An analysis of more than 240 studies of the Earth’s climate over the past 1,000 years suggests the twentieth century was not the hottest and that climate varies naturally. The analysis adds to the growing body of knowledge challenging previous assessments, based on less-comprehensive data, that suggest climate was relatively stable until a sharp warming in the late twentieth century.
The new study, Reconstructing Climatic and Environmental Changes of the Past 1000 Years: A Reappraisal, does not draw conclusions on potential twenty-first century human impact on climate. However, it presents strong evidence that natural causes have forced significant global changes in climate over the past 1,000 years and that twentieth century climate was not unusual.
That knowledge makes it more difficult to interpret the origins of recent surface warming, and it adds significant uncertainty to computer predictions of man’s future potential impact on climate.
Several scientists, in conjunction with the Harvard-Smithsonian Center for Astrophysics, conducted the study, which was sponsored by NASA, the U.S. Air Force, National Oceanic and Atmospheric Administration, and American Petroleum Institute. The report was published in Volume 14, Nos. 2 and 3 (2003) of Energy & Environment.
The study examined more than 240 earlier studies of proxy climate data from locations around the world, reflecting climatic conditions over the past 1,000 years. The proxy data came from documentary and cultural sources, ice cores, glaciers, boreholes, speleotherms, tree-ring growth, peat cellulose, pollen, phenological data, and seafloor sediments.
Based on these data, the study tallied significant climate “anomalies” of greater cold, warmth, wetness, and dryness sustained for at least 50 years within the thousand-year period.
The study provides strong evidence for the existence of a Medieval Warm Period from before 1000 A.D. to about 1300 A.D. and a Little Ice Age from about 1300 A.D. to about 1900 A.D., both extending broadly across the world. Even within these periods the climate varied substantially.
Temperatures rose in the twentieth century, but the warming was not remarkable compared with the Medieval Warm Period and past climate variability. “Overall,” the study concludes, “the 20th century does not contain the warmest or most extreme [climate] anomaly of the past millennium in most of the proxy records.”
The accompanying figure sorts the findings of the proxy studies to evaluate climate during the Medieval Warm Period, Little Ice Age, and twentieth century.
The Harvard-Smithsonian study adds to other research challenging a much more limited study, relied upon heavily by the United Nations’ Intergovernmental Panel on Climate Change, which suggests temperatures over the past 1,000 years have been relatively stable and that warming in the late twentieth century is unusual. The earlier, limited study relied on a mathematical model of tree ring data from locations primarily in the Northern Hemisphere and showed no evidence of the Little Ice Age or Medieval Warm Period.
The Harvard-Smithsonian analysis suggests natural climate variability can be substantial, making it more difficult to separate natural climate variability from potential human impact on climate. The findings confirm the U.S. National Research Council’s conclusion, “The evidence of natural variations in the climate system–which was once assumed to be relatively stable–clearly reveals that climate has changed, is changing, and will continue to do so with or without anthropogenic influences.”
The Harvard-Smithsonian findings also support increased research on natural climate variability and argue for climate policies that reflect our incomplete understanding of what makes climate change.
Willie Soon is a physicist at the Solar and Stellar Physics Division of the Harvard-Smithsonian Center for Astrophysics and an astronomer at the Mount Wilson Observatory. He is also a senior scientist at the George C. Marshall Institute and science director for TechCentral Station. Sallie Baliunas is an astrophysicist at the Harvard-Smithsonian Center for Astrophysics. She serves as deputy director of Mount Wilson Observatory and as Senior Scientist at the George C. Marshall Institute in Washington, DC.
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