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Updated
12 October, 2003
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Climate
Change from the 14th-20th Centuries: Evidence from the Subsurface and
the Arctic USGCRP Seminar, 20 January 1998  |
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INTRODUCTION: Dr. Richard Poore
SPEAKERS: Jonathan T. Overpeck
Henry N. Pollack
Overview
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 1.1ºF (0.6ºC) since 1910 -- with temperatures peaking
around 1945 [the average surface temperature was approximately 2.2ºF
(1.2ºC) higher in 1945 than it was in 1910], followed by a cooling
trend into the late 1970s, then a significant warming trend from the
late 1970s 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cores, historical documents, and lake and ocean sediments reveal that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 2.7ºF
(1.5ºC), compared to a warming of 0.9-1.1ºF (0.5-0.6ºC)
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 1,000 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 Niño
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
0.9ºF (0.5ºC) 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 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
2.7ºF (1.5ºC) -- with warming in some locations approaching
5.4ºF (3ºC), 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 (CO2) to the atmosphere,
natural, pre-industrial Arctic temperatures were, on average, 1.8ºF
(1ºC) colder than during the 20th century, and varied around this
average by about 0.9ºF (0.5ºC). 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 2.7ºF (1.5ºC) warming from 1850 to the
present affected 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 5.4-9.0ºF (3-5ºC) 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:
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 1.8ºF (1.0ºC),
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 0.4ºF (0.2ºC) 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 1.1ºF (0.6ºC), 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 1.8-2.2ºF (1.0-1.2ºC), 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. 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 5 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 M.S. 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 analyzed have been collected by investigators in many countries. 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.
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