Revisiting 2,000 Years of Climate Change (Bad Science and the “Hockey Stick”)

Near the turn of the 21st century, several climate scientists believed that rising CO2 was the cause of observed warming and sought to support this by showing that excursions in the Earth temperature were small for the past 2,000 years, with a sudden increase in the 20th century at the same time period when CO2 emissions increased (a so-called “hockey stick” shape to the temperature vs. time plot). To do this, they employed a large number of proxies (such as tree rings) to estimate the Earth’s temperature over the last 2,000 years. Each proxy was compared to the measured temperature during the calibration period (the 20th century), and this relationship was used to convert up to 2,000 years of proxy measurements to estimated temperature. A sophisticated algorithm was used to process the large amount of data from multiple proxies into a yearly average temperature for up to 2,000 years. Unfortunately, they made several mistakes in the statistical analysis, one of which produced a significant statistical error. This error, in which they standardized the data against the mean during the calibration period (rather than the entire data set) had the effect of almost ignoring most of the proxies, while heavily weighting those few with an increase in the 20th century – thus artificially producing a hockey stick form to the plot of temperature vs. time. In addition, the innate merit of many proxies is dubious. The climate scientists involved appeared to ignore valid criticisms from outside their peer group, and they continued to claim the hockey stick result. Climate scientists also vigorously opposed any suggestion that factors other than CO2 also affected the climate. Climate scientists introduced elements of political influence into scientific discourse by actions such as loading up a thin paper with a large number of authors – publishing votes of confidence rather than scientific results.