U.S. Global Change Research Program Seminar Series
What's Driving Climate Change in the 20th Century - Changes in Solar
Radiation or the Buildup of Greenhouse Gases?
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?
Public Invited
Tuesday, November 23, 1999, 3:15-4:45 PM
Dirksen Senate Office Bldg., Room G-11
Washington, DC
Reception Following
INTRODUCTIONS:
Dr. Michael Mann, University of Virginia, Department of Environmental
Sciences, Charlottesville, VA
SPEAKERS:
Dr. Judith Lean, Naval Research Laboratory, Washington DC 20375
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 0.6oC to 1oC 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 Watts/Square
Meter (but with large uncertainties such that the range is from 0.2 to 1.2
Watts/Square Meter). For comparison, the change in greenhouse gases since
1650 corresponds to a climate forcing of about 2.5 Watts/Square Meter,
95% of which has occurred since 1850. In the pre-industrial period prior to
the Little Ice Age-- during the12th-13th centuries (the Medieval Warm
Period) - warmer surface temperatures coincided with higher levels of solar
activity (which again, could have manifest 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 Watts/Square
Meter (from 1650 to the present) is calculated globally to be 0.45 degrees C,
but with strong regional signatures. Less than 0.25 degrees C of this warming
occurs from1900-1990 when, for comparison, measured surface temperatures
over this same period of time increased 0.6 degrees C. In suggesting that
the Sun's variability accounts for less than half the 0.6 degrees 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 degrees 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
degrees 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 degrees 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 degrees 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 heavily 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 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.
The Next Seminar is Scheduled for the Week of December 12th
Tentative Topic: To Be Announced
For more information please contact:
Anthony D. Socci, Ph.D., U.S. Global Change Research Program Office, 400
Virginia Ave. SW, Suite 750, Washington, DC 20024; Telephone: (202)
314-2235; Fax: (202) 488-8681 E-Mail: [log in to unmask] Additional
information on the U.S. Global Change Research Program (USGCRP) and this
Seminar Series is available on the USGCRP Home Page at:
http://www.usgcrp.gov. A complete archive of seminar summaries can also be
found at this site. Normally these seminars are held on the second Monday
of each month.
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