SOURCE: Data are compiled
by the University of East Angolia and the United Kingdom Meteorological
Office, and will be published in the 2nd
Assessment Report of the Intergovernmental Panel on Climate Change (1995)
[see Summary for Policymakers]
How is the Earth's temperature
measured? What are the historical trends in the Earth's temperature
as observed from surface measurements and from satellites? Are these
records different? What are the reasons for the differences? Can satellite
and surface temperature records be reconciled? Where do the uncertainties
lie and how can they be addressed? To what extent do the records indicate
that climate is changing due to human influences? What is the evidence
that humans are having a discernible influence on the global climate?
Dr. Michael C. MacCracken,
Director, Office of the US Global Change Research Program, Washington,
Monday, May 20: The Satellite Temperature Record
Dr. John R. Christy, Earth
System Science Lab, University of Alabama, Huntsville, Alabama, on "The
Tropospheric Temperature Record from the Microwave Sounding Units"
Dr. Kevin E. Trenberth, Climate Analysis Section, National Center for
Atmospheric Research, Boulder, Colorado, on "Relating the Satellite
and Surface Temperature Records"
Tuesday, May 21:
The Surface Temperature Record
Dr. Tom M. L. Wigley, Senior
Climate Scientist, National Center for Atmospheric Research, Boulder,
Colorado, on "Interpreting the Global Warming Record"
Dr. Benjamin D. Santer, Program for Climate Model Diagnosis and Intercomparison,
Lawrence Livermore National Laboratory, Livermore, California, on "The
Search for a "Fingerprint" of Human Activities in Observed
Temperature is perhaps the
most common measure of the climate of a region, whether it is the cold
temperatures of winter in Minnesota or the hot temperatures of summer
in Arizona. Temperature, along with precipitation, also controls many
aspects of ecosystems, helping determine spring blooming and the extent
of mosquitoes and other vectors for diseases. For these reasons and
more, the longest records of climate in many areas are of temperature.
Similarly for the globe, records of temperature are the most abundant,
provide the longest quantitative record, and can be most readily compiled
and compared. Analysis of the temperature record, on scales from regional
to global, has thus been a critical part of studies of the patterns
and extent of climatic change.
While temperature is the most complete record, the measurements and
available data sets, nonetheless, have many shortcomings. For surface
measurements, these include changes in measurement techniques, limits
to the coverage of measurements, changes in the surroundings around
a station, and many more. Efforts are therefore being made to measure
the Earth's temperature from space, but again there are many limitations,
including, among others, the inability to measure surface temperature,
the changing sequence of instruments, and the limited length of the
This seminar will provide the opportunity to look closely at the records
of both satellites and surface stations, to consider their relative
strengths and weaknesses, and to consider what these records show and
do not show.
The Satellite Temperature
Since 1979, Microwave Sounding
Units (MSUs) on NOAA polar orbiting satellites have measured the intensity
of upwelling microwave radiation from atmospheric oxygen. The intensity
is proportional to the temperature of broad vertical layers of the atmosphere,
as demonstrated by theory and direct comparisons with atmospheric temperatures
from radiosonde (balloon) profiles. A record that is now more than 17
years long has been created by merging data from nine different MSUs,
each with peculiarities (e.g., time drift of the spacecraft relative
to the local solar time) that must be calculated and removed because
they can have substantial impacts on the resulting trend.
A natural step with such a record is to look for trends over this period,
even though it is quite short compared to the surface temperature record.
Between 20°N-20°S, independent view-angle trends in channel
2 and 4 show a warming trend in the upper troposphere with cooling in
the lower troposphere, implying a non-linear vertical temperature adjustment.
The greatest differences between the satellite and surface records occur
between 30°S-30°N and 52°N-82°N. An important aspect
of deriving trends and looking for any human influence, especially over
short periods, is accounting for what might be irregular and extraneous
natural events. For the MSU record, these include the century's largest
El Nino warming in the early 1980s and the century's largest volcanically
induced cooling in the early 1990s. The tilt in the trend created by
these two events suggests a cooling trend over the period of record.
When the MSU records are adjusted for El Nino events and volcanoes so
that the greenhouse/aerosol effect will likely be the dominant influence,
the resulting temperature trend is positive, rising at a rate of +0.055
to 0.011°C per decade.
Because the MSU observations are measuring the temperature of the atmosphere
and not of the surface, an important question is how the two are related.
While the traditional notion has been that they are closely coupled
at all times and throughout the world, this has turned out not to be
the case. Recent research is starting to provide explanations for the
apparent differences and to explain when and where decoupling of the
two temperatures occurs, and how this is likely to affect comparison
of the two records.
At the May 20 seminar, Dr. Christy will describe the MSU record and
indicate the different indications that it provides of climate change.
Dr. Trenberth will describe how the satellite record compares to the
surface temperature record and what this means with respect to conclusions
that can be drawn.
The Surface Temperature
According to a recently
released report from the World Meteorological Organization, the estimated
global mean surface air temperature for 1995 was the highest since reliable
temperature records began in 1861. The previous warmest year was 1990,
which was just before the Mt. Pinatubo volcanic eruption that has suppressed
temperatures for the past several years. The warmth in 1995, unlike
that for 1990, could not be attributed to an El Nino because the average
Equatorial Pacific Ocean temperature anomalies were near the 1961-90
average. Instead, the warmth was evident over other regions, including
the North Atlantic Ocean, where sea surface temperatures were more than
1°C warmer in an area centered around the Azores. In addition,
parts of Siberia were more that 3°C warmer than the 1961-1990 period.
However, as would be expected because of year-to-year variations, the
warmth was not uniform; Greenland, the northwest Atlantic Ocean, and
the mid-latitudes of the North Pacific Ocean were actually cooler than
average in 1995.
Temperature records for a representative fraction of the Earth go back
to 1861. The temperature record since that time suggests an overall
warming of 0.3 to 0.6°C from the 1860s to the 1990s, with the early
decades of this century being slightly cooler than in the mid-19th century
and with a secondary maximum of temperatures (compared to the 1990s)
in the decades around 1940. Proxy records derived from tree rings, ice
cores, and other indirect measures, combined with the thermometer record,
suggest that the most recent decades are the warmest period since at
least 1400 AD, and perhaps as far back as the last interglacial about
80,000 years ago. The IPCC concluded that this combination of factors
suggested that climate change is occurring.
The fact that there have been natural fluctuations of the climate over
the past millennium of about 0.5°C (about a cooler mean temperature),
this introduces the possibility that the recent warming might be due
to natural processes rather than to human activities. To try to distinguish
the human influence, model simulations have been used to generate the
patterns of climate change to be expected from changes in a range of
different factors, both natural and human-induced. Analyses of these
characteristic patterns (or "fingerprints") indicate that
the patterns of climate change are much more likely to be due to human
activities than to natural factors, leading the IPCC to conclude that
"the balance of evidence suggests that there is a discernible human
influence on global climate."
At the May 21 seminar, Dr. Wigley will describe the records of surface
temperatures, the climate trends that emerge, and compare these to the
model projections of climate change since the 1860s. Dr. Santer will
then describe the recent studies to attribute the observed changes to
specific causes of change, especially to human activities.
Dr. John R. Christy is Associate
Professor of Atmospheric Science at the University of Alabama in Huntsville,
and has studied global climate issues since 1987. In 1989 Dr. Roy W.
Spencer, a NASA/Marshall scientist, and Dr. Christy developed a global
temperature data set from microwave data that had been recorded by the
MSU instrument on NOAA satellites beginning in 1979. For this achievement,
the Spencer-Christy team was awarded NASA's Medal for Exceptional Scientific
Achievement in 1991. In 1995 Dr. Christy and Dr. Spencer received a
Special Award from the American Meteorological Society "for developing
a global, precise record of Earth's temperature from operational polar-orbiting
satellites, fundamentally advancing our ability to monitor climate."
Dr. Christy obtained his B. A. degree from the California State Univ.,
Fresno (Mathematics) in 1973, and later taught science as a missionary
teacher in Nyeri, Kenya. After earning a seminary degree in 1978, he
served four years as a bivocational mission-pastor in South Dakota where
he also taught college math. He subsequently received M.S. and Ph.D.
degrees in Atmospheric Sciences from the University of Illinois (1984,
1987) under Dr. Kevin Trenberth. Dr. Christy has served as a contributing
lead author on climate assessment reports by the Intergovernmental Panel
on Climate Change (1992, 1994 and 1995), and has also published numerous
scientific articles including studies appearing in Science, Nature,
the Journal of Climate and the Journal of Geophysical Research.
Dr. Kevin Trenberth was born in New Zealand, where he remains
a citizen. He is Head of the Climate Analysis Section at the National
Center for Atmospheric Research (NCAR) in Boulder, CO. After completing
a first class honors degree in mathematics at the University of Canterbury,
Christchurch, New Zealand, he obtained his Sc. D. in meteorology in
1972 from Massachusetts Institute of Technology. Following several years
in the New Zealand Meteorological Service, he joined the Department
of Atmospheric Sciences at the University of Illinois as an Associate
Professor and became a full Professor in 1984, before moving to NCAR
in 1984. He continued as an Adjunct Professor until 1989. From 1991
to 1995 he served as Deputy Division Director of the Climate and Global
Dr. Trenberth has served as Editor of the Monthly Weather Review,
Associate Editor for the Journal of Climate, and presently serves
as editor of the new electronic scientific journal Earth Interactions
and is the author of many research papers. He serves on the executive
committee of the National Oceanic and Atmospheric Administration (NOAA)
Advisory Panel on Climate and Global Change, the National Academy of
Sciences Global Ocean Atmosphere Land System (GOALS) panel, the Atmospheric
Observation Panel for Climate of the Global Climate Observing System,
and the International Scientific Steering Group for the CLIVAR (Climate
Variability and Predictability) Program. Dr. Trenberth has been a prominent
author in the Intergovernmental Panel on Climate Change (IPCC) Scientific
Assessment activities and is a lead author for Chapter 1 of the 1995
Scientific Assessment. He is a fellow of the American Meteorological
Society, and was made an Honorary Fellow of the New Zealand Royal Society
Dr. Tom Wigley was born and educated in Australia. After his
undergraduate degree he trained as a meteorologist and worked for a
year as a research meteorologist before returning to the university
to complete a Ph.D. in Mathematical Physics. He then joined the faculty
of the Mechanical Engineering Department at the University of Waterloo,
Ontario, Canada. In 1975, he moved to the United Kingdom to the Climatic
Research Unit of the University of East Anglia, becoming Director in
1978. In 1993, he left the Unit to join the University Corporation for
Atmospheric Research (UCAR) in Boulder, CO. In 1994, he received a Senior
Scientist appointment with the National Center for Atmospheric Research.
Dr. Wigley has published widely on diverse aspects of the broad field
of climatology; from data analysis, to climate impacts on agriculture
and water resources, to climate, sea level and carbon cycle modeling,
to paleoclimatology. Dr. Wigley has concentrated recently on facets
of the greenhouse problem, and has contributed as a lead author to all
of the IPCC assessments of the climate change issue. Dr. Wigley had
a major role in the preparation of the 1995 IPCC Working Group I Second
Assessment Report, and contributed important information to the reports
of the other Working Groups. He was responsible for producing the future
concentration profiles for achieving stabilization of CO2 concentrations
used in Working Groups I and III, he produced the global-mean projections
for temperature and sea level change given in Working Group I, and he
was a lead author for the Working Group I detection chapter.
Dr. Benjamin D. Santer is a senior member of the Program for
Climate Model Diagnosis and Intercomparison (PCMDI) at the Lawrence
Livermore National Laboratory (LLNL), Livermore, CA. His research interests
include detection of anthropogenic climate change and climate model
validation. He received his B.Sc. in environmental sciences in 1977,
graduating with first class honors, and his Ph.D. in climatology in
1987. Both were obtained at the University of East Anglia, Norwich,
U.K. Dr. Santer's doctoral work focused on the use of Monte Carlo methods
(randomization) in the regional validation of climate General Circulation
Dr. Santer then served as a postdoctoral research scientist (for two
years), and later as a research scientist (for three years) at the Max-Planck
Institute for Meteorology
(MPI) in Hamburg, Germany, where he worked closely with Dr. Klaus Hasselmann
on climate-change detection. He is the Convening Lead Author for Chapter
8 ("Detection of Climate Change, and Attribution of Causes")
of the 1995 Second Assessment Report of the Intergovernmental Panel
on Climate Change. Dr. Santer is also currently a member of the Science
Advisory Group of NOAA's Climate Change, Data and Detection Program,
and of the International CLIVAR (Climate Variability and Predictability)
Numerical Experimentation Group on anthropogenic climate change.