April 12th Global Change Seminar

[Tony Socci]

         U.S. Global Change Research Program Seminar Series


        The Record of Surface Warming in the 20th Century:
                Recent Observations and Model Results


What does the borehole record of temperature change tells us about climate
change, particularly in the 20th century?  Is the borehole record of
temperature change at the Earth's surface consistent with recent
observations of temperature change and other proxy records of past
temperature changes, regionally and globally?  From an observational
perspective, what are the most notable changes in the surface temperature
in the 20th Century, especially in the U.S.?  From a modeling perspective,
can the warming of the 20th Century be attributed entirely to natural
climate variability?  Are the regional and global warming trends consistent
with a climate change resulting, in part, from a buildup of greenhouse
gases?


                                    Public Invited

                 Wednesday, April 12, 2000, 3:15-4:45 PM
                   Dirksen Senate Office Bldg., Room 138
                                    Washington, DC

                                 Reception Following


INTRODUCTION:

Michael E. Mann, Department of Environmental Sciences, University of
Virginia, Charlottesville, VA

SPEAKERS:

Henry N. Pollack, Professor of Geophysics, Department of Geological
Sciences, University of Michigan, Ann Arbor, MI

David Easterling, Principal Scientist, National Climatic Data Center,
National Oceanic and Atmospheric Administration, Asheville, NC

Thomas R. Knutson, Geophysical Fluid Dynamics Laboratory (GFDL), National
Oceanic and Atmospheric Administration, Princeton, NJ


Temperature Trends Over the Past Five Centuries Reconstructed from
                         Subsurface Temperatures

Temperature changes that occur at the Earth's surface propagate slowly
downward into the rocks beneath the surface.  Thus, rock temperatures at
shallow depths provide evidence of changes that have occurred at the
surface in the recent past.  The pace of heat transfer in rocks is such
that the past 500 years of surface temperature history is imprinted on and
contained within the upper 500 meters of the Earth's crust.

Analyses of underground temperature measurements from more than six
hundred boreholes from all continents except Antarctica show that:

* The global average ground surface temperature has increased by at least
0.9 degrees F (0.5 degrees C) in the 20th century. This is a conservative
estimate of the century-long rate of warming because many boreholes used
in this study were drilled and logged 15 to 20 years ago, prior to the
extraordinary warming of the final decades of the 20th century.

* The 20th century has been the warmest century of the last five centuries.

* The present-day mean temperature is at least 1.8 degree F (1.0 degree C)
warmer than five centuries ago; of this change about half has occurred in
the 20th century alone, and 80% has occurred since the year 1800.

The five-century change can be thought of as a time- and space-averaged
overall measure of climate sensitivity (the response of the global mean
surface temperature to changes in climate forcing factors over this time
interval).

These interpretations provide an historical perspective that indicates that
the 20th century has not been just another century in terms of temperature
change.  In the context of the five-century interval investigated, the 20th
century is clearly unusual.



         Observed Temperature Changes in the 20th Century

Changes in Temperature Extremes

One of several pieces of evidence used to gauge climate change is an
increase in extreme climate events.  The two types of extremes examined
here are: (1) record-breaking average global temperatures, and (2) changes
in the number of days in the U.S. where the temperature exceeds or drops
below a given threshold temperature (e.g., freezing).

Evidence from paleoclimatic data suggests that current temperatures are the
warmest in the past 1000 years, and more recent observations of global
temperatures indicate that temperatures have warmed approximately 0.6
degrees C (1.1 degrees F) over the past 100 years.  However, an important
piece of information related to understanding the sensitivity of the climate
system to increases in carbon dioxide and other greenhouse gases, is the
rate of warming.  Since 1990, society has witnessed some of the warmest
years on record.  In particular, 1997, 1998, and now 1999, are the three
warmest years on record.  Furthermore, embedded within the temperature
records of 1997 and 1998, was a string of sixteen consecutive months where
the monthly global temperature broke the previous record for that month.
In fact, during much of 1998, monthly records were broken that had just
been set the previous year.

Changes in the Rates of Temperature Change

A casual inspection of the global temperature time series reveals that the
increase in global mean temperature has not been constant.  A simple linear
fit to the time series from 1880 to 1999 shows that there are actually two
periods where the rate of change has been much more than the observed 0.6
degrees C (1.1 degrees F)/100 years, and two periods where the rate of change
has been slightly negative or very close to zero.  Three inflection points
(places where trends change direction) in the above time series were
identified using statistical methods, then linear trends were fit to the
sub-sections of the time series as defined by the presence of these inflection
points.

Analysis of these trends show a slight cooling of - 0.38 degrees C (-0.7
degrees F)/100 years from 1880 to 1910, a strong warming trend of 1.2
degrees C (2.2 degrees F)/100 years from 1911 to the 1940s, a slight
cooling of -0.27 degrees C (-0.5 degrees F)/100 years from the 1940s to
mid-1970s, and a very strong warming on the order of 0.2 degrees C (0.4
degrees F)/decade since the mid-1970s.  Using this information, the
string of sixteen consecutive months of record-breaking temperatures was
analyzed for consistency with this observed rate of warming over the past
two decades.  Results of this analysis suggest that this string of
record-breaking temperatures in 1997-98, is not consistent with a rate of
warming of 0.2 degrees C (0.4 degrees F)/decade, but may signal an increase
in this rate of change.  In fact, the observed rate of change since the
1970s is comparable to the 1995 IPCC "business as usual" model scenarios of
human-induced climate change for the 21st century which give a rate of
warming of about 2.0 degrees C (3.6 degrees F)/100 years.

Changes in Daily and Yearly Temperatures in the U.S.

The average climate warming observed within the continental United States
is about 1 degrees F (0.5 degrees C) over the past 100 years.  It has been
shown that most of the warming represented by the global average
temperature is associated more with warming in minimum temperatures
(nighttime lows) than in maximum temperatures (daytime highs).  Analysis
of changes in the number of days where the minimum temperature dips
below freezing indicates that, for the U.S. as a whole, there has been a
decline of two fewer days per year where temperatures fall below 0
degrees C (32 degrees F).  However, since the southeastern U.S. is one of
the few places in the world that has exhibited a cooling, there has been an
increase in this region in the number of days below freezing.  In contrast,
the western U.S. has witnessed significant decreases in the number of days
below freezing.


20th Century Surface Temperature Trends: Models Vs Observations

The ability of global climate models to reproduce the observed surface
temperature trends over the 20th century represents an important test of
the models.  Confidence in the ability of climate models to anticipate
future climate changes rests in part on such evaluations.  A recent set of
global climate model experiments involving the use of a GFDL model, and
driven by past concentrations of greenhouse gases and an estimate of the
forcing by anthropogenic sulfate aerosols, was compared with historical
temperature observations at various geographic or spatial scales: 1) global
mean surface temperatures; 2) latitudinally-averaged temperature changes
for various latitudes; and 3) maps of temperature trends for regions of the
globe with sufficient observations.

The model used provides a fairly realistic simulation of 20th century
surface temperatures in terms of both global averages and
latitudinally-averaged temperatures for various latitudes.  Comparison of
smaller-scale regional details of trends over the last half-century
indicates that some significant discrepancies remain between model output
and observations. In other words, in some regions, the difference between
the model's trend from the greenhouse + sulfate experiments and the
observed trend is greater than the "margin of error" as estimated by the
internal climate variability in the model.  However, for all spatial scales
examined (including regional scales) the aggregate model results suggest
that these regional warming trends are unlikely to be the result of
internal climate variability alone, and suggest a role for a sustained climate
forcing resulting from the buildup of greenhouse gases in the 20th century.

1. Global Mean Temperature: The model driven only by estimates of the
varying concentrations of greenhouse gases and sulfate aerosols over time
is capable of reproducing the 20th century observed global-mean surface
warming quite well.  Five such simulations using different initial ocean
states simulate an overall 20th century warming that closely approximates
the observed warming.

2. Latitudinally-Averaged Temperatures: Observed surface temperature
changes, averaged for different latitudes, show that the warming since the
1970s has been fairly uniform across different latitudes.  In contrast, the
early 20th century warming was largest in high latitudes of the Northern
Hemisphere.  This difference in spatial structure suggests that the early
20th century warming may have resulted from a different set of causal
factors than the recent warming.  All five of the model experiments show a
warming since the 1970s that is fairly uniform across different latitudes,
similar to the observations.  This result suggests an interpretation of the
late 20th century record as a  greenhouse-gas-induced warming "signal"
emerging from the background "noise" of internal climate variations.  One
model experiment suggests that internal climate variability may well have
played a substantial role in the early 20th century warming.

3. Geographical Pattern of Trends: The most stringent of the tests applied
is the comparison of the complete spatial pattern of the observed and
simulated temperature trends.  For the model to be in agreement with
observations, it must agree not only in terms of the globally averaged
temperature changes, but also in terms of the regional details.  According
to this test, the climate model forced by greenhouse gases and sulfate
aerosols is statistically consistent with the observed trends over more
than 2/3 of the globe (considering only regions with sufficient
observations).  However, significant discrepancies exists between model and
observations over about 30% of the area examined.  Nonetheless, the
observed warming trends appear to be clearly outside the range of internal
climate variability alone.

In the case of the greenhouse + sulfate experiments, a number of factors
may contribute to the regional discrepancies between observed and simulated
trend patterns.  These factors (ordered in terms of our estimate as to
their relative importance, from most to least important), include possible
deficiencies in:1) specified radiative forcings such as indirect sulfate
aerosol effects, for example; 2) climate model sensitivity to the forcings;
3) simulated internal climate variability, especially regionally; or 4)
observational records.


                                        Biographies

Dr. Henry Pollack is a professor of geophysics in the Department of
Geological Sciences at the University of Michigan in Ann Arbor.  He has
engaged in research on all seven continents, addressing the dynamics and
evolution of the Earth and its climate.  His current research focuses on
the record of global climate change as recorded by the temperatures of the
rocks beneath the Earth's surface.

Dr. Pollack has served on National Science Foundation advisory panels on
Continental Dynamics, the Global Digital Seismograph Network, the San
Andreas Fault, and Earth Science Instrumentation and Facilities.  From
1991-95, he served as Chairman of the International Heat Flow Commission of
the International Association of Seismology and Physics of the Earth's
Interior.  He is presently a member of the U.S. Geodynamics Committee of
the National Research Council, and the Committee on Global and
Environmental Change of the American Geophysical Union.

Dr. Pollack received his undergraduate degree in geology from
Cornell University, an M.S. degree from the University of Nebraska, and a
Ph.D. in geophysics from the University of Michigan.  He has also held
visiting teaching and/or research positions at Harvard University, the
University of Zambia, the Universities of Durham and Newcastle (UK), and
the University of Western Ontario.


Dr. David Easterling is currently Principal Scientist at the National
Climatic Data Center (NCDC) in Asheville, NC. Prior to that he served as an
Assistant Professor in Climate and Meteorology, in the Department of
Geography at Indiana University-Bloomington.  He has authored or
co-authored numerous research articles in such peer-reviewed journals as
Science  and the Journal of Climate.  He is also a contributor to the
upcoming Intergovernmental Panel on Climate Change (IPCC) 3rd  Assessment
Report, and serves as a member of the National Assessment Synthesis Team.

Dr. Easterling's research interests include the detection of climate change
in the observed record, particularly changes in extreme climate events; the
development of statistical methods for improving the quality of climate
data; and the application of General Circulation Model simulations in
developing climate change scenarios for use in assessing the potential
effects of climate change on the environment and society.  He received his
Ph.D. from the University of North Carolina at Chapel Hill in 1987.


Thomas Knutson is a research meteorologist in the Climate Dynamics Group at
the National Oceanic and Atmospheric Administration's Geophysical Fluid
Dynamics Laboratory, one of the world's leading climate modeling centers.
He has been author or co-author of a number of publications in major
climate research journals, including two recent papers in Science  on
future hurricane intensities under a global warming, and on a model
simulation of early 20th century global warming.  His recent research
interests include: detection of climate change; simulation of internal
climate variability; and the impact of climate change on El Nino and
hurricanes.  He has been an invited expert on climate change and extreme
events at the Aspen Global Change Institute, the Risk Prediction Initiative
of the Atlantic Global Change Institute at the Bermuda Biological Station
for Research, and at a recent Environmental Protection Agency workshop on
climate change and extreme storm events.  More recently, he was an invited
speaker at the American Meteorological Society's 23rd Conference on
Hurricanes and Tropical Meteorology.


The Next Seminar is scheduled for May 17, 2000

Tentative Topic: The Earth's Surface Temperature in the 20th Century:
Coming to Grips with Satellite and Surface-Based Records of Temperature


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:
http://www.usgcrp.gov.  A complete archive of seminar summaries can also be
found at this site under the link: "Second Monday Seminars."