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Temperature for the last 11,000 years

Temperature in the distant past is inferred from "proxies"--a proxy is some measurable quantity that is known to vary in a known way with temperature. One such proxy is the ratio of the two oxygen isotopes ¹⁸O and ¹⁵O in the atmosphere. Since Arctic and Antarctic ice contains trapped air bubbles, this isotope ratio can be measured for as long ago as ice cores can be recovered. Temperature data has been deduced from the kilometer-long ice core drilled as part of the Greenland Ice Sheet Project GISP2. The zero point of temperature is arbitrarily set to the temperature in 1950. We will show the resulting temperature data in a series of graphs over different time periods.

The first graph goes back 2400 years. Observe that for a century around 1000 AD, it was warmer than it is now. This is called the Medieval Warm Period. It was also briefly warmer around 1150 and around 1250. But then it was colder from around 1300 on until around 1900. That cold period is called the Little Ice Age. You might think a half a degree wouldn't make much difference, but it does (as will be discussed on another page, but is hinted at by the remarks in red on the first graph). Right now we are just trying to get the facts straight as to what the temperature actually was at different times and places.

[Source of the graph]

It's interesting how the temperature is related to the isotope ratio of oxygen, but we won't discuss that here, because it doesn't seem to be at all controversial.

The second graph goes back 11,000 years, to the end of the last Ice Age, or glaciation:

You can still see the Medieval Warm Period and the Little Ice Age at the right of the graph, but you can also see that they were nothing very unusual, as there were other warm and cold periods in the preceding millennia. But those variations (all but one less than one degree) are nothing compared to the warming immediately after the end of the glaciation. Maybe the time when humans "discovered" agriculture wasn't so accidental.

The commentary on this graph by Muller et. al. is worth repeating:

The dip near 6000 BC is not understood. It actually appears to be coincident with a short term increase in temperature that took place in Antarctica! So we can't easily interpret everything in these plots, at least not without studying other records. Fluctuations are evident all over the plot, and crying to be understood. Agriculture began about 7,000 BCE, as marked on the plot. All of civilization was based on this invention. Agriculture allows large groups of people to live in the same location. It allows a small number of people to feed others, so that the others can become craftsmen, artists, historians, inventors, and scientists. The sudden rise at the left side of the plot, at about 9,000 BCE (i.e. 11,000 years ago), was the end of the last ice age. The abruptness of the termination is startling. Agriculture, and all of our civilization, developed since this termination. The enormous glacier, several kilometers thick, covering much of North America and Eurasia, rapidly melted. Only small parts of this glacier survived, in Greenland and Antarctica, where they exist to this day. The melting caused a series of worldwide floods unlike anything previously experienced by Homo sapiens . (There had been a previous flood at about 120 kyr, but that was before Homo sapiens had moved to Europe or North America.) The flood dumped enough water into the oceans to cause the average sea level to rise 110 meters, enough to inundate the coastal areas, and to cover the Bering Isthmus, and turn it into the Bering Strait. The water from melting ice probably flooded down over land in pulses, as ice-dammed lakes formed and then catastrophically released their water. These floods left many records, including remnant puddles now known as the Great Lakes, and possibly gave rise to legends that persisted for many years. As the glacier retreated, it left a piles of debris at its extremum. One such pile is now known as New York's Long Island.