During the 10 years between the ICPP's inagural report and its latest report in 2000, key questions have been raised regarding the ICPP's evidence, methodologies, and forecasts. A major issue is whether there is, in fact, a linkage between increasing levels of atmospheric CO2 concentrations and rising surface temperatures. By examining various "proxies,"" such as tree rings and ice core drillings, scientists since 1990 have discovered that the increase in global surface temperatures over the last 100 years was not linear, but occurred mostly during the period 1900 to 1940, when there was negligible human induced CO2 activity. Furthermore, these studies show that there was actually a cooling period from 1940 through 1970 during which global mean temperatures actually declined. The ICPP's climate models failed to accurately plot these temperature patterns, raising obvious concerns about the model's utility.
The IPCC's 2000 report counters by arguing that surface-temperature readings over the past 25 years indicate a sustained global warming. Because this time period correlates with the growth of atmospheric CO2 concentrations, the ICPP concluded in 2000 that human activity over the past 20 years has had "a discernable influence- on global climate. But other scientists are not convinced. Professor Fred Singer, a meteorologist at the University of Virginia and a leading critic of the IPCC, published a report in 2000 challenging the IPCC's ground-based temperature data (which underpins its central global warming models and thesis). According to Singer, "proxy- records such as tree rings and core drillings show no global warming since 1940. Moreover, satellite measurements taken over the past two decades have also failed to detect any sign of global warming - despite taking measurements during supposedly the warmest period on record. The IPCC acknowledges these satellite readings, but theorizes that perhaps some as yet unknown factor in the upper and lower troposphere is distorting the satellite readings.