What is the scientific case for global warming and its causation? Is the scientific case for global warming built upon multiple lines of internally consistent evidence, or is the evidence thin and contradictory? How significant is the role of humans in global warming and what is the evidentiary basis for this claim?
Dr. Anthony Socci, Senior Science Fellow, American Meteorological Society
Dr. David R. Easterling, Chief, Scientific Services Division, National Oceanic and Atmospheric Administration, National Climatic Data Center, Asheville, NC
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Dr. Lonnie G. Thompson, Distinguished University Professor and Senior Research Scientist, School of Earth Sciences, Byrd Polar Research Center, Ohio State University, Columbus, OH
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Dr. Michael E. Mann, Associate Professor, Departments of Meteorology and Geosciences, and Director of the Earth System Science Center (ESSC), Pennsylvania State University, University Park, PA
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Dr. Benjamin D. Santer, Research Scientist, Program for Climate Model Diagnosis and Intercomparison, Lawrence Livermore National Laboratory, Livermore, CA
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Evidence from Observations – Dr. Easterling Observed Changes in Climate in the United States and the Globe.
Observed changes in the globally averaged annual temperature shows a linear increase of approximately 0.6oC/century since the late 1800s, but the rate of increase since the mid-1970s has itself increased to approximately 2oC/century. Seasonally the strongest increases have occurred in the boreal winter (DJF) and spring (MAM). Regionally, in the U.S. the southeastern U.S. and northern Atlantic continue to show a trend to cooling, but the southeastern U.S. trend appears to be getting smaller with each additional year of data and the largest increases in temperature have occurred in the high latitudes of the Northern Hemisphere. Global changes in precipitation shows an increase from 1901 to approximately 1950, then little trend in the period since 1950.
Research into possible changes in extremes in the United States has focused on heavy precipitation, drought, and hurricanes, and following is a summary of the current state of knowledge. Over the past several decades increases in the amount of precipitation occurring in the heaviest daily precipitation events has been observed in many areas of central and eastern United States over the past several decades. However, U.S. droughts show pronounced multiyear to multi-decadal variability but there is no convincing evidence for systematic long-term trends toward more or fewer events. Atlantic hurricane activity, on the other hand, has significantly increased since 1995, with more hurricanes, and more intense hurricanes, compared to the two previous decades and this is also reflected in those hurricanes striking the U.S. However, earlier periods, such as the 1945 to 1970 period were nearly as active. Recent analyses of precipitation rates from hurricanes over the Southeast U.S. show that daily precipitation amounts increase with tropical cyclone strength, while hourly precipitation does not. This means that the more intense hurricanes have longer periods with heavy rainfall.
Evidence from Observations of Galciers
and Ice Sheets – Dr. Thompson
Over the last 30 years ice core records have been recovered systematically from eleven highelevation ice fields, ten of which are located in middle and tropical latitudes. Analyses of these ice cores and of the glaciers from which they have been drilled have yielded three lines of evidence for abrupt climate change both past and present. They are: (1) the temperature and precipitation histories recorded in the glaciers as revealed by the climate records extracted from the ice cores; (2) the accelerating loss of the glaciers themselves, and; (3) the uncovering of ancient plants and animals from the margins of the glaciers as a result of their recent melting, thus illustrating the significance of the current ice loss. Each of these lines of evidence are explained here in more detail.
First, the analysis of oxygen from the glacier ice (used as a temperature proxy) and the measurement of the amount of ice that forms each year (used as a precipitation proxy) have given us histories from ice cores that demonstrate that the current warming at high elevations in the middle and tropical latitudes is unprecedented for at least the last two millennia. Second, the continuing retreat of most middle latitude and tropical glaciers, many of which have persisted for thousands of years, signals a recent and abrupt change in the Earth’s climate system. Changes documented in 2006 on Mt. Kilimanjaro in Africa and on the Quelccaya ice cap in the tropical Andes of Peru clearly demonstrate that not only are these mountain glaciers continually retreating, but that the retreat is accelerating. Finally, as the glacier margins disappear, plants and animals that have been entombed in the ice, in some cases for thousands of years, are being revealed. For example, rooted, soft-bodied wetland plants are currently being exposed along the margins of the Quelccaya ice cap as it melts away. These plants have been radiocarbon-dated and, when coupled with other proxy evidence, provide strong evidence for an abrupt climate event that marked the transition from warmer conditions in Peru from about 10,000 to 5000 years ago to cooler conditions afterwards. This cold climatic episode, which occurred roughly 5200 years ago, was widespread and spatially coherent through much of the world and was coincident with structural changes within several civilizations. These three lines of evidence, when viewed together, argue that the present warming and associated glacier retreat are unprecedented in some areas for at least 5200 years. The ongoing global-scale, rapid retreat of mountain glaciers is not only contributing to global sea level rise, but is also threatening fresh water supplies in many of the world’s most populous regions. The current and present danger posed by ongoing climate change is clear.
Evidence from the Earth’s Paleoclimate Record – Dr. Mann
Earth’s climate exhibits variations on all resolvable timescales, from the interannual (year to year) to the geological (millions of years and longer). This variability is known to result from both internal and external (including humann or ‘anthropogenic’) factors. Over the past two millennia, the basic boundary conditions of Earth’s climate (e.g., the continental arrangement, orography, Earth-orbital parameters, and the spatial extent of continental ice sheets) have not changed significantly. This time interval thus provides an appropriate context for estimating the envelope of natural climate variability within which modern climate change should be interpreted. Because the instrumental record can only provide information regarding large-scale (e.g., hemispheric) climate changes over only the past one-and-a-half centuries, and selected regions for only the past few centuries, it is essential that we turn to other lines of evidence to evaluate the longer-term changes over the past one or two millennia.
One line of evidence is provided by so-called proxy climate data, natural or historical archives of information that describe, albeit imperfectly, climate variations in prior centuries. Those proxy data with relatively high (decadal or better) resolution, such as tree rings, corals, ice cores, historical records, and in some cases speleothems, and lake and marine sediments can be used to reconstruct climate variations over past centuries and, in some cases, as far back as the past two millennia. At hemispheric scales, the warmth of recent decades appears unprecedented in the context of at least the past 1000 years, and probably longer.
It is also possible to estimate from proxy and historical sources the external “forcings” of climate in past centuries (specifically, volcanic and solar natural radiative forcing, and anthropogenic greenhouse gas, aerosol, and land-use changes). These estimates can be used to drive theoretical climate model simulations. A comparison of proxy-based reconstructions with model simulation affirm the role of natural (solar and volcanic) radiative forcing changes in climate changes over past centuries. However, the anomalous warmth of recent decades can only be explained by modern anthropogenic forcing.
Model-Based Evidence of Global Warming – Dr. Santer
Human activities have significantly altered not only the chemical composition of Earth's atmosphere, but also the climate system. Human influences have led to increases in well-mixed greenhouse gases, decreases in stratospheric ozone, and changes in the atmospheric burdens of sulfate and soot aerosols. All of these atmospheric constituents interact with incoming solar and outgoing terrestrial radiation. Human-induced changes in the concentrations of these constituents modify the natural radiative balance of Earth's atmosphere, and therefore perturb climate.
Quantifying the size of the human effect on climate is a difficult statistical problem. "Fingerprint" methods are typically used for this purpose. These methods involve rigorous statistical comparisons of modeled and observed climate change patterns. Fingerprinting assumes that each individual influence on climate has a unique signature in climate records. The climate fingerprints in response to different forcing factors are typically estimated with computer models, which can be used to perform the controlled experiments that we cannot conduct in the real world.
One criticism of the findings of previous scientific assessments is that they have relied heavily on fingerprint studies involving changes in near-surface temperature. Recent fingerprint work, however, has considered a variety of other climate variables, such as ocean heat content, stratospheric temperatures, Northern Hemisphere sea ice extent, sea level pressure, and tropopause height. These studies illustrate that a human-induced climate change signal is identifiable in many different aspects of the climate system. Such internal and physical consistency, manifest in both observed and simulated climate changes, effectively refutes criticism that previous "discernible human influence" conclusions rest on a single variable only.
Dr. David R. Easterling is currently Chief of the Scientific Services Division at NOAA’s National Climatic Data Center in Asheville, NC. He received his Ph.D. from the University of North Carolina at Chapel Hill in 1987 and served as an Assistant Professor in the Climate and Meteorology Program, Indiana University-Bloomington from 1987 to 1990. In 1990 he moved to the National Climatic Data Center as a research scientist, was appointed Principal Scientist in 1999, and Chief of Scientific Services in 2002. Dr. Easterling was also a Contributing Author to the Intergovernmental Panel on Climate Change (IPCC) Second and Third Assessment Reports, and is currently a Lead Author for the IPCC Fourth Assessment Report and a Convening Lead Author for the U.S. Climate Change Science Plan (CCSP) Synthesis and Assessment Product (SAP) 3.3 on Climate Extremes. His research interests include the detection of climate change in the observed record, particularly changes in extreme climate events. Dr. Easterling has authored or co-authored more than sixty research articles on climate change issues in journals such as Science, Nature and the Journal of Climate.
Dr. Lonnie G. Thompson is one of the world’s foremost authorities on paleoclimatology and glaciology. He has led more than 50 expeditions during the last 30 years, conducting ice-core drilling programs in the world’s polar regions as well as in tropical and subtropical ice fields. Recently, Dr. Thompson and his team developed lightweight solar-powered drilling equipment for the acquisition of histories from ice fields in the high Andes of Peru and on Mount Kilimanjaro in Tanzania. The results of these histories, published in more that 200 articles, have contributed greatly toward the understanding of the Earth’s past, present and future climate system. Other Thompson-led expeditions have recovered a 460-meter-long ice core, the world’s longest from a mountain range (Alaska, 2002); the first tropic ice core (Peru, 1983); and cores containing the entire sequence of the Last Glacial Stage (duration?/Chronology?) as well as cores dating over 750,000 years in age, the oldest outside the polar regions (Tibet, 1992).
Dr. Thompson’s research has resulted in major revisions in the field of paleoclimatology, in particular, by demonstrating how tropical regions have undergone significant climate variability, countering an earlier view that higher latitudes dominate climate change. Thompson has received numerous honors and awards. In 2005, he was elected to the National Academy of Sciences and was awarded the John and Alice Tyler Prize for Environmental Achievement. He has been selected by Time magazine and CNN as one of “America’s Best” in science and medicine. His research has been featured in hundreds of publications, including National Geographic and the National Geographic Adventure magazines.
He and his team are the subject of a new book entitled: Thin Ice: Unlocking the Secrets of Climate in the World’s Highest Mountains by Mark Bowen published in late 2005. In 2006, he has been elected member of the American Philosophical Society, Alumni member of Phi Beta Kappa and chosen to receive the Roy Chapman Andrews Society, 2007 Distinguished Explorer Award.
Dr. Michael E. Mann is an associate professor in the Departments of Meteorology and Geosciences, and Director of the Earth System Science Center (ESSC) at Penn State University. He received his undergraduate degrees in Physics and Applied Math from the University of California at Berkeley, an M.S. degree in Physics from Yale University, and a Ph.D. in Geology & Geophysics from Yale University. Current areas of research include model/data comparisons aimed at understanding the long-term behavior of the climate.
Dr. Mann was a Lead Author of the Intergovernmental Panel on Climate Change (IPCC) Third Scientific Assessment Report, and has served as chair for the National Academy of Sciences 'Frontiers of Science'. He has received the outstanding publication award from NOAA, and in 2002 was selected as one of the 50 leading visionaries in science and technology by Scientific American. He is author of more than 90 peer-reviewed and edited publications. He was also a cofounder of the website “RealClimate.org”.
Dr. Ben Santer is an atmospheric scientist at Lawrence Livermore National Laboratory (LLNL). His research focuses on such topics as climate model evaluation, the use of statistical methods in climate science, and identification of natural and anthropogenic "fingerprints" in observed climate records. His work on the climatic effects of combined changes in greenhouse gases and sulfate aerosols contributed to the "discernible human influence" conclusion of the 1995 Report by the Intergovernmental Panel on Climate Change (IPCC). Dr. Santer holds a doctorate in climatology from the University of East Anglia in England, where he studied under Prof. Tom Wigley. After completion of his Ph.D. in 1987, he spent five years at the Max-Planck Institute for Meteorology in Germany, where he worked with Prof. Klaus Hasselmann on the development and application of climate fingerprinting methods. In 1992, Dr. Santer joined the Program for Climate Model Diagnosis and Intercomparison at LLNL. He served as convening Lead Author of the climatechange detection and attribution chapter of the 1995 IPCC report, an experience best described as "character building". His awards include a MacArthur Fellowship (1998), the Norbert Gerbier- MUMM international award from the World Meteorological Organization (1998), the U.S. Dept. of Energy's E.O. Lawrence Award (2002), and a U.S. Dept. of Energy Distinguished Scientist Fellowship (2005). Dr. Santer has over sixty publications in the peer-reviewed scientific literature, and has contributed to ten books.
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