Millennial Climate Cycles
One way to get a handle on the scale of the global warming forecast is to compare it with natural climate changes in the past. Climate varies naturally, for several different reasons that tend to act on different timescales. In this chapter, we will consider natural climate changes of about a thousand years’ duration, and in the two chapters after that, climate changes on successively longer timescales, ultimately reaching millions of years in Chapter 6. Chapter 7 puts everything back together again, to compare climate changes in the past to those that are forecast for the future.
The first meteorological observation network was established in 1653 in Northern Italy. By the middle 1800s, weather observations including temperatures were recorded throughout the inhabited world. The earliest reliable record of global average temperature dates to about 1860.
Climate changes before this time are pieced together using what are called proxies of past climates. The first proxy method used to shed light on past climates was derived from grains of pollen preserved in lake sediments. A pine forest prefers a cooler climate than would suit a grove of maple trees, for example, so