January 25th US Global Change Seminar: "The Earth's SurfaceTemperature in the 20th Century: Coming to Grips with Satellite andSurface-Based Records of Temperature"

Tony Socci tsocci at usgcrp.gov
Wed Jan 19 15:09:13 EST 2000

           U.S. Global Change Research Program Seminar Series

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

Is the satellite-derived record of temperature an accurate or reasonable
estimate of the observed temperature at the Earth's surface, or is the
satellite record of temperature fundamentally different from the observed
temperature recorded at the Earth's surface?  If the satellite record of
temperature is not a measure of temperature at the Earth's surface, what is
it a measure of?  How are these records of temperature different and how
are they related, if at all?

                                          Public Invited

                      Tuesday, January 25, 2000, 3:15-4:45 PM
                        Dirksen Senate Office Bldg., Room G-11
                                        Washington, DC

                                     Reception Following


Dr. Elbert W. (Joe) Friday, Jr., Director, Board on Atmospheric Sciences
and Climate, National Research Council, National Academy of Sciences,
Washington, DC


Dr. John M. Wallace, Department of Atmospheric Sciences, University of
Washington, Seattle, WA

Dr. Kevin E. Trenberth, Head of the Climate Analysis Section, National
Center for Atmospheric Research, Boulder, CO

        Findings of the National Academy of Sciences' Expert Panel

During the past 20 years, global mean surface temperature has been rising
at a rate as large as any that has been observed within the historical
record.  Such rapid warming at the Earth's surface is in contrast to the
trend in the global-mean temperature of the lowest 8 kilometers of the
atmosphere (within that portion of the atmosphere referred to as the
troposphere) as inferred from measurements of radiation emitted by oxygen
molecules (a proxy for troposheric temperature) sampled by the microwave
sounding unit (MSU) carried aboard the NOAA polar-orbiting satellites.

Until as recently as two years ago, the latest estimates of this so- called
"tropospheric temperature trend" based on satellite data since 1979, were
indicating a slight cooling from 1979 onward.  About a year ago, the
algorithms used to process the satellite data were modified to take into
account changes in viewing geometry due to the decay in the satellite
orbits.  As a result of these rather small corrections, together with the
extraordinary warmth associated with the 1997-98 El Nino, the satellite
data are now indicating a warming trend, but it is still much smaller than
the trend in surface temperature.

In an effort to reconcile these seemingly contradictory sets of
measurements, the National Academy of Sciences (NAS) convened an ad-hoc
Panel whose members included the developers of the MSU and experts in
remote temperature sensing, ground-based and balloon-borne atmospheric
temperature measurements, and specialists in the detection and modeling of
global climate change.  Represented among the eleven Panel members were
three lead authors of chapters of the1995 and forthcoming IPCC
(Intergovernmental Panel on Climate Change) scientific assessments, and
individuals representing a wide spectrum of viewpoints with respect to the
greenhouse warming issue.

The NAS Panel assessed the uncertainties inherent in the satellite,
balloon-borne and ground-based measurements, and it considered the
possibility that systematic biases might still remain despite the numerous
corrections that have been made thus far.  It also considered the various
technical issues that arise in comparing data sets with different sampling
characteristics.  The Panel's task was rendered more complicated by the
fact that satellites and ground-based thermometers do not measure the same
physical quantity: for a variety of reasons surface temperatures and
temperatures aloft do not track one another perfectly, either locally or in
the global average

The episodic periods of warmth associated with El Nino events and the
periods of global cooling that follow in the wake of major volcanic
eruptions influence temperatures at different levels of the atmosphere to
varying degrees.  Likewise, humans exert influences on climate that result
in a warming at the surface due to the buildup of greenhouse gases and a
cooling in the stratosphere brought on by the depletion of stratospheric
ozone.  In the presence of natural climate variability operating on a
variety of timescales, a 20-year period of record such as the
satellite-based record of temperature examined by the panel of scientific
experts, cannot yet be regarded as representative of the longer-term
behavior of the climate system.

The Panel of experts affirmed the conclusion of the 1995 IPCC report that
global mean surface temperature has warmed rapidly since 1979; and it noted
that the upward temperature trend has continued and accelerated in the
years since the 1995 report went to press.  A larger degree of uncertainty
remains with regard to the tropospheric temperature measurements, but the
Panel believes it is more likely than not that the troposphere has been
warming, but at a rate less than that of the temperature at the Earth's
surface.  It is conceivable that there will need to be further adjustments
to the estimates of global-mean surface and tropospheric temperature trends
to account for any additional sources of bias that have not yet been

The Panel stressed that even if the current estimates of surface,
radiosonde, and satellite measurements prove to be correct, there is no
basis for expecting that the surface will continue to warm at a rate faster
than the troposphere in future decades.  The Panel noted that the high
degree of uncertainty inherent in the tropospheric temperature measurements
underscores the need for more comprehensive global observations for
monitoring climate change.

                       Evidence in Support of the Findings

Land-based surface temperatures are measured directly, while sea-surface
temperatures (SSTs) are used to establish the monthly temperature of the
air just above the ocean surface.  Because daily temperature variability of
the ocean surface is relatively small, SSTs can be reliably determined with
fewer observations than would be required to establish marine air
temperatures.  Coverage increases with time and is better after 1950, and
global after 1982, when the capability of satellites to measure sea surface
temperatures was added.  Biases occur through changes in observing
practices and changes in land use, such as the urban heat island effect.
The advantages are the long record of ground-based measurements of
temperature from the mid 1800s, many independent measurements, several
independent analyses, and many cross checks such as Northern versus
Southern Hemisphere values, rural vs urban, global vs land-based vs ocean
vs marine air temperatures.  The disadvantages are the less than global
coverage in sampling, which changes with time.  For example,
underrepresentation of the Antarctic and the Southern Ocean might result in
a slight overestimate in estimates of the present temperature trend.
Nonetheless, there is a high degree of confidence that the observed surface
temperature trends are robust.

Balloon-borne temperatures of the lower atmosphere, assembled from up to
nine hundred radiosonde stations that began operating in the mid-1940s, are
at best twice daily, and were standardized in July, 1957.  These radiosonde
stations provide good vertical resolution of temperature profiles.  The
biases stem from many changes in instrumentation and observing methods,
many of which have poor or no documentation.  Known biases occur in some
brands of radiosonde equipment, often due to radiation effects.  The
advantages are that each sounding is with a new instrument; there are
dozens of instrument types; and a few groups provide independent analyses
of the record.  The disadvantages are the dozens of instruments that are
inadequately calibrated, with biases, often unknown, and that change with
time; and much less than global coverage.

The satellite temperatures are estimated from microwave radiation emissions
from oxygen which are proportional to temperature and are known as the "MSU
2LT" temperatures (otherwise referred to as the Microwave Sounder Unit,
Channel 2, Lower Troposphere).  MSU data retrieval requires measuring
microwave radiation in the troposphere from a variety of angles in order to
calculate the 2LT temperature record.  Coverage is global over a few days,
two or four times per day, and began in December, 1978.  Observation times
vary from one satellite to another, and as each satellite drifts in orbit.
Only very broad vertical layers of the atmosphere can be sensed.  The
biases arise from the use of nine different satellites which have been
deployed to construct the satellite temperature record; orbital decay
affects the 2LT temperature data retrieval because it changes the angles at
which measurements are taken; and east-west drift alters the time of day at
which measurements are taken.  A shift in the sampling time from early to
mid afternoon, for example, would produce a spurious warming.  Instrument
calibration and solar heating of the MSU instrument platform require
corrections.  The retrieval process itself amplifies the background noise,
which interferes with the temperature signal that one is trying to detect.

The advantages are the long-term stability of microwave radiation emissions
from oxygen (the proxy for tropospheric temperatures), and the global,
fairly uniform, coverage.  Biases are well determined if there is adequate
satellite overlap and millions of observations to help reduce the random
noise.  The disadvantages are that the temperature signal one is trying to
measure includes a signal from 20% of the land surface (ideally, 100% of
the signal should derive from the atmosphere); contamination by
precipitation-sized ice; and biases are not constant.  Consequently,
continuity of measurements across different satellites is an issue, and
overlap of measurements between and among the NOAA satellites is
inadequate.  Only one group of scientists, for the most part, has processed
the satellite temperature data, making it difficult, if not impossible, to
independently check the methodology, the data and the conclusions.

As data sets have been improved, discrepancies among them have been reduced
and there seems to be good agreement between the radiosonde and MSU
tropospheric temperatures, although the radiosonde record is inadequate in
the tropics and Southern Hemisphere.  The fact that the surface temperature
and the tropospheric temperature are two physically different quantities is
believed to account for a considerable part of the differences between
them.  In particular, observed stratospheric ozone depletion cools the MSU
but not the surface; episodic volcanic eruptions cool MSU more than the
surface; increasing greenhouse gases warm the MSU more; solar effects are
small but make for an added complication; and tropospheric aerosols are
changing and have complex regional and vertical profile effects that are
not well known.  El Nino and other natural climate variability is likely to
produce a larger temperature signal in the troposphere, while day-night
differences are greatest at the surface. Land-ocean differences are also
greater at the surface where winds are weaker.


Dr. John M. Wallace  is a professor in the Department of Atmospheric
Sciences, and co-director of the Program on the Environment, at the
University of Washington, Seattle.  From 1981-98 he served as director of
the (University of Washington/NOAA) Joint Institute for the Study of the
Atmosphere and Ocean.  His research interests and expertise include the
study of atmospheric general circulation, El Nino, and global climate.  He
is a member of the National Academy of Sciences; and a fellow of the
American Association for the Advancement of Science, the American
Geophysical Union, and the American Meteorological Society.  He is also a
recipient of the Rossby medal of the American Meteorological Society and
the Roger Revelle medal of the American Geophysical Union.  He has served
on numerous panels and committees of the National Research Council.

Dr. Kevin E. Trenberth is a senior scientist and Head of the Climate
Analysis Section of the National Center for Atmospheric Research (NCAR) in
Boulder, Colorado.  Prior to joining NCAR in 1984, he was a Professor of
Atmospheric Sciences at the University of Illinois and earlier worked in
the New Zealand Meteorological Service.  Dr. Trenberth has also contributed
significantly to the Intergovernmental Panel on Climate Change (IPCC), as a
lead author for Chapter 1 of the 1995 Scientific Assessment, and as a lead
author for the 2000 IPCC assessment.

Dr. Trenberth has published over 280 peer-reviewed scientific articles or
papers, including 26 books or book chapters.  He has served on a number of
advisory committees and panels, including several dealing with El Nino
research.  He is a member of the International Scientific Steering Group
for the World Climate Research Programme's Climate Variability and
Predictability Programme, for which he recently (1996-1999) served as
co-chair; he serves on the Joint Scientific Committee of the WCRP; he is a
member of the National Research Council's Climate Research Committee Panel
on Reconciling Temperature Observations and the Committee on Global Change
Research; he is a member of the National Oceanic and Atmospheric
Administration (NOAA) Advisory Panel on Climate and Global Change and
NOAA's Council on Long-term Monitoring; he serves on the National Science
Foundation's Climate System Modeling Advisory Board; and he serves as a
member of the ECMWF's (European Center for Medium Range Weather Forecasts)
Reanalysis Project Advisory Group.

Dr. Trenberth is a Fellow of the American Meteorological Society and the
American Association for the Advancement of Science, as well as an Honorary
Fellow of the New Zealand Royal Society.  He is also the most recent
recipient of the Jule G. Charney award from the American Meteorological

The Next Seminar is scheduled for Monday, February 28, 2000

Tentative Topic: "Climate Change in the Arctic and Antarctic: The Latest
Observation Evidence on Changes in Sea Ice and Ice Shelves"

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."

More information about the Coral-list-old mailing list