TCS Daily

Natural History

By Sallie Baliunas - May 30, 2003 12:00 AM

History, Herodotus wrote, is recorded "in the hope of preserving from decay the remembrance of what men have done." Fortunately, nature also writes its history. Coupled with that written by men, nature's records can ameliorate the tendency to sensationalize current trends in climate into something unusual or dangerous.

The historical record - both man and nature's - does not support the view from catastrophism.

It is true that, according to thermometer measurements, the Earth's surface was warmer by some 0.6 degrees Celsius at the end of the 20th Century than it was in the second half of the 19th century. And because the 20th Century also coincided with an increased concentration of human-made greenhouse gases in the air, some have concluded that the 20th Century's warmth - and future global warming - was and will be caused, at least in part, by the burning of fossil fuels.

But to try to accurately gauge the amount of human-made global warming, a demonstration is required that the 20th Century actually was unusually warm, and that the 19th century was normal. Were they?

To work on finding the answer, we need to go back several centuries to a period when greenhouse gases emitted from human activities were minimal. Man's instrumentally-measured record of global temperature is insufficient as it dates only to the mid-19th Century. But nature's historical record, when coupled with other human historical information, goes back much further.

What information is available on past climate? Indicators - or proxies - of climate information come from repositories that chronicle the response of the ecosystem to past climate change. Information on climate can be derived from natural sources, such as glaciers, boreholes, coral, tree growth, sediments of pollen, insects or sea organisms, river effluvia, dune migration, stalactites and stalagmites. Information on climate can also be gotten from human documentary evidence such as weather diaries or crop accounts.

The technique of synthesizing the results from proxies is not easy. There are many differences among types of proxies. For example, one proxy may record summer conditions, another annual. One may be sensitive primarily to temperature, another to precipitation, and still another to several climate variables. One proxy may give quantitative results, and another may give a qualitative sense of climate change. Thus, averaging across many proxies remains tricky. Another difficulty is that each type of proxy is not widely available to make a meaningful global average.

Because of these limitations, proxies are best viewed as records of local climate, with each accounted for in the context of its limits and uncertainties - in time, geographical extent and sensitivity to different climate variables.

Despite the problems, there is much climate information from proxies that can now be found using modern technology to provide a history of climate at many locations worldwide.

A recent review by a team from Harvard University of more than 240 scientific articles by over 1,000 researchers using the various proxy data shows that the climate in most locations was not extreme or unusual during the 20th Century. Instead, the warmest, or most extreme, climate for those locations over approximately the last 1000 years tended to occur sometime between the 9th and 14th centuries, in what is called the Medieval Warm Period.

That period of extreme climate - long before the air's significant increase in greenhouse gas concentration from human activities - must have natural explanations. Whatever they are, the consequences of the warming, as far as man was concerned, were scarcely dangerous. Vikings made their way to Greenland, Iceland and North America in that period. England had vineyards. H.H. Lamb, the founder of the climatic research unit at East Anglia University, found that England's climate was warm enough in the 12th and 13th Centuries to support more than 50 vineyards, signifying that May frosts were rare. William of Malmesbury noted in De Pontificibus: "No county in England has so many or so good vineyards as this Gloucester."

By the 14th Century, that warmth had eroded, heralding a period now known as the Little Ice Age, lasting approximately from 1300 to 1900 C.E. Europe, for example, experienced more acute winters, frost and year-to-year climate variability, and the worst of the Little Ice Age from 1550 to 1700. The human effects were tragic: In Scotland severe weather in seven of the last eight years of the 18th century produced one of Scotland's deadliest famines of the last 1000 years.

The last millennium has seen naturally-warmer periods than the 20th Century in many parts of the world where information is available. That means that the 20th Century was not unusual. Meanwhile, the 19th century, during which world-wide thermometer records began, seems to have been the tail of an unusual and natural cold period that had started perhaps as far back as the 14th century in some areas. The 19th Century was not so normal.

The scientific history drawn from nature and man's records over the last millennium suggests that a trend of catastrophic human-caused warming does not exist for the 20th century. Costly policies to combat global warming trends through proposed small cuts in carbon dioxide emission are unlikely to mitigate any of climate's sure-to-occur natural risks, but they could reduce society's economic ability to cope with them.

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