Because this can be done for any arbitrary segment of years, it becomes difficult to give meaning to an ‘average’ value. The arithmetic mean of temperature during one decade might, by comparison, show that decade to be above average for the two centuries before and after it, but below average for the four centuries which lie ahead of it and behind it.
Thus it is interesting to note that a New York Times article, published in May 2015, says of California’s weather that
According to climate scientists, it may be the worst arid spell in 1,200 years.
The article is quite correct: around the year 800 AD, the globe was in the midst of a decades-long hot and dry spell known to historians as the ‘Medieval Warm Period,’ a time much hotter around the globe than anything within living memory.
The records which reveal this phenomenon are both sentient and organic. Written records of rainfall, snowfall, temperature, the crests of rivers, and the advance or retreat of glaciers goes back to Roman times and even earlier.
In terms of non-human records, tree rings, core samples, and the courses of rivers offer evidence about climate over the centuries and millennia.
As the Times notes, the Medieval Warm Period was a statistical outlier that stretched over several centuries, significantly, observably, and quantifiably hotter than our era. The Medieval Warm Period was also not anthropogenic in cause.
Interestingly, the Times asserts that the current climate, while not uniformly warming, is also not anthropogenic:
The California drought of today is mostly nature’s hand, diminishing an Eden created by man.
The statistical counterpiece to the medieval warm period is known as the ‘Little Ice Age,’ and began sometime between 1300 and 1350. Lasting until around 1850 or 1870, it was several centuries of global cooling, with averages temperatures measured to be lower than the average temperatures of preceding or succeeding centuries.
Like the Medieval Warm Period, the Little Ice Age was not anthropogenic. The current time, the early twenty-first century, is a time neither hotter nor colder than what has been verified over millennia of recorded history. The climate remains as it persistently has been: erratic and unstable.
It would be a worrying violation of pattern if the climate were stable. That hasn’t ever been observed in more than 6,000 years of human experience.
