The identification of natural components in climate change during the last millennium and
last century is an important task in order to accurately estimate the manmade components of
climate change. A few semi-quantitative methods are found in this endeavor. Once one
recognizes that the global average temperature change during the last century can be
approximated by a linear increase of 0.5°/100 years, a new view of climate change during the last
century and beyond emerges. Based on this consideration and an extensive literature review, it is
found that the linear change of approximately the same gradient can be extended to 1800~1850,
which is 100 years before CO2 in the atmosphere began to increase rapidly, strongly suggesting
that the linear change is a natural change. The Earth had experienced a period ~1°C cooler than
the present between 1400 and 1800, namely the Little Ice Age (LIA), so that the linear rise of
temperature can be considered as the recovery of the Earth from the LIA. In addition, the linear
increase is superposed by the multi-decadal oscillation of amplitude 0.2°C, with a period of
50~60 years. It caused a prominent temperature rise from 1910 to 1940, which is similar to that
from 1975 to 2000; the IPCC considers that the latter rise is mostly caused by the greenhouse
effect of CO2, but I here propose that the latter rise was the combination of the linear change and
the positive trend of the multi-decadal oscillation, similar to that from 1910 to 1940. In 1940 and
2000, the temperature change reached a maximum. I suggest that the halting of the rise during
the first decade of this century is caused by the beginning of the negative trend of the multidecadal
oscillation, similar to that in 1940, taking over the linear rise.