claim: Two American researchers allege that U.S. government scientists have  skewed global temperature trends by ignoring readings from thousands of  local weather stations around the world, particularly those in colder  altitudes and more northerly latitudes, such as Canada.
evidence: There were a small number of weather stations in the 1850s, and the number didn't reach the current 3000+ until the 1951 to 1990 period The 2001 IPCC Third Assessment Report ( TAR ) acknowledged that the urban heat island is an important local effect, but cited analyses of historical data indicating that the effect of the urban heat island on the global temperature trend is no more than 0.05 °C ( 0.09 °F ) degrees through 1990.