TCS Daily

Not So Hot

By Sallie Baliunas - April 29, 2003 12:00 AM

History is written, Herodotus says, "in the hope of preserving from decay the remembrance of what men have done, and of preventing the great and wonderful actions of the Greeks and the barbarians from losing their due meed of glory."

The history of man concerns itself not only with a recitation of events, people and places, but also with what our actions say about the essence of humankind.

But the history of ancient climates - or paleoclimates - is wholly simpler. It is largely the detailing of climate variations, which are gotten from repositories contained in physical components of the environment that have responded to climate change. With modern technology, the tales of the environment can be told from changes in chemical, biological and geological signals that are stored in the ecosystem. From those changes, hypotheses on the causes of climate change - past and future - can be tested.

The 20th century, according to thermometer measurements, is warmer than the second half of the 19th century. The mid-19th century is the earliest time from which researchers have established a global record of instrumentally-measured temperature change. Because the 20th century also coincides with the increased concentration of human-made greenhouse gases in the air, the 20th century warmth might be suspiciously eyed as having been caused, at least in part, by the air's increased greenhouse gas content. To estimate the amount of human-made global warming requires showing that the 20th century unusually warm, and that the 19th century was normal. But were they, given climate's vicissitude and the shortness of the thermometer record?

The answer requires going back several centuries - perhaps a millennium or longer - in order to gauge natural climate change, especially from a period when greenhouse gases emitted from human activities were minimal, thereby reducing one variable in the constellation of causes of climate change. Researchers who study ancient climates have found many physical reservoirs that contain climate information. Such indicators - or proxies - fill in the instrumental gap and give a rich history of natural climate change.

The proxies come from many different sources. A few examples are glaciers, boreholes, coral, tree growth, sediments of pollen, insects or sea organisms, river effluvia, dune migration, stalactites and stalagmites, plus documentary evidence such as weather diaries or crop accounts.

The technique of studying proxies has difficulties that introduce uncertainty when trying to produce a global record of climate change. First, a proxy may be affected not only by temperature, but also by other climate variables, such as precipitation or wind patterns. Second, one type of proxy often has different limits to its information than another, making it hard to compare results between, for example, tree growth and boreholes. Tree growth may faithfully record changes from periods of years to decades of local temperature and precipitation. Boreholes rely on measurements of the shallow crust of the earth that have responded slowly (usually over a century) to surface air temperature changes. In this case, the two proxies are sensitive to very different time periods and to somewhat different climate variables. As a result, averaging across many proxies remains tricky. Another difficulty is that each type of proxy is not widely available to make a meaningful global average.

Thus, proxies should be viewed as records of local climate. Each proxy must be remembered in the context of its limits and uncertainties - in time, geographical extent and climate sensitivity. And the results from proxies need to be expanded to other areas of the world, in order to develop a fuller picture of climate change in the last 1,000 years. Researchers are actively doing so.

But there is something to be done with the wealth of climate information from proxies. They can be queried, one by one, to ask what the history of climate has been in that location. Sometimes several types of proxies are available to deliver a more informed history at a particular location.

An assessment by us and colleagues of over 240 published results produced by over 1,000 researchers took the view of asking about climate history in each location. There are several findings. First, the ensemble of proxies suggest that widespread warmth occurred for at least 50 years during the period 800 through 1200 C.E., called the Medieval Warm Period by previous researchers. Because climate is complex, it is unsurprising that the warmth did not occur simultaneously at all locations (just as 20th century warmth did not occur everywhere - but it did in many locations). That period of extreme climate - long before the air's increase in greenhouse gas concentration from human activities - must have natural explanations. Moreover, in the comparison of results from each proxy, the 20th century was neither the most extreme or the warmest over most of the locations where such information has become available.

After the warmth 1,000 years ago, climate largely deteriorated over much of the world's sampled environment into the Little Ice Age (approximately 1300 to 1900 C.E.) which is identified as at least a 50-year cold period between the 14th and 19th centuries.

The last millennium had seen periods warmer than the 20th century in many parts of the world where there is information, and so the 20th century is not unusual. The 19th century, the start of the thermometer records, seems to have been the tail of an unusual cold period that had persisted for some centuries, perhaps as far back as the 14th century in some areas. Those past changes have natural explanations that remain largely speculative, and the forefront of research that seeks to improve the forecasts of future climate.

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