Understanding changes in the Earth's climate in the past 100-200 years, or the slightly longer interval extending back through the last glacial interval, rely on several types of data. Instrumental records of Earth's climate extend back to about the year 1850, when recording devices were put into widespread use. Long cores of ice obtained from Greenland and other locations are also widely used to measure past climate conditions, with this record extending back for about 650,000 years.
The IPCC issued the following statement in November 2007:
Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level. Global atmospheric concentrations (of greenhouse gases) have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial levels. The global increases in carbon dioxide concentration are due primarily to fossil fuel use and land-use change, while those of methane and nitrous oxide are primarily due to agriculture.
This bold and controversial statement was based on rigorous analysis of data from the past 1,000 years, showing that temperatures remained fairly steady at about 0-0.5 degrees below the 1990 average value from the year 1000 to about 1910, then began a sharp upward turn that flattened off for a short time in the 1950s, and has turned sharply up again since about 1976. Temperatures are now about 0.5-1.0
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Year 2000 constant concentrations 20th century
B1 A1T B2 A1B A2 A1F1
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