INTRODUCTION:

Dr. Herman Zimmerman
Paleoclimate Program Director, National Science Foundation, Arlington, VA

SPEAKERS:

Dr. Michael E. Mann
Department of Geosciences, University of Massachusetts, Amherst, MA

Dr. Mark F. Meier
Professor Emeritus, Department of Geological Sciences, and Fellow of the Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO

Assessing the significance of the global warming of the 20th century has traditionally been hampered by a sketchy knowledge of climate variations during past centuries. Widespread instrumental climate data are only available during the 20th century (and even during this period, provides nearly complete coverage only for the Northern Hemisphere and tropical Southern Hemisphere). To go back much further in time, indirect measurements of climate variations derived from natural archives or "proxy" climate indicators such as tree rings, corals, and ice cores, must be used to characterize climate variations.

By using modern statistical techniques to match the widespread instrumental record of the 20th century to natural archives or "proxy" climate indicators such as tree-ring, coral, and ice-core records, combined with the suite of long historical climate records, global patterns of annual temperature have been reconstructed several centuries back in time, with relatively small uncertainties. These uncertainties have been accurately estimated, providing a faithful assessment of the level of certainty in reconstructions of the climate during past centuries. With the longer term perspective afforded by this reconstructed climate history, this reassessment of the Earth's temperature record provides an arguably more robust assessment of recent global warming because of its long term context. The evidence suggests that the decade of the 1990s, especially the years 1990, 1993, and 1997, are almost certainly the warmest back to AD 1400 for the Northern Hemisphere as a whole. The El Niño phenomenon also appears to have increased in intensity in recent decades relative to its pre-20th century behavior. This trend is not, however, as dramatic as that seen in hemispheric temperatures, and the evidence is more tentative given the larger uncertainties inherent in reconstructing this phenomenon. The reconstructed climate patterns were tested for their reliability through a battery of statistical "verification" experiments which demonstrated that the proxy-based climate patterns could reliably ÒpredictÓ early thermometer measurements. The success of the comparison provides compelling evidence that the reconstructions can be trusted back in time. These tests also indicated that, with the proxy data networks presently available, it is difficult as yet to draw conclusions about global climate variations further back in time than AD 1400.

The changes in hemispheric temperature over several centuries were related to possible influences or "forcing agents" through comparisons with the estimates of changes in the three most physically plausible external factors governing climate change over timeÐÐchanges in the brightness of the Sun back in time as estimated by solar physicists, the documented history of explosive volcanic eruptions, and human-caused increases in greenhouse gas concentrations as represented through long-term records of carbon dioxide trapped in ancient ice cores and more recently recorded by humans. These comparisons sought to determine which of these three forcings of climate were most closely related, in a statistical sense, to the variations in hemispheric temperatures over time. The results of this analysis suggests that the significant temperature variations in past centuries likely have their origins in natural climate forcingÐÐvariations in the brightness of the Sun in particular. While these natural factors will no doubt continue to play a role in governing the natural climate variability that operates in the backdrop of human-induced changes in climate, the anomalous warmth of recent decades cannot be explained in terms of these natural factors. Instead, the recent warming shows a sharply emerging significant correlation with increasing greenhouse gases during the past couple of decades. In this sense, the human-enhanced greenhouse warming signal now appears to be detectable above the background of natural climate variability. The recent IPCC conclusion that the "fingerprint" of human activity is discernible in the most recent climate trends is consistent with independent studies that have compared model-predicted and observed trends during the 20th century.

Glaciers and ice caps have been retreating, thinning, and disappearing all over the world during this century, interrupted by short periods of growth in some areas. This recession (shrinkage) rate is now accelerating. Glaciers are thought to be sensitive indicators of climate. This recession therefore, is a tangible, highly visible indicator of a changing climate. Glacier wastage also has direct environmental and societal impacts, especially in regard to sea-level rise and river flow. A new mathematical technique called "scaling", now allows characterization and generalization of glacier changes on a global basis.

Current changes in the two huge continental ice sheets (Greenland and Antarctica) are not fully understood, and are not considered here. Instead, these analyses focus on the behavior of the 160,000 or so "small" glaciers and ice caps that are of major importance as climate indicators, and as sources of runoff that contribute to sea-level change.

What Do Measurements on the Glaciers Tell us about Climate?

Measurements of mass balance (snow input, minus melting and runoff output) provide information on glacial (and climate) change, especially during the period since the early 1960s when many observational programs began. These data are important because they record changes in precipitation and temperature in high mountains and at high latitudes where other data are sparse. On a global basis, snow accumulation appears to be increasing slightly, especially at higher altitudes. This implies a slight increase in winter precipitation. However, melting rates are increasingly significant, especially at lower altitudes, suggesting a major increase in summer air temperature. Both data sets show increasing year-to-year variability. Combining these data, the net mass balance of glaciers is increasingly negative (snow or ice loss exceeds accumulation). On average, the world's glaciers lost 0.18 meters (0.60 feet) per year of water-equivalent of thickness from 1961 to 1990, and the rate of ice loss has been increasing with time. This averaged trend of a net negative glacial mass balance roughly follows the trend in the Northern Hemisphere of increasing air temperatures, but there are many year-to-year differences

What is Happening to the Glaciers of the World?

Changes in size (length, thickness, volume) of glaciers are easier to record than mass balance. In general, glaciers appear to have been equal or larger than today during the 16th through 19th centuries, but with numerous small fluctuations that are incompletely documented. Since 1900, depending on the region, glacial recession (shrinkage) has been the rule and the current recession rates are greater than those inferred during earlier centuries. Compilations of area and volume change of the glaciers in mid-latitude locations show major changes in the last 100 years: about 1/2 of the volume of glacier ice in the European Alps has disappeared since the end of the 19th century. Nearly 1/4 of the ice in the glacier-covered Tien Shan has been lost in the last 40 years. Similar data from the Western Hemisphere does not exist, but some individual glaciers have been studied. For instance, Grinnell Glacier, one of the larger glaciers in Glacier National Park, decreased in area from 2.2 to 1.0 square kilometers (0.9 - 0.4 square miles) from 1900 to 1981, and calculations indicate that it will be gone in 50 to 70 years, and with it virtually all the glaciers in this National Park. In other areas, some glaciers are rapidly disappearing: in 1980, Spain boasted 27 glaciers; by 1994 the number was down to 13.

By way of contrast, the ice caps and glaciers in the Arctic have changed only slightly (e.g., a loss of 13% of their volume since 1880, in Svalbard). The ice caps in the High Arctic of Russia and Canada decreased only a few per cent in the last century. This phenomena begs some explanation because studies by Lachenbruch, Overpeck, and others have shown that the 20th century temperature rise in the Arctic and sub-Arctic has substantially exceeded the global average. One reason for the difference is that these glaciers are so cold that meltwater refreezes and does not run off. Obviously, the glacier/climate relation shows strong regional differences not simply related to air temperature; these have not yet been well defined but the regional variations may aid our understanding of the spatial pattern of climate change.

What are the Present and Future Implications of this Glacier Recession?