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, DC

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"

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 Climate Records"

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 record.

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.

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.

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 Dynamics Division.

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 in 1995.

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 Models.

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.