May 17th US Global Change Seminar: "Surface Temperature Changes and Biospheric Responses in the Northern Hemisphere during the Last 1,000 Years"

Tony Socci tsocci at
Wed May 12 12:30:08 EDT 1999

                            U.S. Global Change Research Program Seminar Series

            Surface Temperature Changes and Biospheric Responses in the
                   Northern Hemisphere during the Last 1,000 Years

What is the record of surface temperature fluctuations in the Northern
Hemisphere over the last 1,000 years?  How does the observed climate
warming of the 20th century compare with this 1,000-year record?  What do
tree-rings tells us about the climate of the last 1,000 years relative to
that of the 20th century?  How much of the observed warming in the 20th
century can be attributed to natural climate variability?  How much of that
warming is likely to be attributable to human activities?  What has the
biospheric response been to these changes, especially in the 20th century?
What can be said about the rate of temperature change over the last 100
years or more?  Was 1998 the warmest year in the last millennium?  To what
extent can the observed warming of 1998 be attributable to El Nino?

                                                    Public Invited

                                   Monday, May 17, 1999, 3:15-4:45 PM
                                   Dirksen Senate Office Bldg., Room G-11
                                                   Washington, DC

                                               Reception Following


Dr. Joseph Friday, Director, Board on Atmospheric Sciences and Climate,
National Research Council of the National Academy of Sciences, Washington,


Dr. Michael E. Mann, Department of Geosciences, University of
Massachusetts, Amherst, MA

Dr. Malcolm K. Hughes, Laboratory of Tree-Ring Research, University of
Arizona, Tucson, AZ

Dr. James Hansen, Head, NASA Goddard Institute for Space Studies, New York, NY

                    Large-scale Temperatures During the Past One-Thousand Years


Without knowledge of natural climate variability at century and longer
timescales, it is difficult to determine the significance of 20th century
warming evident in the relatively short instrumental record of global
surface temperature. To obtain a longer-term perspective on observed
climate variability and change, one must resort to indirect measurements of
climate variations derived from  natural archives or "proxy" climate
indicators such as tree rings, corals, and ice cores, supplemented with the
few available long instrumental and historical climate records.  Using such
proxy data networks, research published in 1998 led to estimates of annual,
global surface temperature patterns dating back to AD 1400. Averaging these
reconstructions allowed the calculation of estimates of Northern Hemisphere
mean temperatures back through AD 1400, as well as estimates of the
uncertainties in these estimates. The warmth of the 1990s appeared to be
unprecedented in this reconstruction, with three years during this decade
(1990, 1995, and 1997) that were likely to be warmer than any other year
since AD 1400.  Reconstructions further back in time were not then possible
with the available data networks.

Two significant events have occurred since that research was done that
allow those original conclusions to be expanded upon. Based upon careful
consideration of the sparse proxy data available for the years AD 1000 - AD
1400, estimates of yearly Northern Hemisphere mean temperatures have been
made dating back to AD 1000, albeit with considerably larger uncertainties.
Not withstanding these uncertainties, and taking into account the slightly
warmer temperature estimates for the early part of the millennium, it would
be difficult to argue that the 1990s were as anomalous when viewed in the
context of the temperature history of the entire millennium, were it not
for the record warmth of 1998.  The year 1998 was observed to have been
significantly warmer (by about 0.2 degrees C) than any other year in the
instrumental record.  In the context of the last 1,000 years, one can say
with a high degree of confidence that 1998 was warmest year for the
Northern Hemisphere.

The Millennial Temperature Record:

Prior to AD 1400, surface temperature estimates depend upon certain key
climate proxy data series, especially those in the higher elevations of the
western U.S. which show a marked temperature sensitivity.  These data also
show non-climatic influences during the 19th and 20th century that may be
related to CO2 increases or perhaps other factors. As such, these
non-climatic influences must be subject to further research and analysis,
before the data can be used in long-term climate reconstruction.  Prior to
about AD 1000, the sparseness of the available data preclude a meaningful
estimate of hemispheric mean temperatures.

Based on the millennial hemispheric temperature estimates, conditions
during the earlier centuries of the past millennium appear somewhat higher
than those of the 15th-19th centuries.  However, the data do not support
the notion of the existence of a hemisphere-wide "Medieval Warm Period"
relative to the late 20th century warming.  Rather, the evidence suggests
that the warmest decades of the Medieval era were comparable to early and
mid 20th century temperatures, but not those of the late 20th century.
Some evidence suggests that certain regions (e.g., the North Atlantic and
Greenland) may have exhibited somewhat greater warmth, but at the
hemispheric scale, the evidence does not support the notion of sustained
periods of warmth during the past 1,000 years, comparable to the warmth of
the late 20th century.  Due to the sparseness of the proxy climate data for
the years prior to AD 1400, and the difficulty in resolving temperature
variations this far back in time, the uncertainties in hemispheric
temperature estimates become considerably larger in the earlier centuries
of the past millennium.  Even with these expanded uncertainties, however,
the 1990s, and 1998 in particular, appear to have exhibited hemispheric
warmth that is unprecedented at least over the last 1,000 years.

Causal Factors of Temperature Change:

In searching for a likely cause or causes that explain variations in the
Earth's surface temperature changes over the last six centuries, a suite of
plausible, candidate, climate-forcing influences such as changes in the
brightness of the Sun, changes in the frequency and magnitude of volcanic
eruptions, and human-caused increases in greenhouse gas concentrations,
have been evaluated.  This analysis suggested that only an increase in the
concentration of greenhouse gases could explain the anomalous warmth of the
late 20th century.  With longer-term, millennial estimates of surface
temperature change, other factors may yet prove to play a role.  On
timescales of tens to hundreds-of-thousands of years, the "astronomical
theory" of climate change holds that changes in the geometry of the Earth's
orbit relative to the Sun, bring about subtle changes in the distribution
of solar radiation at the Earth's surface that may drive slow, but
significant, long-term changes in climate.  A number of recent climate
modeling experiments suggest that such astronomical factors should have led
to a slow cooling of the climate since about 6,000 years before present, at
a rate of cooling of between 0.01 and 0.04 degrees C/century.  Such
theoretical considerations are in remarkable agreement with the observed
cooling trend (about 0.02 degrees C/century) from AD 1000 through the mid
19th century, as observed in this temperature reconstruction.  However,
this long-term cooling trend undergoes a dramatic reversal over the course
of the 20th century.  Thus, the 20th century warming trend appears to be
that much more anomalous when viewed in the context of the natural,
long-term climate variability of the last millennium, and is therefore
again, unlikely to be due to natural factors alone.

Tree-Ring Records of Temperature, Precipitation and the Biosphere's
                         Response to Climate Change

The measured record from thermometers, rain gauges  and barometers, does
not provide an adequate sample of the ways in which climate could vary
under recent or present conditions, even if there were no human influence
on climate. Planning that ignores this will be inadequate, whether its
focus is resource use  (e.g. energy or water) or mitigating the
consequences of natural disturbances such as drought, floods and wildfires.
This is because the instrumental record is often too short to represent the
different ways climate can behave, and because this record was hardly
started by the time human action had made measurable changes in the
composition of our atmosphere.  One therefore, cannot rely solely on the
twentieth century instrumental record to assess the character of climate

Research using tree rings to derive estimates of climate variability
provides many interesting insights. Examples include:

* The Northern Hemisphere has very likely been markedly warmer in the late
twentieth century than at any time in the preceding 900 years.
* Major explosive volcanic eruptions have played a much larger part in
affecting climate in earlier centuries than recently - they have been
relatively rare this century.
* Droughts in the western U.S. have been more frequent, more intense and
sometimes much longer at various times in the last two or three thousand
years than in the twentieth century.
* The time period between years in which El Niño/La Niña strongly affects
conditions in Texas, neighboring states and northern Mexico has varied over
recent centuries.

Work on using tree rings to detect the biosphere's response to climate
variability and climate change is at a much earlier stage than their use as
natural climate recorders, but some intriguing fragments of evidence have
already emerged. In some regions as widely separated as the southern
Rockies in the U.S., and Tasmania, high elevation trees have shown growth
spurts in the last two to three decades that are unprecedented in at least
the last thousand years.  In the case of the southern Rockies, this seems
to have been caused by an unusual combination of climatic conditions,
rather than by any direct fertilization by increased carbon dioxide
concentrations in the air.  Trees growing at the highest elevations in the
mountains in and around the Great Basin have been growing at an accelerated
rate since the middle of the nineteenth century, and the link that did
exist between their growth rate and local climate broke down at that time.
There is no convincing climatic explanation for this, and alternatives,
such as direct carbon dioxide fertilization have been proposed, but none
has gained wide acceptance.  An integrated program of research combining
tree-ring records and vegetation remote sensing is needed to record and
better assess the biosphere's response to climate change and variability.

             Analysis of Surface Temperature Change in the Past Century

The NASA/GISS (Goddard Institute for Space Studies) has developed a data
set that provides estimates of global surface temperature change for
1880-1998, the period with significant global coverage of instrumental
data.  Urban influence on the record is substantial in certain locations,
but is found to have only a small effect on the global estimates.
The record shows global warming this century that is unambiguous and
unusual.  The five-year mean global temperature has increased about 0.7
degrees C since the late 1800s.  The global surface temperature in 1998
was the warmest in the period of instrumental data.  The rate of temperature
change is higher in the past 25 years than at any previous time in the
period of instrumental data.  The warmth of 1998 is too large and pervasive
to be fully accounted for by the recent El Nino.  This analysis suggests
that the
global temperature may have moved to a higher level, analogous to the
significant increase that occurred in the late 1970s.

The record of surface temperature change can be compared with satellite
measurements of tropospheric temperature for the period since 1979.  The
satellite record is sometimes interpreted as being contradictory to the
surface measurements.  The GISS analysis indicates that the differences are
actually small and within estimated measurement errors, and that the
results are consistent with a long term warming trend at the surface and in
the troposphere.  This analysis further indicates that there has been a
slight cooling in the United States in the past 50 years, particularly in
the eastern half of the country.  The latter observation raises questions
regarding the likelihood of the observed temperature change in the U.S.
catching up with the rest of the world, and the observational data on
global climate forcings and the ocean necessary to answer the questions.


Dr Michael E. Mann currently holds an adjunct faculty position at the
University of Massachusetts, in the Department of Geosciences. In the Fall
of 1999, he will become an Assistant Professor at the University of
Virginia, in the Department of Environmental Sciences.  Dr. Mann's research
focuses on the application of statistical techniques to understanding
climate variability and climate change from both empirical and climate
model-based perspectives.  A specific area of current research is
paleoclimate data synthesis and statistically based climate pattern
reconstruction during past centuries using climate "proxy" data networks.
Other areas of active research include model-based simulation of natural
climate variability, climate model/data intercomparison, and long-range
climate forecasting.

Dr. Mann is a Lead Author of the Observed Climate Variability and Change
chapter of the IPCC Third Scientific Assessment Report, and a contributor
on several other chapters of the report.  He is a frequent participant in
government agency-sponsored panels and workshops dealing with climate
variability and paleoclimate, and is heavily involved with international
climate research programs such as PAGES (Past Global Changes) and CLIVAR
(Climate Variability and Predictability).

Dr. Mann received his undergraduate degrees in physics and applied math
from the University of California at Berkeley, an MS degree in physics from
Yale University, and a Ph.D. in geology and geophysics from Yale
University.  Dr. Mann is the author of more than 30 peer-reviewed journal
publications or book chapters, and has been the recipient of numerous
fellowships and prizes.  His work in the area of global climate change has
also been widely described in the popular media.

Dr. Malcolm K. Hughes is a professor of dendrochronology and director of
the Laboratory of Tree-Ring Research at the University of Arizona in
Tucson. His research interests include natural climate variability on
inter-annual to century time scales, and regional to global spatial scales,
primarily using tree rings.

Dr. Hughes has served as a member of the executive committee of the
Institute for the Study of Planet Earth, University of Arizona; the
Committee on Geophysical and Environmental Data at the National Research
Council; the Biometeorology Committee, American Meteorological Society
(AMS); the CLIVAR/PAGES  working group; and the U.S. delegation to the
World Climate Research Program conference, Geneva, Switzerland.

In 1998, Dr. Hughes was selected as a Fellow of the American Geophysical
Union, and in 1999, he was awarded a Bullard Fellowship by Harvard

He received his B.Sc. degree in botany and zoology in 1965, and his Ph.D.
degree in ecology in 1970, from the University of Durham, United Kingdom.

Dr. James Hansen heads the NASA Institute for Space Studies in New York
City, which is a division of Goddard Space Flight Center's (Greenbelt, MD)
Earth Sciences Directorate.  He was trained in physics and astronomy in the
space science program of Dr. James Van Allen at the University of Iowa.
His early research on the properties of clouds of Venus led to their
identification as sulfuric acid.  Since the late 1970s, he has worked on
process studies and computer simulations of the Earth's climate, focusing
on understanding the human impact on the global climate.  Dr. Hansen has
also testified before Congress on the issue of global warming.  In 1995, he
was elected to the National Academy of Sciences.

In 1963, Dr. Hansen received his Bachelor of Arts degree with highest
distinction in physics and mathematics from the University of Iowa.  He
participated in the NASA Graduate Traineeship from 1963-1966, and received
a Masters of Science degree in astronomy from the University of Iowa, in
1965.  Dr. Hansen was a visiting student at the Institute of Astrophysics,
University of Kyoto and the Department of Astronomy, Tokyo University,
Japan from 1965-1966.  He received his Ph.D. in physics from the University
of Iowa in 1967.

The Next Seminar is scheduled for Wednesday, June 16, 1999

Tentative Topic: The Status of Coral Reefs:  An Update

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: TSOCCI at USGCRP.GOV.

Additional information on the U.S. Global Change Research Program (USGCRP)
and this Seminar Series is available on the USGCRP Home Page at:  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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