The Great and Abominable Church: Environmentalism

[_Runtu]

Loran has already admitted that his evaluation of the evidence is driven entirely by his Right-Wing ideology.

Ah, perhaps I just have poor reading comprehension skills, or perhaps I'm blinded by my less-reactionary right-wingedness. It is kind of humorous to have someone criticize me for being too liberal. lol

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I'll answer any question about AGW, its scientific merits or lack thereof, and its functionality as an ideology from anyone who wishes to engage in a serious discussion about it.

Now, just ONE MORE TIME, I will clarify why I do not believe in AGW. This is really, very, very simple, and I've now made this clear at least twice and this will be the last time. Fort will continue to play his clever courtroom games, but perhaps someone here will be able to parse this at face value and pursue the debate from there without the introduction of needless verbal and semantic smog.

I don't believe in AGW because there is no substantive or compelling scientific evidence supporting it. That is the end of the matter on that point.

I don't like the theory of AGW because it is a tool of political and cultural warfare used by the Left as a major aspect of its ongoing program to transform society from one based on deliberative representative Democracy to a centralized, command and control, authoritarian nanny state based upon various notions of collectivist social organization (in the case of AGW as an ideology, the need to again, save the planet from free market capitalism).

Fortigurn's claim that AGW does not function as an ideology is patently unsupportable. A working knowledge of the history of the theory, form its first incarnation in the early eighties through its mature period over the last ten years or so, will demonstrate quite clearly that for the cultural Left, AGW functions as an ideology; as a system or body of claims about the economic, social, and cultural organization and structure of society. This ideology makes clear and vigorous statements about that society, based upon the claimed threat AGW poses to the earth as a function of its creation by human beings engaging in capitalistic (economically free) industrial activities.

The ideological component of AGW (AGW as a both a totemic symbol and as a series of alleged scientific claims that are yet understood to have clear and unmistakable political implications) is a separate yet interconnected aspect of the issue. The two can only be truly separated in the specialized professional literature. The strictly scientific and strictly ideological aspects cannot be separated in the popular media, in much non-scientific academic discourse, and in the environmental movement, which cares little about the scientific veracity of AGW (except to the extent it hopes its true) but is concerned primarily with the degree with which AGW can be used to further Kulturkampf.

I arrived at my views of the scientific validity of AGW over roughly 15 plus years of studying the issue from both sides. Reading analysis of evidence from the professional literature by proponents of both views, studying the claims of the environmental movement and its critics, and of course, immersing myself in the pop news media version of things.

The primary problem with AGW, from a scientific standpoint, is that the entire scenario is appears to be the artifact of the computer models from which it was derived. The severe limitations of those models notwithstanding, heady pronouncements from people with impressive sounding degrees seem weighty. Are they? Perhaps, but over the last decade, actual temperature measurements at both ground, upper atmosphere, and from satellites have supported precisely none of the GCM predictions. Further, actual empirical climate history, including paleoclimatic data, throw just too many wrenches into too many gears of the AGW hypothesis and pose a plethora of intermeshed questions regarding the nature of climate variation that push AGW assumptions to the periphery.

The ideology of AGW, which is a part of the larger ideology and religion of environmentalism, is another question, and is approached in a somewhat different manner, as demonstrating the huge scientific uncertainties endemic to AGW claims will not convince such people to abandon their beliefs in these matters.

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And furthermore...

[_Fortigurn]

It would be helpful if you would only answer my simple question.

[_Coggins7]

And uh, precisely what question was that?

[_Fortigurn]

And uh, precisely what question was that?

I'll make it really easy for you.

[_Coggins7]

I am asking you to explain the process by which you have personally evaluated the available evidence, and dismiss certain claims whilst adhering to others.

By evaluating, over a roughly 15 to 20 year period, the competing claims of environmentalists and their critics, comparing and contrasting the scientific claims on both sides of the debate, looking, as a non-specialist, as best I can, at the peer reviewed data and interpretations of that on both sides, and then applying both inferential and deductive logic; the tools of critical analysis, to the questions, and concluded over time that the case against AGW is much stronger than the case for it.

[_Fortigurn]

I am asking you to explain the process by which you have personally evaluated the available evidence, and dismiss certain claims whilst adhering to others.

By evaluating, over a roughly 15 to 20 year period, the competing claims of environmentalists and their critics, comparing and contrasting the scientific claims on both sides of the debate, looking, as a non-specialist, as best I can, at the peer reviewed data and interpretations of that on both sides, and then applying both inferential and deductive logic; the tools of critical analysis, to the questions, and concluded over time that the case against AGW is much stronger than the case for it.

I'm sorry, you haven't actually explained the process of evaluation. You've simply told me that you carried out a process of evaluation. Nor have you actually answered the additional details I provided in that post which describe exactly what I want you to explain. I even provided a test case for you.

You claim to have spent 15-20 years evaluating the various claims, and yet in one of your posts you quoted without correction part of an article which describes the 'Little Ice Age' as a global phenomenon, when recent assessments of this event ('recent' in this case being within the last 5 years), confine the 'Little Ice Age' to the Northern hemisphere. This doesn't convince me that you actually have your finger on the pulse of this issue.

[_Coggins7]

You claim to have spent 15-20 years evaluating the various claims, and yet in one of your posts you quoted without correction part of an article which describes the 'Little Ice Age' as a global phenomenon, when recent assessments of this event ('recent' in this case being within the last 5 years), confine the 'Little Ice Age' to the Northern hemisphere. This doesn't convince me that you actually have your finger on the pulse of this issue.

All this actually shows is that, again, you haven't done your homework.



Little Ice Age (Global)


Summary

In light of the desperate attempts of certain scientists and politicians to convince the nations of the world that our current warmth is so great as to be unprecedented over the past millennium or more, and that our current high temperatures are therefore the result of the rise in the air's CO2 content that has been driven by the burning of fossil fuels associated with the development of the Industrial Revolution and our expanding global population, we routinely review scientific journal articles that reveal the worldwide existence of the even warmer Medieval Warm Period of a thousand years ago, as well as the subsequent global impact of the Little Ice Age, demonstrating thereby that there is nothing unusual or "CO2-induced" about what we could call the Modern Warm Period, it being but the most recent manifestation of the pervasive warm node of a millennial-scale oscillation of climate that reverberates throughout glacial and interglacial periods alike [see Climate Oscillations (Millennial Variability) in our Subject Index]. As part of this endeavor, we also list and discuss -- under the heading of Little Ice Age in our Subject Index -- numerous scientific papers that document the existence of this latest several-century cool phase of the 1300-1500-year climatic cycle that affects all parts of the planet. In addition, we have a subheading under which we file Editorials and Journal Reviews of papers that individually provide evidence for the global extent of this relatively cold period of earth's climatic history, as well as its likely cause; and in this Summary we describe the results of this group of globally-oriented papers.

Huang and Pollack (1997) searched the large database of terrestrial heat flow measurements compiled by the International Heat Flow Commission of the International Association of Seismology and Physics of the Earth's Interior for measurements suitable for reconstructing an average ground surface temperature history of the planet over the last 20,000 years. Based on a total of 6,144 qualifying sets of heat flow measurements obtained from every continent, they produced a truly global climate reconstruction, which they describe as being "independent of other proxy interpretations [and] of any preconceptions or biases as to the nature of the actual climate history." And what did they find? They found strong evidence that the Medieval Warm Period was indeed warmer than it is now, by perhaps as much as 0.5°C, and that the Little Ice Age was as much as 0.7°C cooler than it is currently worldwide.

Broecker (2001a) began his analysis of the subject by noting that glacial evidence for the Little Ice Age can be found all the way from the Swiss Alps in the Northern Hemisphere to the Southern Alps of New Zealand's South Island, stating that "the Little Ice Age cooled not just Europe but the world." He too notes the existence of alternating warm and cold periods that have occurred, in his words, "virtually unchanged, in both amplitude and duration," with a "nearly regular, 1,500-year cycle" that monotonously repeats itself as far back in time as we have been able to discern. Hence, as he continues, "we cannot assume that in the absence of human intervention, earth's temperatures would have remained stable." Like us, therefore, he concludes there is plenty of "room for maneuvering ... for those who doubt that the buildup of carbon dioxide and other greenhouse gases constitutes a substantial threat."

Another major step in the right direction was provided by Soon and Baliunas (2003) and Soon et al. (2003), who reviewed an immense body of evidence pertaining to the climatic history of the earth over the last millennium. And what did they find? As Soon and Baliunas describe it, "the assemblage of local representations of climate establishes both the Little Ice Age and Medieval Warm Period as climatic anomalies with worldwide imprints, extending earlier results by Bryson et al. (1963), Lamb (1965), and numerous intervening research efforts." In addition, they say that "across the world, many records reveal that the 20th century is probably not the warmest nor a uniquely extreme climatic period of the last millennium."

Krenke and Chernavskaya (2002) also present an impressive review of what is known about the Medieval Warm Period and Little Ice Age throughout the world, based upon glaciological, hydrologic and written historical evidence, as well as dendrological, archaeological and palynological data, concluding that "from the 9th century to, apparently, the mid-15th century, the climatic conditions were warmer than during most of the subsequent five centuries, including the early 20th century." In some places, in fact, they report it was even warmer during this Medieval Warm Period than it was during the latter part of the 20th century. For example, they note that "the northern margin of boreal forests in Canada was shifted [north] by 55 km during the Medieval Warm Period, and the tree line in the Rocky Mountains in the southern United States and in the Krkonose Mountains was higher by 100-200 m than that observed at the present time."

Concentrating on data from within Russia, the two members of the Russian Academy of Sciences report large differences in a number of climate variables between the Little Ice Age (LIA) and the preceding Medieval Warm Period (MWP). With respect to the annual mean temperature of northern Eurasia, they note an MWP to LIA drop on the order of 1.5°C. They also say that "the frequency of severe winters reported was increased from once in 33 years in the early period of time, which corresponds to the MWP, to once in 20 years in the LIA," additionally noting that "the abnormally severe winters [of the LIA] were associated with the spread of Arctic air masses over the entire Russian Plain." Finally, they remark that the data they used to draw these conclusions were "not used in the reconstructions performed by Mann et al.," which perhaps explains why the Mann et al. (1998, 1999) temperature history of the past millennium does not reproduce the Little Ice Age nearly as well as does the more appropriately derived temperature history of Esper et al. (2001). Hence, and in complete contradiction of the contentions of Mann et al., the Russian Academicians unequivocally state that "the Medieval Warm Period and the Little Ice Age existed globally."

Yet another study to provide a proper perspective of the Little Ice Age and the preceding Medieval Warm Period was that of Loehle (2004), who used a pair of well-dated and lengthy proxy climate records to characterize the pattern of temperature change over the past three millennia. The first of these records was derived by Keigwin (1996) from a study of the oxygen isotope ratios of foraminifera and other organisms contained in a Sargasso Sea sediment core retrieved from a deep-ocean drilling site on the Bermuda Rise. This record provides sea surface temperature data for about every 67th year from 1125 BC to 1975 AD. The second temperature series was derived by Holmgren et al. (1999, 2001) from studies of color variations of stalagmites found in a cave in South Africa, which variations are caused by changes in the concentrations of humic materials entering the region's ground water that have been reliably correlated with regional near-surface air temperature.

So why does Loehle use these two specific records … and only these two records? By way of explanation, he says that "most other long-term records have large dating errors, are based on tree rings, which are not reliable for this purpose (Broecker, 2001b), or are too short for estimating long-term cyclic components of climate." Also, and in repudiation of the approach employed by Mann et al. (1998, 1999) and Mann and Jones (2003), he reports that "synthetic series consisting of hemispheric or global mean temperatures are not suitable for such an analysis because of the inconsistent timescales in the various data sets," noting further, as a result of his own testing, that "when dating errors are present in a series, and several series are combined, the result is a smearing of the signal."

But can only two temperature series reveal the pattern of global temperature change? Feeling a need to reassure us on this matter, Loehle reports that "a comparison of the Sargasso and South Africa series shows some remarkable similarities of pattern, especially considering the distance separating the two locations," and he says that this fact "suggests that the climate signal reflects some global pattern rather than being a regional signal only." He also notes that a comparison of the mean record with the South Africa and Sargasso series from which it was derived "shows excellent agreement," and that "the patterns match closely," concluding that "this would not be the case if the two series were independent or random."

Proceeding with his plan of attack, which was to fit simple periodic models to the temperature data as functions of time, with no attempt to make the models functions of solar activity or any other physical variable, Loehle fit seven different time-series models to the two temperature series and to the average of the two series, using no data from the 20th century. In all seven cases, he reports that good to excellent fits were obtained. As an example, the three-cycle model he fit to the averaged temperature series had a simple correlation of 0.58 and an 83% correspondence of peaks when evaluated by a moving window count.

Comparing the forward projections of the seven models through the 20th century leads directly to the most important conclusions of Loehle's paper. He notes, first of all, that six of the models "show a warming trend over the 20th century similar in timing and magnitude to the Northern Hemisphere instrumental series," and that "one of the models passes right through the 20th century data." These results clearly suggest, in his words, "that 20th century warming trends are plausibly a continuation of past climate patterns" and, therefore, that "anywhere from a major portion to all of the warming of the 20th century could plausibly result from natural causes." In addition, Loehle's analyses reveal the existence of the Medieval Warm Period of 800-1200 AD, which is shown to have been significantly warmer than the portion of the Modern Warm Period we have so far experienced, as well as the existence of the Little Ice Age of 1500-1850 AD, which is shown to have been the coldest period of the entire 3000-year record.

So what is the cause of the reliable and well-defined millennial-scale climatic oscillation that has alternately produced the Roman Warm Period, Dark Ages Cold Period, Medieval Warm Period, Little Ice Age and, last of all, the Modern Warm Period? This is the question Bond et al. (2001) set out to answer in a study of ice-rafted debris found in North Atlantic deep-sea sediment cores and its temporal correlation with concentrations of the cosmogenic nuclides 10Be and 14C, which can be used as proxies for solar activity, as noted long ago by Lal and Peters (1967). In this particular case, they utilized measurements of 10Be sequestered in the Greenland ice cap and 14C contained within Northern Hemispheric tree rings, demonstrating, in their words, that "over the last 12,000 years virtually every centennial time-scale increase in drift ice documented in our North Atlantic records was tied to a solar minimum." Hence, they concluded that the underlying climatic cycle is solar-induced, and that "a solar influence on climate of the magnitude and consistency implied by our evidence could not have been confined to the North Atlantic."

In support of their contention that the phenomenon is global in scope, Bond et al. cite a number of studies that confirm the presence of the millennial-scale climatic oscillation in Scandinavia, Greenland, the Netherlands, the Faroe Islands, Oman, the Sargasso Sea, coastal West Africa, the Cariaco Basin, equatorial East Africa, and the Yucatan Peninsula, demonstrating thereby that "the footprint of the solar impact on climate we have documented extend[s] from polar to tropical latitudes." They also note that "the solar-climate links implied by our record are so dominant over the last 12,000 years … it seems almost certain that the well-documented connection between the Maunder solar minimum and the coldest decades of the LIA could not have been a coincidence," and that both the Little Ice Age and Medieval Warm Period "may have been partly or entirely linked to changes in solar irradiance."

In a similar approach to the subject, Bard et al. (2000) created a 1200-year history of cosmonuclide production in earth's atmosphere from 10Be measurements of South Pole ice (Raisbeck et al., 1990) and atmospheric 14C measured in tree rings (Bard et al., 1997), which they converted to Total Solar Irradiance (TSI) values by "applying a linear scaling using the TSI values published previously for the Maunder Minimum." This approach resulted in an extended TSI record that suggests, in their words, that "solar output was significantly reduced between 1450 and 1850 AD, but slightly higher or similar to the present value during a period centered around 1200 AD." Hence, they say "it could thus be argued that irradiance variations may have contributed to the so-called 'little ice age' and 'medieval warm period'," additionally noting that TSI variations "would tend to force global effects."

Noting that "the most direct mechanism for climate change would be a decrease or increase in the total amount of radiant energy reaching the earth," Perry and Hsu (2000) developed a simple solar-luminosity model by summing the amplitude of solar radiation variance for fundamental harmonics of the eleven-year sunspot cycle throughout an entire 90,000-year glacial cycle. Their model output was well correlated with the amount of 14C in well-dated tree rings gong back to the time of the Medieval Warm Period (about A.D. 1100), as well the sea-level curve developed by Ters (1987); and present in both of these records over the entire expanse of the Holocene was a "little ice age"/"little warm period" cycle with a period of approximately 1300 years. As a result, they too concluded that the idea of "the modern temperature increase being caused solely by an increase in CO2 concentrations appears questionable." And at the other end of the cause-and-effect spectrum, Hunt (1998) used a global climate model to demonstrate that most of the observed features in the climatic record of the earth "can be reproduced by processes associated with internal mechanisms of the climatic system," concluding on this basis also that "the modern temperature increase being caused solely by an increase in CO2 concentrations appears questionable.

In light of the results of these several studies, there would appear to be no question but what the Little Ice Age was global in scope [for even more evidence, see Little Ice Age (Regional - Africa, Antarctica, Arctic, Asia, Australia/New Zealand, Europe, North America, Oceans, South America, Tropics, as was the Medieval Warm Period. There is also considerable evidence that suggests these two climatic states were produced by a millennial-scale climatic oscillation driven by similar variations in solar activity, or possibly by no external forcing at all. Hence, there is no compelling reason to believe, as climate alarmists claim, that the historical increase in the air's CO2 content is responsible for the development of the Modern Warm Period. Earth's current high (but not unprecedented) temperatures could possibly have developed without any forcing at all, but are more likely the result of cyclical variations in solar activity.

References
Bard, E., Raisbeck, G., Yiou, F. and Jouzel, J. 1997. Solar modulation of cosmogenic nuclide production over the last millennium: comparison between 14C and 10Be records. Earth and Planetary Science Letters 150: 453-462.

Bard, E., Raisbeck, G., Yiou, F. and Jouzel, J. 2000. Solar irradiance during the last 1200 years based on cosmogenic nuclides. Tellus 52B: 985-992.

Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G. 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294: 2130-2136.

Broecker, W.S. 2001a. Glaciers That Speak in Tongues and other tales of global warming. Natural History 110 (8): 60-69.

Broecker, W.S. 2001b. Was the Medieval Warm Period global? Science 291: 1497-1499.

Bryson, R.A., Arakawa, H., Aschmann, H.H. and Baerris, D.A. plus 36 others. 1963. NCAR Technical Note. In: Bryson R.A., and Julian P.R. (Eds.) Proceedings of the Conference on Climate of the 11th and 16th Centuries, Aspen CO, June 16-24 1962, National Center for Atmospheric Research Technical Notes 63-1, Boulder, CO.

Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253.

Holmgren, K., Karlen, W., Lauritzen, S.E., Lee-Thorp, J.A., Partridge, T.C., Piketh, S., Repinski, P., Stevenson, C., Svanered, O. and Tyson, P.D. 1999. A 3000-year high-resolution stalagmite-based record of paleoclimate for northeastern South Africa. The Holocene 9: 295-309.

Holmgren, K., Tyson, P.D., Moberg, A. and Svanered, O. 2001. A preliminary 3000-year regional temperature reconstruction for South Africa. South African Journal of Science 99: 49-51.

Huang, S. and Pollack, H.N. 1997. Late Quaternary temperature changes seen in world-wide continental heat flow measurements. Geophysical Research Letters 24: 1947-1950.

Hunt, B.G. 1998. Natural climate variability as an explanation for historical climate fluctuations. Climatic Change 38: 133-157.

Keigwin, L.D. 1996. The Little Ice Age and Medieval Warm Period in the Sargasso Sea. Science 274: 1504-1508.

Krenke, A.N. and Chernavskaya, M.M. 2002. Climate changes in the preinstrumental period of the last millennium and their manifestations over the Russian Plain. Isvestiya, Atmospheric and Oceanic Physics 38: S59-S79.

Lal, D. and Peters, B. 1967. Cosmic ray produced radio-activity on the Earth. In: Handbuch der Physik, XLVI/2. Springer, Berlin, Germany, pp. 551-612.

Lamb, H.H. 1965. The early medieval warm epoch and its sequel. Palaeogeography, Palaeoclimatology, Palaeoecology 1: 13-37.

Loehle, C. 2004. Climate change: detection and attribution of trends from long-term geologic data. Ecological Modelling 171: 433-450.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.

Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 10.1029/2003GL017814.

Perry, C.A. and Hsu, K.J. 2000. Geophysical, archaeological, and historical evidence support a solar-output model for climate change. Proceedings of the National Academy of Sciences USA 97: 12433-12438.

Raisbeck, G.M., Yiou, F., Jouzel, J. and Petit, J.-R. 1990. 10Be and 2H in polar ice cores as a probe of the solar variability's influence on climate. Philosophical Transactions of the Royal Society of London A300: 463-470.

Soon, W. and Baliunas, S. 2003. Proxy climatic and environmental changes of the past 1000 years. Climate Research 23: 89-110.

Soon, W., Baliunas, S., Idso, C.D., Idso, S.B. and Legates, D.R. 2003. Reconstructing climatic and environmental changes of the past 1000 years: A reappraisal. Energy & Environment 14: 233-296.

Ters, M. 1987. Variations in Holocene sea level on the French Atlantic coast and their climatic significance. In: Rampino, M.R., Sanders, J.E., Newman, W.S. and Konigsson, L.K. (Eds.) Climate: History, Periodicity, and Predictability. Van Nostrand Reinhold, New York, NY, pp. 204-236.


The page at which all the peer sourced material upon which this analysis is based can be found at: www.co2science.org/scripts/CO2ScienceB2 ... iceage.jsp

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The Little Ice Age:
When global cooling gripped the world
By Alan Cutler

THE YEAR WAS 1645, and the glaciers in the Alps were on the move. In Chamonix at the foot of Mont Blanc, people watched in fear as the Mer de Glace (Sea of Ice) glacier advanced. In earlier years, they had seen the slowly flowing ice engulf farms and crush entire villages.

They turned to the Bishop of Geneva for help, and he made the journey to Chamonix. At the ice front he performed a rite of exorcism.

Little by little, the glacier receded.

But before long the threatening ice returned, and once again the bishop was summoned. The struggle against the glacier continued for decades.

Similar dramas unfolded throughout the Alps and Scandinavia during the late 1600s and early 1700s, as many glaciers grew farther down mountain slopes and valleys than they had in thousands of years. Sea ice choked much of the North Atlantic, causing havoc with fisheries in Iceland and Scandinavia. Eskimos paddled their kayaks as far south as Scotland. At the same time in China, severe winters in Jiang-Xi province killed the last of the orange groves that had thrived there for centuries.

These and many similar events, bewildering and disruptive to the societies of the time, are pieces of a global climatic puzzle that scientists and historians today call the "Little Ice Age."

Throughout the world, from Norway to New Zealand, glaciers in mountainous areas advanced. Elsewhere, particularly in parts of Europe and North America, temperatures plummeted and harsh weather set in. It was a time of repeated famine and cultural dislocation, as many people fled regions that had become hostile even to subsistence agriculture.

Experts disagree on the duration of the Little Ice Age. Some mark its inception as early as the 1200s, others view the Little Ice Age "proper" as beginning around 1450 or even later.

Disagreements arise because the phenomenon was not simply a giant cold snap. The cooling trend began at different times in different parts of the world and often was interrupted by periods of relative warmth.

All agree, however, that it lasted for centuries, and that the world began emerging from its grip between 1850 and 1900.

Most of the Little Ice Age occurred well before the Industrial Revolution and the widespread burning of fossil fuels, so scientists are confident that its climatic convulsions had purely natural causes. The event fascinates scientists because it gives them a glimpse of how Earth's climate system operates when left to its own devices.

"It's important because we're trying to understand the warming over the past 100 years," says Alan Robock of the University of Maryland's Department of Meteorology. "Some people have said it's just a `recovery from the Little Ice Age.' Well, what does that mean?"

In the 10,000 years since the end of the last major ice age, which closed the Pleistocene Epoch, Earth's climate has undergone a series of global warmings and global coolings. Though far smaller than the temperature swings of the Pleistocene, during which vast ice sheets expanded over large parts of continents and melted away several times, these oscillations nonetheless left their marks on human cultures and natural ecosystems.

With each climate change, whether global or local, ecological communities shifted north or south or were disrupted, leading to the creation of new groupings of species. Likewise, human cultures were uprooted and driven to more favorable locales, or people adapted by changing their technologies and behaviors.

About 6,000 years ago, for example, during a period known as the "Holocene Maximum," global temperatures were about 2 degrees Fahrenheit warmer than today. Rainfall patterns also were different. For example, in what is now the arid core of the Sahara desert, hippopotamuses and crocodiles thrived in lakes and swamps. Moister conditions in Mesopotamia and the Indus Valley aided the development of agriculture and humanity's first great civilizations in these regions.

Then global cooling dropped the temperatures to a little cooler than they are now, and living things shifted again. Earth didn't warm appreciably until about 2,000 years ago.

During the present millennium there was a period of relatively mild climate called the Medieval Warm Period, lasting from about 1000 to 1300 AD. As with the Little Ice Age, its timing and effects varied from region to region, and many experts doubt that the Medieval Warm Period was a truly global phenomenon. In East Asia, for example, temperatures were cooler.

Europe, though, enjoyed an undeniably balmy climate during the early medieval period. Agriculture flourished farther north and at higher elevations on mountains than is possible even in today's warmish climate, and harvests generally were good.

Farmers raised wine grapes in England 300 miles north of present limits, and in what now are icebound parts of Greenland, Norse settlers grazed sheep and dairy cattle. In his book Climate History and Modern Man, H.H. Lamb noted that the great burst of cathedral-building and population expansion in medieval Europe coincided with the peak of the Medieval Warm Period.

By about 1400, the climate had cooled to temperatures comparable to today. Over the next century or two, the world would cool still further, bringing on the Little Ice Age.

TRACKING CLIMATE

Unlike many earlier climate swings, the Little Ice Age was abundantly documented by human observers. Records include the first readings from meteorological instruments such as rain gauges and thermometers. Galileo invented the thermometer in the midst of the ice age, and in central England, reliable, monthly temperature records begin in 1659.

For most places, however, and for times before the 1600s, it takes some sleuthing to deduce past weather conditions. Climate historians search old journals and public documents for descriptions of events such as snowstorms, frosts and droughts at unusual times of year.

The prices of wheat and other grains in a given year sometimes are used to estimate the size of the harvest and, by another step in logic, the favorableness of the weather that year. One researcher even made a statistical study of the skies depicted in landscape paintings to trace the changes in cloudiness from the 1500s to the present.

Where human records are absent or unreliable, researchers turn to a host of natural climate indicators. Foremost among these are tree rings, which are formed by the annual growth of wood in the trunk. During warm years, trees grow fast, adding thick rings; during cool years, rings are thin. After correcting for idiosyncrasies of tree growth, a tree-ring researcher can use the pattern of thick and thin rings to reconstruct the temperatures during the tree's lifetime.

Researchers also drill into glacial ice at the poles and on high mountains to obtain records of snowfall, dust and atmospheric chemicals contained in the ice. These can give information on temperature, precipitation and even global wind patterns, if the source of the dust can be determined. Layers within lake sediments, coral reefs and cave formations can be analyzed by sophisticated chemical techniques to determine the temperatures at which they formed.

LIFE IN THE FRIGID TIME

From all these data sources, climate researchers have assembled a broad picture of a world that was, on average, one to two degrees cooler than it is today. For comparison, during the Pleistocene, when the ice cap in eastern North America reached as far south as Pennsylvania, the world was about nine degrees cooler.

Averages, however do not tell the story. The effects of the Little Ice Age were anything but uniform. Cooling was much more pronounced (or at least better documented) in the Northern Hemisphere than in the Southern Hemisphere. In some places and some years, winter temperatures were colder, but not summer temperatures. In France, for example, the harsh winter of 1788-89 added to the misery and discontent of the peasants, but Paris warmed up pleasantly in time for the storming of the Bastille that summer.

Cold and erratic weather patterns produced numerous crop failures in northerly areas such as Scotland and Norway. Native American tribes such as the Iroquois relocated their villages to escape the cold. These migrations stirred up political conflict among tribes, leading to the creation of nonaggresssion pacts like the famous League of the Iroquois, adopted in the 1500s.

Perhaps hardest hit were the Norse settlements in Iceland and Greenland. The population of famine-ridden Iceland dwindled during the Little Ice Age to half its previous numbers.

Greenlanders fared even worse. Growing sea ice cut off communication with the outside world beginning about 1370, and when German ships landed in Greenland more than a century later, they found a single frozen corpse but no living colonists among the ruins.

Despite all the hardships, there was a lighter side to the Little Ice Age. In London, freezings of the Thames River were celebrated with carnival-like "Frost Fairs" with food, drink and entertainment on the ice. The cold, snowy winters of the early 1800s may have inspired Charles Dickens' sentimental vision of the "old-fashioned" white Christmas.

In the fledgling United States, New York harbor froze over in winter, allowing people to walk from Manhattan to Staten Island.

So, what caused the Little Ice Age?

Because the sun is the ultimate source of Earth's warmth, some researchers have looked to it for an answer. In the 1970s, solar researcher John Eddy, now at Saginaw Valley State University in Michigan, noticed the correlation of sunspot numbers with major ups and downs in Earth's climate. For example, he found that a period of low activity from 1645 to 1715, called the Maunder Minimum, matched perfectly one of the coldest spells of the Little Ice Age.

Judith Lean, a solar physicist at the Naval Research Laboratory in Washington, estimates that the sun may have been about a quarter of 1 percent dimmer during the Maunder Minimum. This may not sound like much, but the sun's energy output is so immense that 0.25 percent amount to a lot of missing sunshine -- enough to cause most of the temperature drop, she says.

Other researchers have examined earthly causes. Volcanic eruptions are known to meddle with climate by injecting a veil of sun-blocking aerosols into the atmosphere -- the so-called parasol effect. Remember Mount Pinatubo? Its eruption in 1991 dropped Earth's average air temperature by about 1 degree -- an effect that lasted about two years. The University of Maryland's Robock points out that there were more frequent eruptions during the Little Ice Age than during the 20th century.

Most prominent was the 1815 eruption of Tambora in Indonesia. It pumped into the atmosphere vast amounts of ash -- ten times that of Krakatoa, another famous Indonesian volcano. The following year has been called the "Year Without a Summer." In June and July of 1816, New England and northern Europe suffered frost and even snow.

Scientists dispute the importance of these two causes, and of other possibilities such as shifts in ocean currents. But it seems possible that during the Little Ice Age Earth's climate was hit by a one-two punch from a dimmer sun and a dustier atmosphere.

What about the greenhouse gases -- mainly carbon dioxide and methane -- that have been so much in the news lately? These heat-trapping gases have been important players in the climate system since our planet's beginnings, but their natural variations in recent centuries have been too tiny to have had much impact.

That, however, may be changing. Greenhouse gases in the atmosphere have been increasing steadily on account of the burning of fossil fuels and other human activities. Lean, at the Naval Research Lab, says that while changes in solar output and volcanic dust seem to have driven the fluctuations of the past, this century's rise in temperature may have been influenced by humans.

The amount of influence, however, remains in dispute. Carbon dioxide concentrations have increased by about 28 percent since pre-industrial times and are growing at the rate of 0.4 percent per year. There is no dispute about this. There is, however, disagreement about whether the increase is warming the climate and by how much.

"We're lucky to have the phenomenon of the Little Ice Age," says climatologist Jonathan Overpeck of the National Oceanic and Atmospheric Administration in Boulder, Colo. Earlier periods of climate change are not nearly so accessible for study. "By studying the last several centuries we should really be able to narrow down the uncertainty with regard to what's going to happen next year or 50 years from now."

AN ICE AGE LEGACY

One thing that happened during the Little Ice Age was that it spoiled the 1816 summer vacation of poet Percy Bysshe Shelley and his wife, Mary, with friends at Lake Geneva in Switzerland. The weather was so cold that they stayed indoors much of the time, entertaining one another with horror stories. Mary Shelley's contribution was Frankenstein, the immortal fable of human tampering with the forces of nature.

In Shelley's tale, a legacy of the Little Ice Age, the monster and his creator meet their fates in a frozen Arctic sea. Today she might have chosen a parched greenhouse desert.

Science fiction aside, the clear message of science and history is that climate change has always been a natural phenomenon on Earth and a matter of vital human interest.

Alan Cutler is a visiting scientist at the National Museum of Natural History.