a) Industrial
pollution from factories, power plants, and cars caused global warming
b)
Natural variations in global temperatures may occur in roughly 500-years
cycles
c) Global cooling occurred as a result of the Renaissance
Period
CHART INFORMATION and SOURCE:
Example of regional variations in surface air temperature
for the last 1000 years, estimated from a variety of sources, including
temperature-sensitive tree growth indices and written records of various
kinds, largely from western Europe and eastern North America. Shown are
changes in regional temperature in ° C, from the baseline value for
1900. Compiled by R. S. Bradley and J. A. Eddy based on J. T. Houghton
et al., Climate Change: The IPCC Assessment, Cambridge UniversityPress,
Cambridge, 1990 and published in EarthQuest, vol 5, no 1, 1991. Courtesy
of Thomas Crowley, Remembrance
of Things Past: Greenhouse Lessons from the Geologic Record