A careful study of climate history showed that there is a strong likelihood that
the major portion of temperature change in the 20th century was due to natural variations.
Evidence: the Holocene records up to 8000 years before present, from several ice
cores were examined.
. . . The average standard deviation of temperature was 0.98 ± 0.27 °C. This suggests
that while some portion of the temperature change observed in the 20th century was
probably caused by greenhouse gases, there is a strong likelihood that the major
portion was due to natural variations.
Global Temperature Record Can Be Explained By A Random Walk.
Random Walk analysis of NOAA global temperature anomaly data
The global temperature record doesn’t demonstrate an upward trend. It doesn’t demonstrate
a lack of upward trend either. Temperature readings today are about 0.75°C higher
than they were when measurement began in 1880, but you can’t always slap a trendline
onto a graph and declare, “See? It’s rising!” Often what you think is a pattern is
actually just Brownian motion. When the global temperature record is tested against
a hypothesis of random drift, the data fails to rule out the hypothesis. This doesn’t
mean that there isn’t an upward trend, but it does mean that the global temperature
record can be explained by simply assuming a random walk. The standard graph of temperatures
over time, despite showing higher averages in recent decades than in earlier ones,
doesn’t constitute a “smoking gun” for global warming, neither natural nor anthropogenic;
merely drawing a straight line from beginning to end and declaring it a trend is
a grossly naive and unscientific oversimplification, and shouldn’t be used as an
argument in serious discussions of environmental policy.