The Heat Is Online

Borehole Measurements Confirm Surface Warming

U.S. Global Change Research Program Seminar Series

Climate Change from the 14th-20th Centuries: Evidence from the Subsurface and the Arctic

How have surface temperatures changed over the few centuries prior to the Industrial Revolution compared to since that time? What types of records are available? What do observational and proxy records of climate change in the Arctic region show? Are the observed recent changes natural or the result of human activities? Could warming in the Arctic trigger other natural responses that might result in the future climate being even warmer than currently projected? What are the subsurface (borehole) records of

climate change over the last few centuries? Are these records of climate change consistent with the instrumental record of surface temperature change over the past century? What do these records suggest about the future direction, magnitude, and rate of climate change?

Tuesday, January 20, 1998


Jonathan T. Overpeck, Head, U.S. National Oceanic and Atmospheric Administration's Paleoclimatology Program, National Geophysical Data Center, Boulder, CO

Henry N. Pollack, Dept. of Geological Sciences, University of Michigan, Ann Arbor, MI



Thermometer records from the Arctic are somewhat rare, and shorter in length than records at lower latitudes, but they do indicate that the Arctic has warmed by about 1.1 degrees F (0.6 degrees C) since 1910, with temperatures peaking around 1945 (the average surface temperature was approximately 2.2 degrees F [1.2 degrees C] higher in 1945 than it was in 1910), followed by a cooling trend into the late 70's, and then with a significant warming trend from the late 70's to the present. Since 1860, proxy data from tree-rings, ice-cores, historical documents, and lake and ocean sediments, reveal that the average surface temperature in the Arctic has increased by 2.7 degrees F (1.5 degrees C), compared to a warming of 0.9-1.1 degrees F (0.5-0.6 degrees C) over this period. Arctic warming has been twice that of the average warming for the Northern Hemisphere. The proxy data also indicate that the magnitude and extent of Arctic warming over the past 150 years are unprecedented when compared to climate records extending back through much of the last 1000 years.

Subsurface (borehole) temperature records from around the globe constitute yet another independent set of indicators (or measures) of global climate change over the past several centuries. These records provide a basis for evaluating the relative influence of natural climate variation versus human-induced changes in climate. Borehole records of climate change are generally insensitive to short-term fluctuations in climate and weather (i.e., El Nino events, etc.), but are generally reliable and stable

indicators of long-term climate changes and trends. These borehole records indicate that the surface temperature around the globe has increased, on average, by about 0.9 degrees F (0.5 degrees C) in the 20th century; that the 20th century is the warmest century since at least the year 1500; and that the rate of temperature change in the 20th century is four times greater than the average rate of change over the previous four centuries.


What the Centuries-Long Arctic Perspective Reveals About the 20th Century

When the short thermometer record of the 20th century is compared to proxy temperature data spanning the last few centuries, it becomes clear that the 20th century is not at all representative of conditions this millennium.

Where thermometer and proxy records overlap in the time, the records are in agreement. The longer records reveal, however, that the dramatic Arctic warming prior to 1950 was a continuation of a trend that began in the mid-19th century. Between 1845 and the present, the Arctic warmed by over approximately 2.7 degrees F (1.5 degrees C), with warming in some locations approaching 5.4 degrees F (3 degrees C), the largest Arctic-wide warming trend on record for the last 400 years and possibly the last millennium.

Moreover, the period spanning the last 150 years appears to be the only period during which the entire Arctic warmed above the pre-1920 level. Thus, both the magnitude and spatial extent of Arctic warming during the last 150 years are unprecedented in the context of the past several centuries.

Before humans had added significant amounts of carbon dioxide (CO-2) to the atmosphere, natural, pre-industrial Arctic temperatures were, on average, 1.8 degrees F (1 degrees C) colder than during the 20th century, and varied around this average by about 0.9 degrees F (0.5 degrees C). When the centuries-long record of Arctic temperature variation is compared to reconstructed records of candidate climate "forcing" mechanisms, it appears that changes in the Sun's output and variations in volcanic activity can explain much of this "natural" variability. However, these same "cause and effect" comparisons suggest that solar and volcanic variability cannot account for all of the warming after 1920. This observation thus agrees with climate theory and models, suggesting that Arctic warming in the 20th century is increasingly being dominated by human influences (i.e., greenhouse gases) relative to natural (solar and volcanic) forces.

What the Centuries-Long Arctic Perspective Suggests About the Future

Observations of historic Arctic temperature changes are in agreement with a number of global climate model simulations of temperature, suggesting that the Arctic (and the globe) will continue to warm dramatically as humans continue to increase the concentrations of greenhouse gases in the atmosphere. The patterns of past warming suggest that Arctic warming will continue to be at least double that of the Northern Hemisphere as a whole. Observations on how the roughly 2.7 degrees F (1.5 degrees C) warming from 1850 to the present impacted the Arctic reveal unprecedented changes in glaciers and ice-caps, ocean conditions, and permafrost, as well as terrestrial and lake ecosystems.

These results suggest that the predicted additional warming of 5.4-9.0 degrees F (3-5 degrees C) by the end of the next century would have unprecedented impacts on forestry, engineering, transportation, fishing, water resources, hunting and natural preserves in the Arctic. Continued climate warming in the Arctic might also have a large impact on the rest of the globe, particularly via changes in the global water cycle, ocean circulation, and impacts on the ability of plants and soils to either absorb or release atmospheric greenhouse gases.

Subsurface Temperatures Reveal Five Centuries of Climate Change

Temperature changes that occur at the Earth's surface propagate slowly downward into the rocks beneath the surface. Thus, rock temperatures at shallow depths provide evidence of changes that have occurred at the surface in the recent past. The pace of heat transfer in rocks is such that the past 500 years of surface temperature history is imprinted on and contained within the upper 500 meters of the Earth's surface.

Analyses of underground temperature measurements from several hundred boreholes from around the world show that:

     * The global average surface temperature has increased about 0.9 degrees F (0.5 degrees C) in the 20th century, an amount consistent with the estimate from surface observations given by the 1995  Intergovernmental Panel on Climate Change (IPCC).

     * The rate of temperature change in the 20th century is four times greater than the average rate of change over the previous four centuries.

     * The 20th century has been the warmest century of the last five centuries.

     * The present-day mean temperature is a little more than 1.8 degrees F (1.0 degrees C) warmer than five centuries ago; of this change about half has occurred in the 20th century alone, and 80% has occurred since 1750.

These interpretations provide an historical perspective that indicates the 20th century has not been just another century in terms of temperature change. In the context of the five-century interval investigated, the 20th century is clearly unusual.

What Do Observed Subsurface Temperature Changes Over the Past Five Centuries Suggest About the Future?

The magnitude of the surface temperature change since the year 1500, globally about 1.8 degrees F (1.0 degrees C), provides some information about climate sensitivity, i.e. the way the temperature responds to changes in factors that affect it. Human impacts on the atmosphere, in terms of changing greenhouse gas concentrations, can first be observed around 1750 when greenhouse gases began to increase from pre-industrial levels.

Thus, prior to 1750, temperature changes are likely to be wholly from natural causes, although land use changes in some regions may have had a minor global effect. These results show an increase in temperature of about 0.4 degrees F (0.2 degrees C) from 1500 to 1750. If that rate of natural warming continued from 1750 to the present, it would account for about 40% of the total change of temperature since 1500, leaving 60%, or about 1.1 degrees F (0.6 degrees C), attributable to anthropogenic causes. That latter amount can be thought of as a time- and space-averaged overall measure of the way in which the global mean temperature has responded to the changes in greenhouse gas concentrations, anthropogenic aerosols, and other human factors since 1750. If this observed rate of warming continues, the warming over the next half-century will likely be about 1.8-2.2 degrees F (1.0-1.2 degrees C), rather close to the IPCC's "best estimate" of the rise in mid-21st century temperatures.



Dr. Jonathan Overpeck is a physical scientist for the National Oceanic and Atmospheric Administration's (NOAA) National Geophysical Data Center (NGDC) in Boulder, Colorado. As the head of NOAA's Paleoclimatology Program, his work focuses on using natural archives (e.g., trees, land and ocean sediments, coral reefs, and ice cores) to reconstruct and understand the full range of climate variability, particularly with reference to anticipating future climatic changes on societally-relevant time scales.

Dr. Overpeck is a Fellow at the Institute of Arctic and Alpine Research, and an Adjunct Associate Professor in the Department of Geological Sciences at the University of Colorado. He serves on numerous national and international scientific committees, including those associated with the International Geosphere-Biosphere and the World Climate Research Programmes. He was a contributor to the 1995 assessment report of the Intergovernmental Panel on Climate Change (IPCC). He is also the author of over 30 peer-reviewed scientific publications.

Dr. Overpeck received his Ph.D. in Geological Sciences from Brown University in 1986, after which he spent five years at Colombia University's Lamont-Doherty Earth Observatory as an Associate Research Scientist before his appointment with the National Oceanic and Atmospheric Administration.

Acknowledgments: The data and work presented in this seminar are derived primarily from a recently published article in "Science" (v. 278, pp. 1251-1256), with a large number of American and Canadian collaborators.

Dr. Henry Pollack is Professor of Geophysics in the Department of Geological Sciences at the University of Michigan in Ann Arbor. He has engaged in research on all seven continents, addressing the dynamics and evolution of the Earth and its climate. His current research focuses on the record of global climate change as recorded by the temperatures of the rocks beneath the Earth's surface, seeking to identify the human impact on climate.

Dr. Pollack has served on National Science Foundation advisory panels on Continental Dynamics, the Global Digital Seismograph Network, and the San Andreas Fault. In 1992 he presented aspects of his research on global climate change to the Senate Commerce Science and Transportation Committee. From 1991-95 he served as Chairman of the International Heat Flow Commission of the International Association of Seismology and Physics of the Earth's Interior. He is presently a member of the U.S. Geodynamics Committee of the National Research Council, and the Committee on Global and Environmental Change of the American  Geophysical Union.

At the University of Michigan, his home institution for the past 34 years, he has taught at every level of the curriculum, from introductory Earth science courses for non-scientists to specialized graduate seminars. He has been a frequent lecturer for the  University of Michigan Alumni Association around the USA, and on special excursions to Alaska and Antarctica.

Professor Pollack received his undergraduate degree in geology from Cornell University, an MS degree from the University of Nebraska, and a Ph.D. in geophysics from the University of Michigan. He has also held visiting teaching and/or research positions at Harvard University, the University of Zambia, the Universities of Durham and Newcastle (UK), and the University of Western Ontario.

Acknowledgments: The results discussed in this seminar derive from a long collaboration with Dr. Shaopeng Huang, a research scientist and close colleague at the University of Michigan. The data we have analyzed have been collected by investigators in many countries, and we are grateful that this information has been readily shared with us. Our research has been funded by the National Science Foundation, the National Oceanic and Atmospheric Administration, the Czech-U.S. Cooperative Science Program, and the University of Michigan.