Climate researchers have reliable temperature records from weather stations dating back to about 1850. To study variations in climate that pre-date the advent of weather stations, climatologists rely on reconstructions based on a variety of proxy and direct measurements.
Reconstruction of global temperature changes based on 695 boreholes in the Northern Hemisphere. Blue-shaded area indicates the range of values for various reconstructions. After Pollack and Smerdon 2004.
The historical temperature record based on ice core proxy measures indicates that Earth’s climate during the past million years has been punctuated with about 8 cycles of relatively long, colder periods (glacials) interrupted by relatively short, warmer periods (interglacials). Each of nine interglacial periods correlates with higher concentrations of the greenhouse gases CH4, CO2, and N2O. Current levels of these gases, however, greatly exceed those at any time during the past 650,000 years.
Tree ring and coral proxy measures provide clues to changes within the past millennium. A compilation of tree ring data from the Northern Hemisphere indicates that air temperatures today are as warm as any time during the past 1,200 years. Cores samples from coral reefs at the Great Barrier Reef, Australia dating as far back as 1565 show that temperatures near the corals during the last half of the twentieth century are as warm as they have been in over 400 years.
Assessments of changes in glacial extent support this account. Assuming a simple relationship (one that ignores variation in precipitation) between glacier length and average air temperature, one can reconstruct the temperatures near various glaciers over the past 400 years.  These reconstructions indicate that temperatures were relatively uniform through the seventeenth, eighteenth, and nineteenth centuries but began to rise worldwide during the twentieth century. Recent reconstructions of temperature records based on studies of shallow ice cores and boreholes suggest a similarly abrupt rise in temperature within the last 150 years.
 Oerlemans, J. (2005) Extracting a climate signal from 169 glacier records. Science 308:675-677.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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