1000 Years of Climate Change

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Science  26 May 2000:
Vol. 288, Issue 5470, pp. 1353-1355
DOI: 10.1126/science.288.5470.1353

The exceptional warmth of the 1990s [HN1]—the warmest decade since instrumental records began—has sharpened concern over the nature of anthropogenic influences on climate. However, the causes of 20th century climate variability are difficult to resolve, because the period of instrumental records coincides with the time during which the atmosphere has increasingly been contaminated by greenhouse gases [HN2]. What are the “natural” variations that preceded the observed warming of the 20th century? To the casual observer, it might seem that this question has a simple answer—20th century warming was preceded by the “Little Ice Age” and before that by a “Medieval Warm Period.” [HN3] But to those who have looked into this question in depth, the answer is not so simple, as was apparent at a recent workshop (1). Many questions remain regarding the climatic changes that occurred over the last millennium.

Lamb was the first to argue for the existence of a “Medieval Warm Epoch” from around A.D. 1000 to 1300, but his evidence was highly anecdotal and largely qualitative (2). Furthermore, his reconstruction of the mild winters and dry summers that led him to characterize the epoch as he did was based on records from western Europe only. Nevertheless, the concept of a Medieval Warm Period (MWP) has taken on a life of its own, with the implicit assumption that it was a global phenomenon and that temperatures were warmer than in recent decades.

There is little evidence that this was so. Hughes and Diaz [HN4] (3) argued a few years ago that the available evidence allowed nothing more significant to be concluded “than the fact that in some areas of the globe, for some part of the year, relatively warm conditions may have prevailed” (p. 28). Subsequent research has not altered that conclusion (4-6). But we cannot entirely rule out the possibility of a globally extensive warm episode (or episodes) for at least part of the period from A.D. 1000 to 1300, because of the paucity of high-resolution records (especially from the oceans and the Southern Hemisphere) spanning that interval. High-precision borehole temperature measurements from Greenland (7), and some from Antarctica [HN5], do point to warmer conditions around A.D. 1000 than in recent decades, but other paleoclimate records provide a different picture. For example, tree ring data [HN6] from the Southern Hemisphere do not provide support for a warmer period ∼1000 years ago. By contrast, high-latitude tree ring data from the Northern Hemisphere show evidence of strong warming at that time, at least in the summer months, and some marine records from the North Atlantic also suggest warmer conditions (8).

Whether there really were warm episodes of global extent in medieval times (though perhaps not as warm as the past few decades) thus deserves further scrutiny. Until a more extensive set of data is available, the absence of evidence does not necessarily mean evidence of absence. This is unfortunate, because the MWP is often pointed to as “proof” that there were periods in the recent past when conditions were “naturally” warm, without anthropogenic influences. Cosmogenic isotope data (10Be and 14C) [HN7] point to relatively high levels of solar activity (and hence perhaps higher irradiance) around A.D. 1000; orbital considerations also suggest that, at least in boreal summers, insolation was slightly higher at that time [HN8] (9, 10). Solar activity levels were generally lower than in medieval times for most of the last millennium, only reaching comparable levels in the latter half of the 20th century, and this issue is therefore very important for understanding our current climate state.

Do we have any better ideas about climatic conditions after the purported MWP? As we move forward in time, the amount of information increases rapidly, and the temporal and geographical patterns of climate change come increasingly into focus as the 20th century is approached. By the mid-16th century, global temperatures were colder than in previous centuries, and alpine glaciers [HN9] around the world—a sensitive bellwether of temperature change—generally advanced. This period of more extensive glacierization, called the Little Ice Age (LIA), lasted until the mid to late 19th century in most regions. But even within this period, there were warmer episodes; for example, the 18th century was warmer than the preceding and subsequent centuries, and in some periods winters were markedly cool while summers remained relatively warm. The geographical patterns also varied; some areas were warm when others were quite cold (11). Furthermore, there is evidence that the first postmedieval advance of glaciers occurred as early as the 13th century in some areas and the 14th and 15th centuries in others (12). But whatever date one selects for the “onset” of the LIA, there is little doubt that it was firmly at an end by the beginning of the 20th century. The reduction in ice masses accumulated over preceding centuries has continued to the present (in fact, the rate has accelerated) in almost all regions of the world (13) (see the figure).

20th century retreat.

Photographs of the Dana Glacier, Sierra Nevada, California (3660 m, 37° 54' N, 119° 13' W) in 1883 (left) and 1985 (right) show how the glacier has retreated since the late 19th century.


How can these changes be quantified, and what can we learn about their causes? Several reconstructions of large-scale temperature changes have been made with various sets of paleoclimate data, from tree rings and ice cores to corals, sediments, and historical records [HN10] (4, 6, 14). These reconstructions point to a general decline in temperature from ∼A.D. 1000 (or earlier) to ∼1900, with rapid warming over the last century that has no counterpart in the millennial record. Independent reconstructions derived from borehole temperatures suggest even colder temperatures about 400 to 500 years ago and/or even greater warming in the 20th century. To what extent these differences in reconstructed temperatures are related to the effect of land use change [HN11] on borehole temperatures remains to be resolved, but it suggests that land use change may be another factor, in addition to changes in atmospheric trace gases and aerosols [HN12], that may have to be taken into account to realistically simulate past (and future) climate change.

There are indications that the long-term variability of the oceans may also have played a key role in climate variations of the last millennium. Broecker et al. hypothesize that there may have been a reduction in North Atlantic deep water formation (and a concomitant reduction in the Gulf Stream/North Atlantic Drift) during the LIA, with opposite conditions during the MWP [HN13] (15). This may have been the result of an oscillation between deep water production in the North Atlantic and the Southern Ocean, implying a period of reduced Southern Ocean deep water formation (with associated colder conditions in that region) during the MWP and enhanced production (warmer conditions) in the LIA. There are very few records from the Southern Ocean to test this idea, but there is intriguing evidence for more open water, at least in the Ross Sea [HN14], during the LIA (16). Borehole temperatures from Taylor Dome [HN15] (but not from other Antarctic ice core sites) also point to warmer conditions in the LIA (17) and lower temperatures in medieval times. How all this fits with mounting evidence for a periodic increase every 1400 to 1500 years in ice-rafted debris across the North Atlantic remains to be resolved. The LIA may have just been one of many oscillations within the Holocene, driven either by internal ocean system dynamics or quasi-regular external forces not yet resolved.

Many questions remain about the nature and causes of climate changes over the last millennium, and it is easy to overlook what we do know that is relevant to the present debate over global warming [HN16]. One fact stands out as indisputable: Temperatures rose in the 20th century at a rate unprecedented in the last millennium. Future research is unlikely to shake that conclusion. It is also very probable that temperatures at the end of the 20th century were higher than at any time in the last millennium; the latter half of the century was probably one of the warmest 50-year periods for several millennia (10). However, more research is needed to consolidate and verify that conclusion. Much remains to be learned about the climate-forcing mechanisms that operated over the last millennium, but current research clearly implicates anthropogenic greenhouse gases in the remarkable temperature changes of the last century (14). Given that the atmospheric CO2 concentration [HN17] is now higher than at any time in (at least) the last 420,000 years (18), that it will almost certainly double within the next century, and that a considerable amount of heat has already been sequestered in the upper ocean during this century (19), the climatic and environmental changes of the last millennium may be trivial compared with those in the near future.

HyperNotes Related Resources on the World Wide Web

General Hypernotes

The World Wide Web Virtual Library of Paleoclimatology and Paleoceanography is maintained by P. Farrar, Remote Sensing Division, Naval Oceanographic Office.

R. H. Cummins, School of Interdisciplinary Studies, Miami University, OH, provides a collection of Internet links to paleoclimate resources.

The Tiempo Climate Cyberlibrary is a resource for Internet information on climate and climate change; it is a collaborative project of the School of Environmental Sciences, University of East Anglia, Norwich, UK, and the International Institute for Environment and Development, London.

R. Ford, U.S. Agency for International Development, provides links to Internet resources for Earth systems education.

The NASA Goddard Institute for Space Studies offers a climate glossary, as well as a collection of nontechnical research summaries.

The Climate Diagnostics Center of the National Oceanic and Atmospheric Administration (NOAA) provides links to climate resources on the Web. The NOAA Office of Global Programs Library offers a collection of links to Internet climate resources. NOAA's National Climatic Data Center provides information about climate research and applications and other climate resources including a Web page about global climate change.

The NOAA Paleoclimatology Program at the National Geophysical Data Center is a central location for paleoclimate data, research, and education. An introduction to paleoclimatology science and a presentation titled “A paleo perspective on global warming” are provided.

NASA's Global Change Master Directory is a comprehensive searchable directory of descriptions of data sets of relevance to global change research; included are links to data on paleoclimate.

The Hadley Centre for Climate Prediction and Research of the UK Meteorology Office provides information resources on climate and climate change. An overview of climate change and its impact and graphs of temperatures since 1860 are provided.

The International Geosphere-Biosphere Programme is an interdisciplinary scientific activity established and sponsored by the International Council for Science; PAGES (Past Global Changes) is one of the programs. CLIVAR (Climate Variability and Predictability) is an interdisciplinary research effort within the World Climate Research Programme that focuses on the variability and predictability of the components of the climate system. The Workshop on the Climate of the Last Millennium, a PAGES/CLIVAR joint initiative, was held in Venice in November 1999; abstracts of the presentations are provided.

The Global Change Research Program of the U.S. Geological Survey (USGS) presents a series of fact sheets about Earth systems dynamics. offers Encyclopædia Britannica articles on climate and climatology.

M. Pidwirny, Department of Geography, Okanagan University College, Kelowna, BC, Canada, includes a chapter on meteorology and climatology in his Web textbook Fundamentals of Physical Geography .

E. Takle, Department of Geological and Atmospheric Sciences, Iowa State University, offers lecture notes and Web links for a Web course on global change.

Lecture notes on the climate system are provided as a component of the Earth system Web course offered by the Department of Earth and Environmental Sciences, Columbia University.

R. Myneni, Department of Geography, Boston University, offers lecture notes and Web links for a course on global climate change and environmental impacts. A presentation on observed climate variability and change is included.

G. Lash, Department of Geosciences, Fredonia State University, NY, provides lecture notes for a course on catastrophic weather and climatic change. Presentations on climate change in the past and mechanisms of climate change are included.

The Good Earth Web site, provided by D. McConnell, Department of Geology, University of Akron, offers presentations on Earth's climate system and on global change.

A Web course on global change, offered by the University of Michigan under the direction of T. Kileen, Department of Atmospheric, Oceanic and Space Sciences, provides lecture notes on the paleoclimate record and climate models and climate patterns and paleoclimates.

History of Climate Change is an educational presentation of the Athena curriculum project.

The 15 February 2000 issue of the Proceedings of the National Academy of Sciences featured an editorial, nine perspectives, and six research articles related to rapid climate change.

The July-August 1999 issue of American Scientist had an article by K. Taylor titled “Rapid climate change.”

The U.S. National Report to International Union of Geodesy and Geophysics 1991-1994 had a contribution by D. Schimel and E. Sulzman titled “Variability in the earth climate system: Decadal and longer timescales” and a contribution by J. Overpeck titled “Paleoclimatology and climate system dynamics.”

Numbered Hypernotes

1. The Warming of the Earth is a presentation offered by the Woods Hole Research Center. The Goddard Institute for Space Studies offers a presentation on global temperature trends and makes available an article titled “A Common sense climate index: Is climate changing noticeably?” and an article titled “The global warming debate,” both by J. Hansen. The Spring 1995 issue of Consequences, available from the US Global Change Research Information Office, had an article by T. Karl et al . titled “Trends in U.S. climate during the twentieth century.”

2. The Athena curriculum project offers a presentation on atmospheric greenhouse gases. The 28 September 1999 issue of Eos, published by the American Geophysical Union, had an article by T. Ledley et al . titled “Climate change and greenhouse gases.” M. Pidwriny's Web textbook on physical geography offers a presentation on greenhouse gases and the greenhouse effect. The Atmospheric Research division of CSIRO Australia offers a presentation about the greenhouse effect with links to other resources.

3. The Encyclopædia Britannica article on the Holocene includes sections on the Medieval Warm Period and the Little Ice Age; the article on climate includes an overview of recent climate change. The NOAA Paleoclimatology Program provides an overview of the Medieval Warm Period. The climate glossary of the Goddard Institute for Space Studies has an entry for Little Ice Age. For a course on Ice Age environments, J. Aber, Earth Science Department, Emporia State University, KS, includes sections on the medieval climatic optimum and the Little Ice Age in his lecture notes on the climatic history of the Holocene; the Little Ice Age, a student presentation for the course by K. Shobe, is also made available. D. McConnell's Good Earth Web site includes information about the Medieval Warm Period and the Little Ice Age in the presentation on climate history. The Center for the Study of Carbon Dioxide and Global Change, Tempe, AZ, offers a presentation titled “Earth's climatic history: The last 1,000 years.” An overview of Earth's climatic history is provided in the section on meteorology and climatology of M. Pidwriny's Web textbook on physical geography.

4. M. Hughes is at the Laboratory of Tree-Ring Research, University of Arizona, Tucson, AZ. H. Diaz is at NOAA's Climate Diagnostics Center, Boulder, CO.

5. Warnings from the Ice from NOVA Online offers a presentation on ice cores. The Ice Core Paleoclimatology Group, Byrd Polar Research Center, Ohio State University, provides an introduction to ice core drilling. The Greenland Ice Sheet Project Two (GISP2) at the Climate Change Research Center, Institute for the Study of Earth, Oceans and Space, University of New Hampshire, offers a presentation titled “Ice cores that tell the past.” Overviews of the GISP2 and the Greenland Ice Core Project (GRIP) ice coring efforts are provided on the Web version of the Greenland Summit Ice Cores CD-ROM. The American Geophysical Union makes available an article by Vostok Project members titled “Deciphering mysteries of past climate from Antarctic ice cores.” Antarctica Online, the Web site of the Australian Antarctic Division, makes available an article by T. Jacka titled “Antarctic ice cores and environmental change” on the information page about glaciology. The USGS National Ice Core Laboratory, Denver, CO, provides a report from the Ice Core Working Group titled “Ice core contributions to global change research: Past successes and future directions,” which includes sections on Greenland and Antarctica, as well as an introduction to the process of drilling and studying ice cores. The 1 April 1999 issue of Environmental Science and Technology had an article by D. Schoen titled “Learning from polar ice core research.”

6. The Ultimate Tree-Ring Web Pages are maintained by H. Grissino-Mayer, Department of Physics, Astronomy and Geosciences, Valdosta State University, GA. “Tree Rings: A Study of Climate Change” is an educational presentation of the Athena curriculum project. K. Briffa, Climatic Research Unit, University of East Anglia, offers a presentation titled “Trees as indicators of climate change.” A press release titled “Tree rings show rising earth temperatures” describes research at the Tree-Ring Laboratory of Lamont-Doherty Earth Observatory, as published in Science in a 9 August 1996 report by G. Jacoby, R. D'Arrigo, and T. Davaajamts titled “Mongolian tree rings and 20th-century warming.” A Web page about the Mongolian climate studies is also provided by the Tree-Ring Laboratory.

7. The Purdue Rare Isotope Measurement Laboratory defines cosmogenic isotopes. The Department of Earth Sciences, University of Bristol, UK, presents lecture notes on cosmogenic isotopes for a course on isotope geochemistry. WebElements, maintained by M. Winter, University of Sheffield, UK, provides information about beryllium and carbon. Table of the Nuclides, developed by Jonghwa Chang, Korea Atomic Energy Research Institute, provides information about 10Be and 14C. W. White of the Keck Foundation Isotope Laboratory, Cornell University, offers extensive lecture notes in Adobe Acrobat format for an isotope geochemistry course; lecture notes on cosmogenic isotopes and on paleoclimatology are included.

8. The Solar Physics Group at the Mullard Space Science Laboratory, Department of Space and Climate Physics, University College London, provides an introduction to solar activity. The Institute of Astronomy, Swiss Federal Institute of Technology Zurich (ETH Zurich), offers a presentation on past solar irradiance variations. C. Vita-Finzi, Department of Geological Sciences, University College London, offers lecture notes on solar variation, which include a discussion of climate effects, for a Web course on Earth and planetary geology; lecture notes on terrestrial paleoclimatology are also provided. The Mt. Wilson Observatory, Pasadena, CA, offers a presentation on the sunspot cycle. The Solar Physics Group of NASA's Marshall Space Flight Center presents information about the sunspot cycle. M. Pidwriny's Web textbook on physical geography includes a discussion of orbital variations and solar insolation in the section on causes of climate change. The Library of NASA's Earth Observatory Web site includes a presentation on Milutin Milankovitch with information about orbital variations. R. Cerveny, Department of Geography, Arizona State University, Tempe, offers a visualization of the Milankovitch orbital theory. The Winter 1996 issue of Consequences had an article by J. Lean and D. Rind titled “The Sun and climate.”

9. The Glacier Web site, made available by the Department of Geology and Geophysics, Rice University, includes a presentation on ice with sections about glaciers, ice cores, and ice ages. J. Aber provides lecture notes on glaciers for a course on Ice Age environments. The USGS Cascades Volcano Observatory offers an information page about glaciers and a presentation on the Mt. Rainier glaciers that discusses the effect of the Little Ice Age. The National Snow and Ice Data Center offers information about glaciers and a a presentation titled “State of the Cryosphere” that provides a summary of cryospheric and related indicators of global climate trends. Scientific American presents an exploration feature titled “Going, Going—Gone?” about the melting of glaciers. The Earth Surface Dynamics division of the USGS Global Change Research Program provides background information on glaciers in a fact sheet on the satellite image atlas of glaciers of the world.

10. The NOAA Paleoclimatology Program archives reconstructions of past climatic conditions derived from climate proxies. For a course on global change, E. Takle provides lecture notes on using proxy data to reconstruct past climates and lecture notes on the instrument-climate records. J. Aber provides lecture notes on paleoclimate reconstructions for a course on Ice Age environments. The Borehole Temperatures and Climate Reconstructions Database Web site, a project initiated by the Geothermal Laboratory, University of Michigan, provides an introduction to the geothermal approach of climate reconstruction. The US Global Change Research Information Office makes available a publication titled “Geoindicators: Tools for assessing rapid environmental changes,” which was developed by the International Union of Geological Sciences; borehole temperature regimes, corals, and glacial fluctuations are among the topics covered. The NOAA Office of Global Programs provides a presentation on coral paleoclimatology. The Laboratory of Paleoclimatology and Climatology, Department of Geography, University of Ottawa, makes available a paper by K. Gajewski titled “Climate changes of the past millennium reconstructed from high-resolution pollen records.” The Ice and Climate Division of the British Antarctica Survey provides information about its research project to construct a paleoenvironmental history of the last 2000 years using information from ice cores.

11. The Center for International Earth Science Information Network (CIESIN) provides a Thematic Guide on Land Use and Global Environmental Change. The Spring 1995 issue of Consequences had an article by W. Meyer titled “Past and present land use and land cover in the USA.” The Library of NASA's Earth Observatory Web site includes a presentation on the changing global land surface.

12. The Library of NASA's Earth Observatory Web site includes a presentation on aerosols and climate by M. Hardin and R. Kahn. Atmospheric trace gases and aerosols are discussed in the chapter on atmospheric chemistry and climate from Atmospheric Chemistry and Global Change, sample chapters of which are made available on the Web by the Atmospheric Chemistry Division, National Center for Atmospheric Research.

13. W. Broecker is at the Department of Earth and Environmental Sciences and the Lamont-Doherty Earth Observatory, Columbia University. The 5 November 1999 issue of Science had a news article by Richard Kerr titled “Has a great river in the sea slowed down?” about the report in that issue by W. Broecker, S. Sutherland, and T.-H. Peng titled “A possible 20th-century slowdown of Southern Ocean deep water formation.” The 15 February 2000 issue of the Proceedings of the National Academy of Sciences had a perspective article by Broecker titled “Was a change in thermohaline circulation responsible for the Little Ice Age?” The Library of NASA's Earth Observatory Web site includes a presentation by D. Herring on oceans and climate.

14. provides an Encyclopædia Britannica article about the Ross Sea. The National Geographic Society offers a map of Antarctica.

15. The Ice Core Paleoclimatology Group at the Byrd Polar Research Center, Ohio State University, provides a map of the location of Antarctica sites including Taylor Dome. The Taylor Dome Ice Core Project Web site, provided by E. Steig, Department of Earth and Environmental Science, University of Pennsylvania, provides information about the project, as well as maps.

16. The May 1997 issue of Scientific American had an article by T. Karl, N. Nicholls, and J. Gregory titled “The coming climate.” The 1997 Annual Report of the Geological Survey of Norway included a chapter by E. Larsen titled “The climate of the past — a key to understanding future climate development.” G. Lash provides lecture notes on the global warming model and dissenting views for a course on catastrophic weather and climatic change. The U.S. Environmental Protection Agency provides a Global Warming Web site. The United Nations Framework Convention on Climate Change provides a Climate Change Information Kit, a collection of fact sheets. The US Global Change Research Information Office, maintained by CEISIN at Columbia University, provides access to data and information on global environmental change research, adaptation/mitigation strategies and technologies, and related educational resources on behalf of the US Global Change Research Program; an introduction to global warming and climate change is provided. The WWF-World Wide Fund For Nature provides a Climate Change Web site. A report on the science of climate change titled “Climate change: State of knowledge” is available on the Web from the Environment Division of the White House Office of Science and Technology Policy.

17. E. Takle provides lecture notes on carbon dioxide for a course on global change. The Carbon Dioxide Information Analysis Center, maintained by the Environmental Sciences Division of Oak Ridge National Laboratory, is the primary global-change data and information analysis center of the U.S. Department of Energy; a FAQ about carbon dioxide is offered and the Trends Online Web page makes available reports on atmospheric carbon dioxide and carbon isotope records.

18. R. Bradley is in the Department of Geosciences and at the Climate System Research Center, University of Massachusetts.

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