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How much of the
observed climate change over the 20th century is attributable to changes
in solar radiation, and how much can be attributed to changes in the
concentration of greenhouse gases in the atmosphere resulting from human
activities? How much influence do changes in solar radiation have on the
Earth's climate system relative to the influence exerted on the climate
system by the concentration of greenhouse gases? How large a role are
changes in solar radiation likely to play in driving climate change in the
future relative to the role greenhouse gases are likely to play?
INTRODUCTION:
Dr. Michael Mann
University of Virginia, Department of Environmental Sciences,
Charlottesville, VA
SPEAKERS:
Dr. Judith Lean
Naval Research Laboratory, Washington, DC
Dr. Jerry D. Mahlman
Director of the National Oceanic and Atmospheric Administration's
Geohpysical Fluid Dynamics Laboratory, Princeton, NJ
The Role of the
Sun in Climate Change
The Sun has long been a
reliable source of the radiant energy for the Earth. But this radiation,
which helps determine our fundamental climate state and enables
habitability, is not constant. Changes in solar radiation must therefore,
be considered as a possible cause of climate change, among a number of
other possible causes.
Space-based measurements
reveal the existence of 11-year cycles in solar radiation upon which are
often superimposed larger, short-term changes with 27-day cycles.
Unfortunately, direct observations of variations in solar irradiance
exist, thus far, only for the last 20 years, which is a very short record
in terms of climatological time scales. Indirect "proxies" of
solar activity (from tree-rings and ice-cores) exhibit 11-year solar
cycles as well as longer-term changes or cycles that exceed the amplitudes
of their 11-year cycles. Comparisons of these proxy records with direct
observations suggest that present levels of solar irradiance are probably
increased relative to periods of anomalously low solar activity commonly
known as the "Dalton", "Maunder" and "Sporer
Minima" (which occurred from 1790 to 1820, from 1645 to 1715 and from
1425 to 1575, respectively). During the first half of the twentieth
century, solar activity increased steadily, but since about 1950 the
activity underlying the 11-year cycles has been essentially constant
exhibiting little or no change. The Sun's radiation is expected to track
trends in solar activity but the true amplitudes (a measure of the energy
output of the sun) of long-term irradiance changes can only speculated
because of the lack of a sufficiently long database of direct
observations. During the past two decades (since 1978), for which direct
observations do exist, there is little evidence for an underlying upward
trend in the Sun's radiation, although the existence of an 11-year solar
cycle is apparent.
An array of empirical
sun-climate relationships imply a consequential role for the Sun in
climate change. Climate parameters of many types often exhibit cycles that
are also common in solar activity proxies, such as near 11-, 22-, 80 and
210-years. Times of cooler climate in past millennia usually coincide with
reduced levels of solar activity. During the Little Ice Age, which
occurred from about 1450 to 1850, surface temperatures were from 0.6�C to
1�C colder than at present (depending on geographic location, as changes
in solar radiation can often result in regional or local responses as
opposed to leaving a global signature). Solar activity was lower than at
present because of the occurrence of the Dalton, Maunder and Sporer
Minima. The speculated decrease of overall levels of solar radiation in
1650 from present levels comprises a climate forcing of 0.6 Wm-2
(but with large uncertainties such that the range is from 0.2 to 1.2 Wm-2).
For comparison, the change in greenhouse gases since 1650 corresponds to a
climate forcing of about 2.5 Wm-2, 95% of which has occurred
since 1850. In the pre-industrial period prior to the Little Ice
Age--during the 12th-13th centuries (the Medieval Warm Period) --warmer
surface temperatures coincided with higher levels of solar activity (which
again, could have manifested itself regionally).
Climate models with
realistic sensitivities simulate surface temperature changes of a few
tenths of a degree C in response to plausible climate forcing due to
changes in solar radiation over the past few hundred years. The
simulations can account for two hundred years of surface temperature
fluctuations prior to the industrial epoch (from 1600-1800). The
solar-related warming in response to a solar forcing of 0.6 Wm-2
(from 1650 to the present) is calculated globally to be 0.45�C, but with
strong regional signatures. Less than 0.25�C of this warming occurs from
1900-1990 when, for comparison, measured surface temperatures over this
same period of time increased 0.6�C. In suggesting that the Sun's
variability accounts for less than half the 0.6�C surface warming in the
industrial period, the climate change model simulations are in good
agreement with the pre-industrial empirical Sun-climate associations. A
larger role for the Sun in explaining the observed climate warming over
the twentieth century, is inconsistent with direct measurements of solar
output, and with proxy evidence of solar variability during the
pre-industrial era.
During the past two decades
(1976-1996) direct observations of solar irradiance suggest negligible
long-term solar forcing of climate. Over this observational period, solar
radiation levels remained approximately constant, exhibiting no change
during two successive solar cycle minima (1986 and 1996) while observed
surface temperatures nevertheless increased by 0.2�C over the same period
of time. Of course, even in the absence of long-term trends, climate
variability may be associated with the 11-year irradiance cycle in ways
implied by empirical associations, but not presently understood or
accounted for in climate models.
Solar activity is presently
at high levels relative to the historical record of the past 8,000 years.
This suggests that future levels of solar radiation will probably be
comparable to or less than present levels, and that future solar forcing
will either be small, or negative relative to the climate forcing due to
greenhouse gases, while projected concentrations of greenhouse gases
continue to increase. Furthermore, solar activity levels during the past
8,000 years have ranged, typically, from low levels similar to the
"Maunder Minimum" (a period of low solar irradiance circa 1650)
to higher levels, on a par with the present. A warming on the order of
0.5�C that present scientific understanding and analyses attribute to
this increase in solar forcing since 1650, possibly represents an upper
limit to the observed surface temperature change attributable to solar
radiation. Were solar activity to undergo another "Maunder
Minimum" type event over the next 200 years, surface cooling would
likely be 0.5�C or less, as a result. However, projections of future
solar activity are exceedingly difficult for even one 11-year solar cycle,
and are essentially impossible for the long-term. Ultimately, a more
refined determination of the Sun's role in climate change requires a much
longer record of measurements of solar radiation than the present meager
20-year database. Continuous, precise solar monitoring is crucial for the
indefinite future. Improved understanding of the processes by which solar
radiation interacts with the Earth is similarly essential to better
specify climate's response to direct solar forcing of climate by
fluctuations in solar radiation, and indirect solar forcing of climate via
solar-related changes in atmospheric ozone.
The Role of Greenhouse
Gases in Climate Change
Identifying and evaluating
possible causes for the observed warming of earth's climate over the 20th
century are matters of considerable importance because of the need for
policy-makers and decision-makers, in general, to evaluate the credibility
of the models being used to calculate future climate change due to
"greenhouse" gases being added to the atmosphere. A series of
model calculations were used to examine the differing effects of natural
variability, carbon dioxide and other greenhouse gases, sulfate particles,
and changing solar output, on the climate of the 20th century. In general,
these calculations make it clear that it is scientifically very difficult
to construct an explanation for the 20th century warming that does not
include a major role for the added greenhouse gases resulting from human
activities.
Based on the above
calculations, and the observational records of climate change for the 20th
century, the following conclusions are drawn:
- Global climate of the
20th century has warmed by 0.7-0.8�C.
- Natural (unforced)
climate variability cannot explain the magnitude of the observed
warming over the 20th century.
- Solar irradiance
variations are large enough to shape, but not dominate, the observed
warming.
- The extended warming
period between 1910-1940 can be explained by natural variability plus
added greenhouse gases. It can also be explained by added greenhouse
gases plus increased solar irradiance.
- Added greenhouse gases
provide, by far, the most plausible hypothesis for explaining the
warming of the 20th century.
BIOGRAPHIES
Dr. Judith Lean is a
Research physicist in the Space Science Division of the Naval Research
Laboratory in Washington, DC, where she has been since 1988. Her research
is focused on the mechanisms and measurements of variations in the Sun's
radiative output at all wavelengths, and the effects of this variability
on the Earth's global climate and space weather. Dr. Lean has been
extensively involved in the Upper Atmosphere Research Satellite (UARS)
program presently monitoring solar radiation, and is also involved in the
Solar Radiation and Climate Experiment (SORCE) scheduled to commence high
precision monitoring of solar radiation in 2002. She has worked closely
with Dr. David Rind of NASA to better understand solar-induced climate
forcing of the Earth's climate system relative to the influences of
greenhouse gases and ozone depletion.
Dr. Lean has published over
60 peer-reviewed scientific papers and over 20 conference proceedings in
the scientific literature, and has given more than 170 presentations at
scientific meetings, seminars, colloquia and lectures. As one of the
leading authorities on solar radiation and its influence on the Earth's
climate, she has been widely quoted over the past decade by magazines and
newspapers such as Science, Science Impact Newsletter, New Scientist,
Newsweek, Space News, Earth, the New York Times, and the
National Public Radio.
She has served as Chair of
the National Academy of Sciences' (NAS) Working Group on Solar Influences
on Global Change, and as a member of NAS's Task Group on Ground-Based
Solar Research. She has also served on various NASA (National Aeronautics
and Space Administration), NSF (National Science Foundation), and NPOESS
(National Polar Orbiting Operational Environmental Satellite System)
panels, committees and working groups.
She is a member of the
American Geophysical Union, the International Association of Geomagnetism
and Aeronomy, the American Astronomical Society, and the Solar Physics
Division of the American Meteorological Society. Dr. Lean has also
testified before Congress on the role of solar radiation and solar
variation in climate change. She received her Ph.D. in Atmospheric Physics
in 1981, from the University of Adelaide, Australia.
Dr. Jerry D. Mahlman is
the Director of the National Oceanic and Atmospheric Administration's
Geophysical Fluid Dynamics Laboratory - one of the world's leading climate
modeling centers. He also holds a Professorship in Atmospheric and Oceanic
Sciences at Princeton University. His research career has been directed at
modeling, diagnosing, and understanding the behavior of the atmosphere and
its implications for climate and chemical change. Over the past decade, he
has occupied a central role in the interpretation of human-caused climate
change to policy-makers and communities. Dr. Mahlman served as the Chair
of the Scientific Advisory Committee of NASA's Mission to Planet Earth, is
a member of the National Research Council's Board on Sustainable
Development, was the US Representative to the World Climate Research
Programme, and was a member of NASA's Advisory Council. From 1989-1991,
Dr. Mahlman served as a member of the US-USSR Joint National Academy of
Sciences Committee on Global Ecology. He is a Fellow of the American
Geophysical Union and of the American Meteorological Society, and has
received the Department of Commerce Gold Medal, the Presidential
Distinguished Rank Award, the American Meteorological Society's Carl-Gustaf
Rossby Research Medal, and an Honorary Alumnus Award from Colorado State
University.
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