Should we take the average temperature of the last 500 years as normal? Or of the 500 years prior to that? How does the 500 years between 1450 and 1950 compare to the 500 years between 1200 and 1700? And which of them might be “normal”? The lack of direct temperature measurements complicates the question further. Prior to a century or two ago, few direct observations of climate were recorded. Even in the last century, large parts of the earth's surface were unmeasured in terms of daily temperatures.
This is true of rainfall as well: it is constantly changing on a micro and a macro scale, over decades and centuries, and observational data are scarce for most regions prior to the last few years.
A wealth of questions and dearth of data: one political question looms over all - could any of this climate change be caused by human activity? There are indeed quite a few policymakers who believe that climatic instability is anthropogenic. How can this claim be verified?
Those who assert that the planet's climate is being disrupted usually assert that this is the result of industrialization. Large-scale use of fossil fuels, and the production of other alleged agents of climate change, began around 1750. The earliest working steam engines date to the late 1600's, and people had burned wood, coal, peat, and charcoal for centuries before that. But the mid-1700's saw an acceleration of industrialization, and if human activity can influence the climate, then this would be the starting date.
One may formulate the question this way: do we see changes in the earth's climate after 1750 which are unlike changes and patterns which occurred prior to that date? If so, then it is at least possible that some climate change is anthropogenic. Jack Goldstone, commenting on research published by Geoffrey Parker, notes that scholarship uses
the geological, biological, and recorded data on regional and global temperatures to show that the “Little Ice Age” of the 17th century was not a mere figure of speech or anachronistic exaggeration; it was in fact an age of dramatically lower temperatures and the advance of ice and glaciers across the world: at one point even the Bosporus froze solid, creating a land bridge from Europe to the Middle East that had not existed for millennia.
Thus documented is a temperature swing, in this case a cooling, greater in magnitude and duration than anything observed after 1750. The early 1600's were the center of this "Little Ice Age," which spanned, according to scholar J.R. McNeill, the years from 1250 to 1850. Nobody is likely to assert that a climate change starting in 1250 was caused by human activity.
The causes and variables in the climate are so many, and their interrelationships so complex, that mathematical and computerized models are still insufficient to reflect them. Current computational models, as complex as they may be, are still oversimplified relative to the actual climate, and do not produce accurate results.
Of the many factors in this complicated climate, Goldstone mentions solar and volcanic activity. He writes that Parker's scholarship
attributes the climate change to both a decline in solar radiation (indicated by a sustained period of low or absent sunspot activity known as the “Maunder minimum” from 1645 to 1710) and to clusters of major volcanic eruptions (in 1600-1609, 1641-1647, 1676-1679, and 1709-1710) whose sulfur and ash further reduced the solar radiation reaching the lower atmosphere. This combination triggered several series of exceptionally cold years in succession, in 1600-1609, 1620-1627, 1641-1647, 1666-1680, and 1695-1699, as measured by severe low summer temperatures in the northern hemisphere.
The example above cites a significant and enduring cold period. Much of the current concerns about climate contemplate, however, the hypothesis of a warming trend. Immediately prior to the “Little Ice Age” was a period of several centuries of atypically warm weather. Here is an instance of “global warming” which occurred at a time which prevents it from being labeled as anthropogenic in any sense. Under the wordy title Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, the IPCC writes:
Continental-scale surface temperature reconstructions show, with high confidence, multi-decadal periods during the Medieval Climate Anomaly (year 950 to 1250) that were in some regions as warm as in the late 20th century.
Dramatic changes in temperature and rainfall are documented, then, at points in time prior to any possible effects of large-scale industrialization. If current climate trends have yet to reach either the magnitude or the duration of these previous naturally-occurring climate disruptions, then there is little reason to hypothesize that such trends are anthropogenic.
