Human civilizations have experienced many of their worst trials during periods of cooling and cold temperatures over the past 8,000 – 10,000 years. These were episodes associated with droughts as well. Conversely, civilizations have tended to prosper during warm, wet periods. These associations between human progress and the natural environment are discussed in a pair of articles by Andy May: “Climate and Human Civilization Over the Past 18,000 Years“, and  “Climate and Human Civilization for the Past 4,000 Years“. The articles are part climate science, part history, and part anthropology, with many fascinating details.

May presents large charts that can be downloaded, and they are especially interesting to ponder. He uses historical temperature proxies from Antarctica and Greenland to construct the charts, along with more recent data on measured surface temperatures in Greenland. According to May, the proxies are highly correlated with other proxy data from less extreme latitudes. Several important takeaways are the following:

1. Warm periods in the historical record are associated with wet conditions, and cold periods are associated with dry conditions. This is intuitive, as warm air holds more moisture than cold air.
2. There are estimates of temperatures going back more than 800 million years; apparent cyclical regularities in temperatures have lasted as long 150 million years. Cycles within cycles are evident: a 100,000 year cycle is prominent as well as a 25,000 year cycle (see #4 below).
3. Today’s temperatures are not as high as those prevailing during about 200 years of the so-called Roman Warm Period, or during a span of similar length in the so-called Minoan Warm Period, about 3,300 years ago. Today’s temperatures are much lower than estimates for much of the earth’s pre-human history.
4. The southern hemisphere has more volatile temperatures than the northern hemisphere due to the tilt of the earth’s axis at perihelion in January, when the earth is closest to the sun. That means the southern hemisphere tends to have warmer summers and colder winters. That will reverse over the next 10,000 years, and then it will reverse again. There is more land mass in the north, however, so it’s not clear that less extreme weather in the north helps explain the hugely lopsided distribution of development and population in that hemisphere.
5. Recent increases in sea levels have been small relative to the years following the Little Ice Age. Projected increases over the next 50 years are of a magnitude that should be easily manageable for most coastal areas.
6. Atmospheric carbon concentration seems to lag major increases in temperatures by about 800 years, raising a question of causality. Today’s carbon concentration is low relative to earlier epochs; it has been increasing for thousands of years, clearly independent of human activity, and is now near 400,000 year highs.
7. Civilizations have blossomed with warm temperatures and they have collapsed or hit extended periods of retarded progress with declines in temperatures. Human agriculture was born as temperatures rose out of the depths of a glacial period about 10,000-12,000 years ago. Rome flourished during a warm cycle and collapsed as it waned. The Vikings settled in Greenland and Newfoundland during the Medieval Warm Period and were eliminated by the Little Ice Age. May cites a number of other examples of temperature cycles bringing on major shifts in the course of human progress. There are many possible explanations for the decline of past civilizations, but extremely low temperatures, droughts, and lengthy periods of weather inhospitable to agriculture have been important.

The fashion today is to insist that only dramatic changes in our use of energy can avert a global warming catastrophe. It is not clear that any effort by humans to manipulate global temperatures can overcome the natural forces that are always driving temperature change. For that matter, it is not clear that carbon dioxide is a bad thing, or that diverting vast quantities of resources to reduce it would be wise. CO2 is certainly not a pollutant in the normal sense of the word. Here is an excerpt from May’s conclusion in his “4,000 years” article, which speaks volumes:

“First, there is no perfect temperature. Man, even in pre-industrial times, adapted to a variety of temperatures and he has always done better in warm times and worse in cold times. Second, why would anyone want to go back to the pre-industrial climate? The Washington Post says the goal of the Paris Climate Conference was get the world to agree to limit global warming to less than two degrees above pre-industrial temperatures. Pre-industrial times? That’s the Little Ice Age, when it snowed in July, a time of endless war, famine and plague. According to the Greenland ice core proxy data, temperatures 180 years ago were nearly the coldest seen since the end of the last glacial period 10,000 years ago! Why measure our success in combating anthropogenic warming, if there is any such thing, from such an unusually cold time?“