptability to new directions and opportunities is therefore
imperative to advance understanding of climate variability and change on
these timescales.\" (Pathways, p. 129)

\"To date, we do not have a comprehensive inventory of global patterns,
nor do we understand their mechanisms, couplings, longevity, or full
implications for climate predictions.\" (Pathways, p. 140)

\"Climate fluctuates in the absence of any change in forcing, just as
weather fluctuates from day to day.\" (Climate Change Science, p. 7)

\"The paleoclimate record contains examples of sudden large climate
changes, at least on regional scales. Understanding these rapid changes
is a current research challenge\...\" (Climate Change Science, p. 7)

\"Climate is continually varying on time scales ranging from seasons to
the lifetime of Earth.\" (Climate Change Science, p. 8)

\"Predictions of global climate change will require major advances in
understanding and modeling\...\" (Climate Change Science, p. 23)

\"The climate record for the past 100,000 years clearly indicates that
the climate system has undergone periodic \-- and often extreme \--
shifts, sometimes in as little as a decade or less. The causes of abrupt
climate changes have not been clearly established, but the triggering of
events is likely to be the result of multiple natural processes.\"
(Abrupt Climate Change, NAP Description)

\"Recent scientific evidence shows that major and widespread climate
changes have occurred with startling speed.\" (Abrupt Climate Change, p.
1)

\"The new paradigm of an abruptly changing climatic system has been well
established by research over the last decade, but this new thinking is
little known and scarcely appreciated in the wider community of natural
and social scientists and policy-makers.\" (Abrupt Climate Change, p. 1)

\"That Earth\'s climate has changed and that it will continue to do so
is well appreciated. Nevertheless, there are persistent questions
regarding present climate trends on a decadal or centenary time
scale\...\" (Integration of Research and Satellites, p. 7)

\"The characteristic scales of climate variability demand long time
series in order to determine the critical processes as well as to
separate natural variability from anthropogenic influences. Unlike
weather forecasting, the interval between stimulus and response can be
years to centuries. With a high level of background variability, subtle
changes in Earth\'s climate system can be difficult to detect.\"
(Integration of Research and Satellites, p. 8)

\"Even without human influence, past records show that climate can be
expected to continue to undergo considerable change over decades to
centuries.\" (Dec-Cen Variability, NAP Description)

\"In 1990, the Intergovernmental Panel on Climate Change (IPCC) released
its monumental scientific assessment on climate change. \... One
significant gap involved our meager understanding and documentation of
natural variability in the Earth\'s climate system which provides a
context for evaluating the significance of human-induced changes.\"
(Dec-Cen Variability, Preface)

\"The climate change and variability that we experience will be a
commingling of the ever changing natural climate state with any
anthropogenic change. While we are ultimately interested in
understanding and predicting how climate will change, regardless of the
cause, an ability to differentiate anthropogenic change from natural
variability is fundamental to help guide policy decisions, treaty
negotiations, and adaptation versus mitigation strategies. Without a
clear understanding of how climate has changed naturally in the past,
and the mechanisms involved, our ability to interpret any future change
will be significantly confounded and our ability to predict future
change severely curtailed.\" (Dec-Cen Variability, Preface)

\"Understanding, predicting, and detecting future climate variability
and change is an immense problem; it will not be solved by anything
short of a well-focused coherent international scientific assault on the
problem.\" (Dec-Cen Variability, Preface)

\"Decadal-to-centennial climate variability is a subtle phenomenon.\"
(Dec-Cen Variability, Summary)

\"Climate is constantly changing and will continue to do so; we cannot
assume a stable climate system even in the absence of anthropogenic
influences.\" (Dec-Cen Variability, p. 124)

\"The complexity and interactive nature of the physical processes in the
atmosphere will usually defy prediction unless supplementary and
organizing principles can be found.\" (Atmospheric Sciences, Summary)

\"Climate is variable on time scales of seasons to centuries and over
longer time intervals.\" (Atmospheric Sciences, p. 272)

\"Climate variability refers to fluctuations in climate statistics with
reference to a very long time average. Thus, the average summer
temperature over a region may differ from year to year (interannual
variability) or may manifest a fluctuation that spans a number of years
(decadal variability). Natural climate variability has been observed on
a range of time scales from months to seasons to centuries and more.\"
(Atmospheric Sciences, p. 277)

\"\...significant progress in achieving the mission of characterizing
and predicting seasonal-to-century time-scale variability in climate,
including the role of human activities in forcing this variability, is
likely to take a decade or more. Some aspects of the problem will
continue to be intractable for considerably longer periods.\"
(Atmospheric Sciences, p. 278)

\"Large gaps in our knowledge of interannual and decade-to-century
natural variability hinder our ability to provide credible predictive
skill or to distinguish the role of human activities from natural
variability. Narrowing these uncertainties and applying our
understanding define the mission of climate and climate change research
and education for the twenty-first century.\" (Atmospheric Sciences, p.
278)

Statements on some of the principle natural climate variations and other
new mechanisms.

[Solar energy variation - direct and indirect.]{.underline}

\"Numerous possible indirect forcings associated with solar variability
have been suggested.\" (Climate Change Science, p. 14)

\"It has been suggested that the solar influence on climate can be
mediated directly by the influence of cosmic ray fluxes (modulated by
the solar wind\'s effect on the earth\'s magnetic field) on cloud
formation, but the magnitude or even sign of the effect has not yet been
quantified.\" (Abrupt Climate Change, p. 87)

\"A number of issues must be addressed regarding the potential role of
changes in solar radiation on climate. For instance, how representative
are proxies for solar activity (e.g., sunspots) of actual total solar
irradiance on dec-cen time scales? To what extent are dec-cen climate
changes related to changes in the sun\'s output?\" (Dec-Cen Variability,
Summary)

[Internal oscillations - PDO, NAO, AO, ENSO, etc.]{.underline}

\"The Northern Hemisphere as a whole experienced a slight cooling from
1946-75, and the cooling during that period was quite marked over the
eastern United States. The cause of this hiatus in the warming is still
under debate.\" (Climate Change Science, p. 16)

\"It has become clear that much of the accelerated global warming that
has occurred since the mid-1970s is associated with a low-frequency
relative phasing of two of the predominant patterns of the Northern
Hemisphere: the Pacific-North American (PNA) teleconnection and the
North Atlantic Oscillation (NAO). This phasing produces a spatial
warming pattern that is similar to one of the \"greenhouse
fingerprints\" predicted by models. This similarity raises several
questions regarding the interaction between anthropogenic change and
natural variability.\"

\"For example, is the accelerated warming the result of natural
variability caused by an unusually persistent coincidence of the NAO and
PNA, or the result of the modification of natural modes (patterns) by
anthropogenic changes in radiative forcing that alter the phasing, or
some combination of both of these? Likewise, there appears to have been
a distinct change in the character (frequency and severity) of El Nino
and La Nina events during this period of accelerated warming. Is this a
consequence of the influence of anthropogenic change on the dominant
natural modes of climate variability, or is it a natural, low-frequency
(dec-cen) modulation of a high-frequency (interannual) mode?\" (Dec-Cen
Variability, Summary)

[Ocean variation.]{.underline}

\"Key research areas for increasing our understanding of abrupt climate
change include:

\-- ocean circulation, especially related to deepwater formation;

\-- sea-ice transport and processes, particularly where they interact
with deepwater formation\...\" (Abrupt Climate Change, p. 3)

\"Climate models that are used to test leading hypotheses for abrupt
climate change, such as altered deep-ocean circulation, can only
partially simulate the size, speed, and extent of the large climatic
changes that have occurred. The failure to explain the climate record
fully suggests either that the proposed mechanisms being used to drive
these models are incomplete or that the models are not as sensitive to
abrupt change as is the natural environment. It is also of concern that
existing models do not accurately simulate warm climates of the past.\"
(Abrupt Climate Change, p. 4)

\"The possibly chaotic nature of THC \[thermohaline circulation\]
changes needs to be addressed.\" (Abrupt Climate Change, p. 106)

\"Analyses of historical records have revealed a number of interesting
cases of longer-period fluctuations for North America and other parts of
the world, while model studies have demonstrated that ocean-atmosphere
and land-biosphere-atmosphere interactions are plausible mechanisms to
explain decade-to-century variability. Historical and paleoclimatic
data, as well as coupled models, indicate the potential for significant
climate variability on long time scales. Such changes can be expected to
occur in the future, irrespective of human impacts on climate.\"
(Atmospheric Sciences, p. 273)

[Biospheric variation.]{.underline}

\"How land contributes, by location and processes, to exchanges of
carbon with the atmosphere is still highly uncertain\...\" (Climate
Change Science, p. 11)

\"A frontier for climate models is the incorporation of more complete
biogeochemical cycles (for example, for carbon dioxide).\" (Climate
Change Science, p. 15)

\"Analyses of historical records have revealed a number of interesting
cases of longer-period fluctuations for North America and other parts of
the world, while model studies have demonstrated that ocean-atmosphere
and land-biosphere-atmosphere interactions are plausible mechanisms to
explain decade-to-century variability. Historical and paleoclimatic
data, as well as coupled models, indicate the potential for significant
climate variability on long time scales. Such changes can be expected to
occur in the future, irrespective of human impacts on climate.\"
(Atmospheric Sciences, p. 273)

[Cryogenic variation.]{.underline}

\"Key research areas for increasing our understanding of abrupt climate
change include:

\-- sea-ice transport and processes, particularly where they interact
with deepwater formation;

\-- land-ice behavior, including conditions beneath ice sheets;

\-- the hydrological cycle, including storage, runoff, and permafrost
changes\...\" (Abrupt Climate Change, p. 3)

[Surface versus satellite temperature variation.]{.underline}

\"The finding that surface and troposphere temperature trends have been
as different as observed over intervals as long as a decade or two is
difficult to reconcile with our current understanding of the processes
that control the vertical distribution of temperature in the
atmosphere.\" (Climate Change Science, p. 17)

[Aerosol forcing mechanisms.]{.underline}

\"Because of the scientific uncertainties associated with the sources
and composition of carbonaceous aerosols, projections of future impacts
on climate are difficult.\" (Climate Change Science, p. 12)

\"The monitoring of aerosol properties has not been adequate to yield
accurate knowledge of the aerosol climate influence.\" (Climate Change
Science, p. 12)

\"Climate forcing by anthropogenic aerosols is a large source of
uncertainty about future climate change. \... The greatest uncertainty
about the aerosol climate forcing \-- indeed, the largest of all the
uncertainties about global climate forcings \-- is probably the indirect
effect of aerosols on clouds. \... The great uncertainty about this
indirect aerosol climate forcing presents a severe handicap both for the
interpretation of past climate change and for future assessments of
climate changes.\" (Climate Change Science, pp. 13-14)

\"Because of the large and still uncertain level of natural variability
inherent in the climate record and the uncertainties in the time
histories of the various forcing agents (and particularly aerosols), a
causal linkage between the buildup of greenhouse gases in the atmosphere
and the observed climate changes during the 20th century cannot be
unequivocally established.\" (Climate Change Science, p. 17)

[The hydrologic cycle.]{.underline}

\"Water is at the heart of climate change and the impacts of climate
variability. Any assessment of climate change, its causes and impacts,
must be based on significantly better observations of the water cycle.\"
(Pathways, p. 523, Finding 2.2.)

\"Key research areas for increasing our understanding of abrupt climate
change include:

\-- the hydrological cycle, including storage, runoff, and permafrost
changes\...\" (Abrupt Climate Change, p. 3)

\"A better understanding needs to be obtained of the nature of dec-cen
changes and variations that have occurred in the portions of the climate
system that are especially sensitive to climate changes. These sensitive
components include cloud cover, sea-ice fields, terrestrial water, and
the subsurface ocean. Improved understanding is also needed of how the
various components of the climate system interact to produce dec-cen
climate variability, as distinguished from the variability that is a
product of variations internal to a single component. The atmosphere
works on very fast time scales, so coherent decadal-to-centennial
variability within the atmosphere is expected to be attributable, at
least in part, to interactions with the other, slower components of the
climate system, or else to be a response to changes in the radiative
forcing. (Nonlinear internal feedbacks are a distinct possibility,
though).\" (Dec-Cen Variability, Summary)

**Sources**

1\. *Global Environmental Change: Research Pathways for the Next Decade*
(1999) 621 pages.

\"Pathways\"

\<http://www.nap.edu/catalog/5992.html\>

2*. Climate Change Science: An Analysis of Some Key Questions* (2001).

\"Climate Change Science\"

\<http://www.nap.edu/catalog/10139.html\>

3\. *Abrupt Climate Change: Inevitable Surprises* (2001).

\"Abrupt Climate Change\"

\<http://www.nap.edu/catalog/10136.html\>

4\. *Issues in the Integration of Research and Operational Satellite
Systems for Climate Research: Part I. Science and Design* (2000).

\"Integration of Research and Satellites\"

\<http://www.nap.edu/catalog/9963.html\>

5\. *Decade-to-Century-Scale Climate Variability and Change: A Science
Strategy* (1998).

\"Dec-Cen Variability\"

\<http://www.nap.edu/catalog/6129.html\>

*6. The Atmospheric Sciences: Entering the Twenty-First Century* (1998).

\"Atmospheric Sciences\"

\<http://www.nap.edu/catalog/6021.html\>