Recent 'hockey stick creation' studies from Oregon State and Penn State have attempted to minimize historical temperature variation by incredible statistical misrepresentation...however, the real-world empirical research confirms natural oscillations cause numerous large global temperature changes, not CO2 - as shown by sea surface temperature variation off Japan
(click on image to enlarge, source)
"Working with three sediment cores retrieved off the coast of central Japan...Isono et al. generated a multidecadal-resolution record of alkenone-derived sea surface temperature (SST) that covers the full expanse of the Holocene...This record, in their words, "showed centennial and millennial variability with an amplitude of ~1°C throughout the entire Holocene," and they state that "spectral analysis for SST variation revealed a statistically significant peak with 1470-year periodicity." At the latter end of the record report that "SST minima centered at ca. 0.3 ka and ca. 1.5 ka are correlated with the Little Ice Age and the Dark Ages Cold Period in Europe, respectively, whereas the SST maximum centered at ca. 1.0 ka is correlated with the Medieval Warm Period." From data presented in the authors' Figure 2, we estimate that the MWP was about 1°C warmer than the Current Warm Period." [Dai Isono, Masanobu Yamamoto, Tomohisa Irino, Tadamichi Oba, Masafumi Murayama, Toshio Nakamura, Hodaka Kawahata 2009: Geology]
From year 1AD, atmospheric CO2 levels were essentially flat until the late 19th century. The superimposed mauve CO2 hsitorical curve (data sources) clearly indicates that CO2 levels were absolutely not responsible for the huge past temperature variation.
Likewise, more recent global warming/cooling cannot be explained by CO2's tiny impact.
The recently discredited study (and widely confirmed science abomination) known as the Marcott et al (2013) study actually incorporated this Isono et al (2009) peer-reviewed field research into their own study. Yet the wide historical temperature variation exhibited by the Isono research was effectively disappeared by Marcott's absurd data/statistical manipulations.