  Monthly instrumental temperature records from 5 stations in the northern
hemisphere are analyzed, each of which is local and over 200 years in length,
as well as two reconstructed long-range yearly records - from a stalagmite and
from tree rings that are about 2000 years long. In the instrumental records,
the steepest 100-year temperature fall happened in the 19th century and the
steepest rise in the 20th century, both events being of about the same
magnitude. Evaluation by the detrended fluctuation analysis (DFA) yields Hurst
exponents that are in good agreement with the literature. DFA, Monte Carlo
simulations, and synthetic records reveal that both 100-year events were caused
by external trends. In contrast to this, the reconstructed records show
stronger 100-year rises and falls as quite common during the last 2000 years.
These results contradict the hypothesis of an unusual (anthropogenic) global
warming during the 20th century. As a hypothesis, the sun's magnetic field,
which is correlated with sunspot numbers, is put forward as an explanation. The
long-term low-frequency fluctuations in sunspot numbers are not detectable by
the DFA in the monthly instrumental records, resulting in the common low Hurst
exponents. The same does not hold true for the 2000-year-long reconstructed
records, which explains both their higher Hurst exponents and the higher
probabilities of strong 100-year temperature fluctuations. A long-term
synthetic record that embodies the reconstructed sunspot number fluctuations
includes the different Hurst exponents of both the instrumental and the
reconstructed records and, therefore, corroborates the conjecture.
