The notion propagated over social media is roughly this, that since the increased use of ‘fossil fuels’ (coal, oil, and natural gas) started in the late 1700s, emissions, specifically carbon dioxide have accumulated in the earth’s atmosphere and thereby initiated changes in the climate.
Long-term trends in the planet’s climate, however, predate large-scale industrialization.
A wide variety of techniques allow researchers to measure conditions prior to the advent of recorded readings from thermometers: core samples from ice or soil; tree-ring evaluation; written records quantifying the expansion and contraction of glaciers, and gauging snowfall and rainfall; and general agricultural and naturalistic records documenting which types of plant thrived in various locations.
From such measurements, it is possible to accurately reconstruct a climatic history of the Earth. The Intergovernmental Panel on Climate Change (IPCC) sets forth a history which includes a Roman Warm Period (ending around 400 A.D.), a Dark Ages Cold Period (ending around 950 A.D.), a Medieval Warm Period (ending around 1250 A.D.), and a Little Ice Age (ending around 1870 A.D.).
Naturally, these dates are not precise, but rather indicate a general and gradual change in a trend. Terminology varies somewhat, as the Medieval Warm Period is, e.g., also known as the Medieval Climate Optimum or the Medieval Climate Anomaly (MCA). The IPCC notes that
New warm-season temperature reconstructions covering the past 2 millennia show that warm European summer conditions were prevalent during 1st century, followed by cooler conditions from the 4th to the 7th century.
What seems, in the short-term context of a century or two, to be a warming trend, is in the larger context of several millennia merely the global climate emerging from end of the Little Ice Age.
A visual graph on a Cartesian plane of the Earth’s climatic temperature over time would look something like a sine wave, warming and cooling patterns following each other over the centuries. The IPCC writes that
Persistent warm conditions also occurred during the 8th–11th centuries, peaking throughout Europe during the 10th century. Prominent periods with cold summers occurred in the mid-15th and early 19th centuries.
This pattern extends backward in time long before the commencement of the large-scale use of fossil fuel. The planet has experienced centuries in which northern Europe and North America had temperatures warm enough to support subtropical plant life.
Conversely, there were centuries in which, e.g., large parts of northern and central Africa experience temperatures consistent with temperate zones. These extremes occurred at a time when the use of coal, oil, and gas was nearly unknown. The IPCC reports that
There is high confidence that northern Fennoscandia from 900 to 1100 was as warm as the mid-to-late 20th century.
The pattern which emerges, then, is a steady sinusoid pattern, with the planet’s climate alternating between warm periods and cool periods every few centuries. This general pattern seems to continue unaffected by any anthropogenic variables.
If the use of fossil fuels were capable of altering the earth’s climate, then other human activities of comparable size and scope should be also capable of producing changes in climate.
But other human actions, like the shockingly large number of atomic bombs and hydrogen bombs detonated during the era of frequent bomb tests, have had no long-term measurable effect on the climate. The explosion of over 2,000 nuclear weapons over a few years was not capable of altering the climate.
If the weapons-testing programs were not capable of producing long-term warming or cooling trends (they might have had temporary localized effects), then it’s much less likely that the comparatively smaller use of fossil fuel would generate any change in climate trends.
The reliability of the sinusoid as a best-fit pattern for global climate stands, even though individual data points are sometimes outliers which depart from the best-fit line, as the IPCC indicates:
The evidence also suggests warm conditions during the 1st century, but comparison with recent temperatures is restricted because long-term temperature trends from tree-ring data are uncertain.It would be noteworthy, even surprising, if the planet weren’t in the midst of a warming or cooling trend. The ‘normal’ condition of the Earth’s climate is to be in constant change.
Going back several thousand years, the Earth’s climate has demonstrated a semi-regular pattern of alternating between several warm centuries and several cool centuries. In light of this historical record, the climate’s current behavior is within ranges established by this pattern.
Because temperature trends in the late twentieth and early twenty-first centuries are consistent with the tendencies observed in previous millennia, there might be no need to search for extraordinary or anthropogenic causes. The IPCC notes that the MCA, a millennium ago, was likely even warmer than the current twenty-first century climate:
In the European Alps region, tree-ring based summer temperature reconstructions show higher temperatures in the last decades than during any time in the MCA, while reconstructions based on lake sediments show as high, or slightly higher temperatures during parts of the MCA compared to most recent decades.
Likewise, the Roman Warm Period, beginning around 250 B.C., was perhaps warmer than the current climate:
The longest summer temperature reconstructions from parts of the Alps show several intervals during Roman and earlier times as warm (or warmer) than most of the 20th century.
In sum, the climate is ‘changing’ only in the sense that it has been continuously and predictably changing for at least the last two millennia. The climate is doing nothing extraordinary.
Observed and measured temperatures, and warming and cooling trends, do not correlate with the onset of industrialization and the widespread use of fossil fuels. The data do not suggest that any generalizations gathered from climatic observation are anthropogenic.
